Patchwork x86: Use pad in pthread_unwind_buf to preserve shadow stack register

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Submitter H.J. Lu
Date April 12, 2018, 11:50 p.m.
Message ID <CAMe9rOqiboX7-Pdiy1i=0Pc7JYT8GEQ8CSFXfWXHj+CXPrNQ2g@mail.gmail.com>
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Permalink /patch/26706/
State New
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H.J. Lu - April 12, 2018, 11:50 p.m.
On Thu, Apr 12, 2018 at 2:36 PM, Carlos O'Donell <carlos@redhat.com> wrote:
> On 04/06/2018 07:59 AM, H.J. Lu wrote:
>> On Thu, Apr 5, 2018 at 9:46 PM, Carlos O'Donell <carlos@redhat.com> wrote:
>>> On 03/30/2018 12:41 PM, H.J. Lu wrote:
>>>> On Thu, Mar 29, 2018 at 1:20 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
>>>>> * H. J. Lu:
>>>>>
>>>>>> On Thu, Mar 29, 2018 at 1:15 PM, Florian Weimer <fw@deneb.enyo.de> wrote:
>>>>>>> * H. J. Lu:
>>>>>>>
>>>>>>>> You need to make a choice.  You either don't introduce a new symbol
>>>>>>>> version or don't save shadow stack for thread cancellation.  You
>>>>>>>> can't have both.
>>>>>>> I don't understand.  We have room to save the shadow stack pointer in
>>>>>>> the existing struct.
>>>>>> No, we don't have room in struct pthread_unwind_buf:
>>>>>>
>>>>>> Note: There is an unused pointer space in pthread_unwind_buf_data.  But
>>>>>> it isn't suitable for saving and restoring shadow stack register since
>>>>>> x32 is a 64-bit process with 32-bit software pointer and kernel may
>>>>>> place x32 shadow stack above 4GB.  We need to save and restore 64-bit
>>>>>> shadow stack register for x32.
>>>>> We have for void * fields.  They are subsequently overwritten by
>>>>> __pthread_register_cancel.  But __sigsetjmp can write to them first
>>>>> without causing any harm.  We just need a private __longjmp_cancel
>>>>> that doesn't restore the shadow stack pointer.
>>>> Here is the patch which does that.   Any comments?
>>>
>>> OK, so I have reviewed https://github.com/hjl-tools/glibc/ hjl/cet/master,
>>> please confirm that this is the correct branch of the required implementation
>>> for CET. It really helps to review the rest of the patch set, you should be
>>> preparing this as a patch set instead of having it reviewed one-at-a-time.
>>> This issue was already raised in the thread with Zack.
>>
>> Thanks for your feedbacks.
>>
>>> Architecture:
>>>
>>> * We avoid a "flag day" with feature_1 TCB flag to switch to a new ABI,
>>>   which we have discussed is a fragile process which should be avoided if
>>>   a supportable alternative solution exists.
>>>
>>> * We avoid versioned symbols, this makes CET backportable, and this has a
>>>   bigger benefit for long-term stable distributions.
>>>
>>> * A key design problem has been that cancellation jump buffers within glibc
>>>   are truncated to optimize on-stack size, and this means that setjmp could
>>>   write beyond the structure because setjmp now tries to save the shadowstack
>>>   pointer into space that the cancellation jump buffer did not allocate.
>>>   For the record this optimization seems premature and I'm sad we did it, this
>>>   is a lesson we should learn from.
>>>
>>> * We have all agreed to the following concepts:
>>>
>>>   * The cancellation process, in particular the unwinder, never returns from
>>>     any of the functions we call, it just keeps calling into the unwinder to
>>>     jump to the next unwound cancellation function all the way to the thread
>>>     start routine. Therefore because we never return from one of these functions
>>>     we never need to restore the shadow stack, and consequently wherever it is
>>>     stored in the cancellation jump buffer can be overwritten if we need the
>>>     space (it's a dead store).
>>>
>>>     * The corollary to this is that function calls made from cancellation handlers
>>>       will continue to advance the shadowstack from the deepest point at which
>>>       cancellation is initiated from. This means that the depth of the shadowstack
>>>       doesn't match the depth of the real stack while we are unwinding. I don't
>>>       know if this will have consequences on analysis tooling or not, or debug
>>>       tools during unwinding. It's a fairly advanced situation and corner case,
>>>       and restoring the shadowstack is not useful becuase we don't need it and
>>>       simplifies the implementation.
>>>
>>>   * The cancellation jump buffer has private data used for chaining the cancel
>>>     jump buffers together such that the custom unwinder can follow them and
>>>     call them in sequence. This space constitutes 4 void *'s which is space
>>>     that setjmp can write to, because we will just overwrite it when we register
>>>     the cancel handler.
>>>
>>>   * If the new shadowstack-enabled setjmp stores the shadowstack pointer into
>>>     the space taken by the 4 void*'s then we won't overflow the stack, and we
>>>     don't need to change the layout of the cancellation jump buffer. The 4 void*'s
>>>     are sufficient, even for x32 to write a 64-bit shadow stack address.
>>>
>>> * After fixing the cancellation jump buffers the following work needs to be reviewed:
>>>
>>>   * Add feature_1 in tcb head to track CET status and make it easily available
>>>     to runtime for checking.
>>>
>>>   * Save and restore shadowstack in setjmp/longjmp.
>>>
>>>   * Add CET support to ld.so et. al. and track runtime status.
>>>
>>>   * Adjust vfork for shadow stack usage.
>>>
>>>   * Add ENDBR or NOTRACK where required in assembly.
>>>
>>>   * CET and makecontext incompatible.
>>>     - Probably need to discuss which default is appropriate.
>>>     - Should the user get CET automatically disabled in makecontext() et. al. silently?
>>>     - Should your current solution, which is to error out during the build, and require
>>>       flag changes, be the default? This forces the user to review the security for their
>>>       application.
>>
>> I'd like to reserve 4 slots in ucontext for shadow stack:
>>
>> https://github.com/hjl-tools/glibc/commit/9bf6aefa8fb45f8df140d42ce9cf890bb24076e1
>>
>> It should be binary backward compatible.  I will investigate if there is a way
>> to support shadow stack with existing API.  Otherwise, we need to add a new
>> API for ucontext functions with shadow stack.
>
> Could you explain in detail how this is binary backwards compatible?
>
> Is the assumption that if you extend ucontext_t, that the kernel will just write
> more to the stack, and users who accesss it via the void* in a signal handler
> setup via sigaction + SA_SIGINFO, will not read past the size they expect?
>
> How is the shadow stack information moved between a getcontext -> setcontext
> set of API calls? The user ucontext_t in existing binaries is too small to hold the
> shadow stack?
>
> Would we again have a "flag day" where CET enablement must be coordinated with
> the definition of a new ucontext_t?
>
>>>   * prctl for CET.
>>
>> We have been experimenting different approaches to get the best implementation.
>> I am expecting that this patch may change as we collect more data.
>>
>
> OK.
>
>>> * The work to review after this patch appears to be less contentious in terms of
>>>   the kinds of changes that are required. Most of the changes are internal details
>>>   of enabling CET and not ABI details, with the exception of the possible pain we
>>>   might cause with makecontext() being unsupported and what default position to take
>>>   there.
>>>
>>> Design:
>>>
>>> * Overall the implementation looks exactly how I might expect it to look, but some
>>>   of the math that places the shadowstack pointer appears to need either commenting
>>>   or fixing because I don't understand it. You need to make it easy for me to see
>>>   that we have placed the shadowstack pointer into the 4 pad words.
>>>
>>> Details:
>>>
>>> * One comment needs filling out a bit more, noted below.
>>>
>>>> 0001-x86-Use-pad-in-pthread_unwind_buf-to-preserve-shadow.patch
>>>>
>>>>
>>>> From f222537447f5ec879427f318b7c0396362b7453a Mon Sep 17 00:00:00 2001
>>>> From: "H.J. Lu" <hjl.tools@gmail.com>
>>>> Date: Sat, 24 Feb 2018 17:23:54 -0800
>>>> Subject: [PATCH] x86: Use pad in pthread_unwind_buf to preserve shadow stack
>>>>  register
>>>>
>>>> The pad array in struct pthread_unwind_buf is used by setjmp to save
>>>> shadow stack register.  We assert that size of struct pthread_unwind_buf
>>>> is no less than offset of shadow stack pointer + shadow stack pointer
>>>> size.
>>>>
>>>
>>> OK.
>>>
>>>> Since functions, like LIBC_START_MAIN, START_THREAD_DEFN as well as
>>>> these with thread cancellation, call setjmp, but never return after
>>>> __libc_unwind_longjmp, __libc_unwind_longjmp, which is defined as
>>>> __libc_longjmp on x86, doesn't need to restore shadow stack register.
>>>
>>> OK.
>>>
>>>> __libc_longjmp, which is a private interface for thread cancellation
>>>> implementation in libpthread, is changed to call __longjmp_cancel,
>>>> instead of __longjmp.  __longjmp_cancel is a new internal function
>>>> in libc, which is similar to __longjmp, but doesn't restore shadow
>>>> stack register.
>>>
>>> OK. Good. I like the use of a __longjmp_cancel name to call out what's
>>> going on in the API (clear semantics).
>>>
>>>>
>>>> The compatibility longjmp and siglongjmp in libpthread.so are changed
>>>> to call __libc_siglongjmp, instead of __libc_longjmp, so that they will
>>>> restore shadow stack register.
>>>
>>> OK.
>>>
>>>>
>>>> Tested with build-many-glibcs.py.
>>>>
>>>>       * nptl/pthread_create.c (START_THREAD_DEFN): Clear previous
>>>>       handlers after setjmp.
>>>>       * setjmp/longjmp.c (__libc_longjmp): Don't define alias if
>>>>       defined.
>>>>       * sysdeps/unix/sysv/linux/x86/setjmpP.h (_JUMP_BUF_SIGSET_NSIG):
>>>>       Changed to 97.
>>>>       * sysdeps/x86/Makefile (sysdep_routines): Add __longjmp_cancel.
>>>>       * sysdeps/x86/__longjmp_cancel.S: New file.
>>>>       * sysdeps/x86/longjmp.c: Likewise.
>>>>       * sysdeps/x86/nptl/pt-longjmp.c: Likewise.
>>>
>>> This looks much better.
>>>
>>>> ---
>>>>  nptl/pthread_create.c                 |  9 ++--
>>>>  setjmp/longjmp.c                      |  2 +
>>>>  sysdeps/unix/sysv/linux/x86/setjmpP.h |  4 +-
>>>>  sysdeps/x86/Makefile                  |  4 ++
>>>>  sysdeps/x86/__longjmp_cancel.S        | 20 ++++++++
>>>>  sysdeps/x86/longjmp.c                 | 45 ++++++++++++++++
>>>>  sysdeps/x86/nptl/pt-longjmp.c         | 97 +++++++++++++++++++++++++++++++++++
>>>>  7 files changed, 176 insertions(+), 5 deletions(-)
>>>>  create mode 100644 sysdeps/x86/__longjmp_cancel.S
>>>>  create mode 100644 sysdeps/x86/longjmp.c
>>>>  create mode 100644 sysdeps/x86/nptl/pt-longjmp.c
>>>>
>>>> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
>>>> index caaf07c134..1c5b3780c6 100644
>>>> --- a/nptl/pthread_create.c
>>>> +++ b/nptl/pthread_create.c
>>>> @@ -427,12 +427,15 @@ START_THREAD_DEFN
>>>>       compilers without that support we do use setjmp.  */
>>>>    struct pthread_unwind_buf unwind_buf;
>>>>
>>>> -  /* No previous handlers.  */
>>>> +  int not_first_call;
>>>> +  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
>>>> +
>>>> +  /* No previous handlers.  NB: This must be done after setjmp since
>>>> +     the same space may be used by setjmp to store extra data which
>>>> +     should never be used by __libc_unwind_longjmp.  */
>>>
>>> Suggest:
>>> ~~~
>>> No previous handlers.  NB: This must be done after setjmp since
>>> the private space in the unwind jump buffer may overlap space
>>> used by setjmp to store extra architecture-specific information
>>> which is never be used by the cancellation-specific
>>> __libc_unwind_longjmp.
>>>
>>> The private space is allowed to overlap because the unwinder never
>>> has to return through any of the jumped-to call frames, and thus
>>> only a minimum amount of saved data need be stored, and for example,
>>> need not include the process signal mask information. This is all
>>> an optimization to reduce stack usage when pushing cancellation
>>> handlers.
>>> ~~~
>>
>> Will fix it.
>>
>>>>    unwind_buf.priv.data.prev = NULL;
>>>>    unwind_buf.priv.data.cleanup = NULL;
>>>>
>>>> -  int not_first_call;
>>>> -  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
>>>
>>> OK.
>>>
>>>>    if (__glibc_likely (! not_first_call))
>>>>      {
>>>>        /* Store the new cleanup handler info.  */
>>>> diff --git a/setjmp/longjmp.c b/setjmp/longjmp.c
>>>> index a2a7065a85..453889e103 100644
>>>> --- a/setjmp/longjmp.c
>>>> +++ b/setjmp/longjmp.c
>>>> @@ -40,9 +40,11 @@ __libc_siglongjmp (sigjmp_buf env, int val)
>>>>  }
>>>>
>>>>  #ifndef __libc_siglongjmp
>>>> +# ifndef __libc_longjmp
>>>>  /* __libc_longjmp is a private interface for cancellation implementation
>>>>     in libpthread.  */
>>>>  strong_alias (__libc_siglongjmp, __libc_longjmp)
>>>> +# endif
>>>
>>> OK.
>>>
>>>>  weak_alias (__libc_siglongjmp, _longjmp)
>>>>  weak_alias (__libc_siglongjmp, longjmp)
>>>>  weak_alias (__libc_siglongjmp, siglongjmp)
>>>> diff --git a/sysdeps/unix/sysv/linux/x86/setjmpP.h b/sysdeps/unix/sysv/linux/x86/setjmpP.h
>>>> index c0ed767a0d..90a6bbcf32 100644
>>>> --- a/sysdeps/unix/sysv/linux/x86/setjmpP.h
>>>> +++ b/sysdeps/unix/sysv/linux/x86/setjmpP.h
>>>> @@ -22,8 +22,8 @@
>>>>  #include <bits/types/__sigset_t.h>
>>>>
>>>>  /* The biggest signal number + 1.  As of kernel 4.14, x86 _NSIG is 64.
>>>> -   Define it to 513 to leave some rooms for future use.  */
>>>> -#define _JUMP_BUF_SIGSET_NSIG        513
>>>> +   Define it to 97 to leave some rooms for future use.  */
>>>
>>> OK.
>>>
>>>> +#define _JUMP_BUF_SIGSET_NSIG        97
>>>
>>> Please provide proof in the way of a comment or rewriting this constant
>>> to show that it places the shadow stack pointer on both x86_64 and x32
>>> into the range of the private pad.
>>
>> sysdeps/x86/nptl/pt-longjmp.c has
>>
>> _Static_assert ((sizeof (struct pthread_unwind_buf)
>>> = (SHADOW_STACK_POINTER_OFFSET
>>      + SHADOW_STACK_POINTER_SIZE)),
>> "size of struct pthread_unwind_buf < "
>> "(SHADOW_STACK_POINTER_OFFSET + SHADOW_STACK_POINTER_SIZE)");
>>
>> If shadow stack pointer is saved in the offset bigger than the size of
>> struct pthread_unwind_buf, assert will trigger at compile-time.
>>
>
> Thanks, I'll review this in the patch you posted.
>
>>> Also, from commit  f33632ccd1dec3217583fcfdd965afb62954203c,
>>> where did this math come from?
>>>
>>> ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
>>>
>>> Why the +7?
>>
>> _JUMP_BUF_SIGSET_NSIG is the biggest signal number + 1.
>
> Agreed.
>
>> _JUMP_BUF_SIGSET_NSIG - 1 gives the biggest signal number.
>
> Agreed.
>
>> _JUMP_BUF_SIGSET_NSIG - 1 + 7 rounds up to the number of bytes
>> which are needed to store the biggest signal number.
>
> It does not.

I changed to

/* Number of bits per long.  */
#define _JUMP_BUF_SIGSET_BITS_PER_WORD (8 * sizeof (unsigned long int))
/* The biggest signal number.  As of kernel 4.14, x86 _NSIG is 64.
   Define it to 96 to leave some rooms for future use.  */
#define _JUMP_BUF_SIGSET_NSIG 96
/* Number of longs to hold all signals.  */
#define _JUMP_BUF_SIGSET_NWORDS \
  (ALIGN_UP (_JUMP_BUF_SIGSET_NSIG, _JUMP_BUF_SIGSET_BITS_PER_WORD) \
   / _JUMP_BUF_SIGSET_BITS_PER_WORD)

> Result  # of signals    # of bits       Current #       Expected #
...

> Luckily for 512 signals (513) the math works out.
>
> For 96 signals it does not.
>
> (96 - 1 + 7) = 102
> 102 / 64 = 1

True.

> That's only a signal word, that only supports 64 signals, not 96.

Lucky for me.  Since unsigned long int __shadow_stack_pointer is aligned
as unsigned long, there is padding before __shadow_stack_pointer.

> Why doesn't the static assert trigger? Because you a < the size of the
> pthread_unwind_buf, and too small actually, writing into other parts of
> the buffer!

Assert

_Static_assert ((sizeof (struct pthread_unwind_buf)
>= (SHADOW_STACK_POINTER_OFFSET
     + SHADOW_STACK_POINTER_SIZE)),
"size of struct pthread_unwind_buf < "
"(SHADOW_STACK_POINTER_OFFSET + SHADOW_STACK_POINTER_SIZE)");

is correct.

> I would expect us to use something like this:
>
> diff --git a/sysdeps/unix/sysv/linux/x86/setjmpP.h b/sysdeps/unix/sysv/linux/x86/setjmpP.h
> index c0ed767a0d..6e1e8f901c 100644
> --- a/sysdeps/unix/sysv/linux/x86/setjmpP.h
> +++ b/sysdeps/unix/sysv/linux/x86/setjmpP.h
> @@ -20,13 +20,15 @@
>  #define        _SETJMPP_H      1
>
>  #include <bits/types/__sigset_t.h>
> +#include <libc-pointer-arith.h>
>
> -/* The biggest signal number + 1.  As of kernel 4.14, x86 _NSIG is 64.
> -   Define it to 513 to leave some rooms for future use.  */
> -#define _JUMP_BUF_SIGSET_NSIG  513
> +/* As of kernel 4.14, x86 _NSIG is 64.
> +   Define it to 512 to leave some rooms for future use.  */
> +#define _JUMP_BUF_SIGSET_NSIG  512
>  /* Number of longs to hold all signals.  */
>  #define _JUMP_BUF_SIGSET_NWORDS \
> -  ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
> +  (ALIGN_UP(_JUMP_BUF_SIGSET_NSIG, (8 * sizeof (unsigned long int)))   \
> +   / (8 * sizeof (unsigned long int)))
>
>  typedef struct
>    {
> ---
> ... but with the size you need and explain *why* it's 96.
>
> You need a comment like this:
> /* The layout looks like this:
>    - N words for this.
>    - N words for that.
>    - N words for shadow stack.
>    Total 96 signals.  */
>
> Align the number of signals up to multiple of signals you can store in
> a hardware word, and then figure out how many words that takes up.

I put comments in sysdeps/x86/nptl/pt-longjmp.c together with static
assert.

> Please review my math.
>
>>>>  /* Number of longs to hold all signals.  */
>>>>  #define _JUMP_BUF_SIGSET_NWORDS \
>>>>    ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
>>>> diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
>>>> index 0d0326c21a..d25d6f0ae4 100644
>>>> --- a/sysdeps/x86/Makefile
>>>> +++ b/sysdeps/x86/Makefile
>>>> @@ -8,3 +8,7 @@ sysdep-dl-routines += dl-get-cpu-features
>>>>  tests += tst-get-cpu-features
>>>>  tests-static += tst-get-cpu-features-static
>>>>  endif
>>>> +
>>>> +ifeq ($(subdir),setjmp)
>>>> +sysdep_routines += __longjmp_cancel
>>>> +endif
>>>
>>> OK.
>>>
>>>> diff --git a/sysdeps/x86/__longjmp_cancel.S b/sysdeps/x86/__longjmp_cancel.S
>>>> new file mode 100644
>>>> index 0000000000..b57dbfa376
>>>> --- /dev/null
>>>> +++ b/sysdeps/x86/__longjmp_cancel.S
>>>> @@ -0,0 +1,20 @@
>>>> +/* __longjmp_cancel for x86.
>>>> +   Copyright (C) 2018 Free Software Foundation, Inc.
>>>> +   This file is part of the GNU C Library.
>>>> +
>>>> +   The GNU C Library is free software; you can redistribute it and/or
>>>> +   modify it under the terms of the GNU Lesser General Public
>>>> +   License as published by the Free Software Foundation; either
>>>> +   version 2.1 of the License, or (at your option) any later version.
>>>> +
>>>> +   The GNU C Library is distributed in the hope that it will be useful,
>>>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>>>> +   Lesser General Public License for more details.
>>>> +
>>>> +   You should have received a copy of the GNU Lesser General Public
>>>> +   License along with the GNU C Library; if not, see
>>>> +   <http://www.gnu.org/licenses/>.  */
>>>> +
>>>> +#define __longjmp __longjmp_cancel
>>>> +#include <__longjmp.S>
>>>
>>> OK.
>>>
>>>> diff --git a/sysdeps/x86/longjmp.c b/sysdeps/x86/longjmp.c
>>>> new file mode 100644
>>>> index 0000000000..a53f31e1dd
>>>> --- /dev/null
>>>> +++ b/sysdeps/x86/longjmp.c
>>>> @@ -0,0 +1,45 @@
>>>> +/* __libc_siglongjmp for x86.
>>>> +   Copyright (C) 2018 Free Software Foundation, Inc.
>>>> +   This file is part of the GNU C Library.
>>>> +
>>>> +   The GNU C Library is free software; you can redistribute it and/or
>>>> +   modify it under the terms of the GNU Lesser General Public
>>>> +   License as published by the Free Software Foundation; either
>>>> +   version 2.1 of the License, or (at your option) any later version.
>>>> +
>>>> +   The GNU C Library is distributed in the hope that it will be useful,
>>>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>>>> +   Lesser General Public License for more details.
>>>> +
>>>> +   You should have received a copy of the GNU Lesser General Public
>>>> +   License along with the GNU C Library; if not, see
>>>> +   <http://www.gnu.org/licenses/>.  */
>>>> +
>>>> +#define __libc_longjmp __redirect___libc_longjmp
>>>> +#include <setjmp/longjmp.c>
>>>> +#undef __libc_longjmp
>>>> +
>>>> +extern void __longjmp_cancel (__jmp_buf __env, int __val)
>>>> +     __attribute__ ((__noreturn__)) attribute_hidden;
>>>> +
>>>> +/* Since __libc_longjmp is a private interface for cancellation
>>>> +   implementation in libpthread, there is no need to restore shadow
>>>> +   stack register.  */
>>>> +
>>>> +void
>>>> +__libc_longjmp (sigjmp_buf env, int val)
>>>> +{
>>>> +  /* Perform any cleanups needed by the frames being unwound.  */
>>>> +  _longjmp_unwind (env, val);
>>>
>>> OK.
>>>
>>>> +
>>>> +  if (env[0].__mask_was_saved)
>>>> +    /* Restore the saved signal mask.  */
>>>> +    (void) __sigprocmask (SIG_SETMASK,
>>>> +                       (sigset_t *) &env[0].__saved_mask,
>>>> +                       (sigset_t *) NULL);
>>>
>>> OK.
>>>
>>>> +
>>>> +  /* Call the machine-dependent function to restore machine state
>>>> +     without shadow stack.  */
>>>> +  __longjmp_cancel (env[0].__jmpbuf, val ?: 1);
>>>
>>> OK.
>>>
>>>> +}
>>>> diff --git a/sysdeps/x86/nptl/pt-longjmp.c b/sysdeps/x86/nptl/pt-longjmp.c
>>>> new file mode 100644
>>>> index 0000000000..7eb8651cfe
>>>> --- /dev/null
>>>> +++ b/sysdeps/x86/nptl/pt-longjmp.c
>>>> @@ -0,0 +1,97 @@
>>>> +/* ABI compatibility for 'longjmp' and 'siglongjmp' symbols in libpthread ABI.
>>>> +   X86 version.
>>>> +   Copyright (C) 18 Free Software Foundation, Inc.
>>>> +   This file is part of the GNU C Library.
>>>> +
>>>> +   The GNU C Library is free software; you can redistribute it and/or
>>>> +   modify it under the terms of the GNU Lesser General Public
>>>> +   License as published by the Free Software Foundation; either
>>>> +   version 2.1 of the License, or (at your option) any later version.
>>>> +
>>>> +   The GNU C Library is distributed in the hope that it will be useful,
>>>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>>>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
>>>> +   Lesser General Public License for more details.
>>>> +
>>>> +   You should have received a copy of the GNU Lesser General Public
>>>> +   License along with the GNU C Library; if not, see
>>>> +   <http://www.gnu.org/licenses/>.  */
>>>> +
>>>> +/* <nptl/descr.h> has
>>>> +
>>>> +struct pthread_unwind_buf
>>>> +{
>>>> +  struct
>>>> +  {
>>>> +    __jmp_buf jmp_buf;
>>>> +    int mask_was_saved;
>>>> +  } cancel_jmp_buf[1];
>>>> +
>>>> +  union
>>>> +  {
>>>> +    void *pad[4];
>>>> +    struct
>>>> +    {
>>>> +      struct pthread_unwind_buf *prev;
>>>> +      struct _pthread_cleanup_buffer *cleanup;
>>>> +      int canceltype;
>>>> +    } data;
>>>> +  } priv;
>>>> +};
>>>> +
>>>> +  The pad array in struct pthread_unwind_buf is used by setjmp to save
>>>
>>> This appears to be SHADOW_STACK_POINTER_OFFSET in subsequent patches.
>>>
>>> However on your hjl/cet/master branch it appears that this offset is not
>>> defined to be *just after* the mask_was_saved?
>>
>> sysdeps/unix/sysv/linux/x86/setjmpP.h has
>>
>> typedef struct
>>   {
>>     unsigned long int __val[_JUMP_BUF_SIGSET_NWORDS];
>>   } __jmp_buf_sigset_t;
>>
>> typedef union
>>   {
>>     __sigset_t __saved_mask_compat;
>>     struct
>>       {
>>          __jmp_buf_sigset_t __saved_mask;
>>           /* Used for shadow stack pointer.  */
>>          unsigned long int __shadow_stack_pointer;
>>       } __saved;
>>   } __jmpbuf_arch_t;
>>
>> __shadow_stack_pointer is placed after __saved_mask, aka mask_was_saved.
>
> OK, I'll re-review.
>

>
> I look forward to a v2 with correct rounding and a new comment.
>

Here is the updated patch.  Please see if

sysdeps/unix/sysv/linux/x86/setjmpP.h
sysdeps/x86/nptl/pt-longjmp.c

address your concerns.

Thanks.
Carlos O'Donell - April 21, 2018, 3:28 a.m.
Lets call this v2. Subject adjusted. Please keep incrementing the version
number on your patches to make the review easier by myself and others.

On 04/12/2018 06:50 PM, H.J. Lu wrote:
> From 1ce2b4f199070a63d9a60bd9b7ca9e33013d4208 Mon Sep 17 00:00:00 2001
> From: "H.J. Lu" <hjl.tools@gmail.com>
> Date: Sat, 24 Feb 2018 17:23:54 -0800
> Subject: [PATCH] x86: Use pad in pthread_unwind_buf to preserve shadow stack
>  register
> 
> The pad array in struct pthread_unwind_buf is used by setjmp to save
> shadow stack register.  We assert that size of struct pthread_unwind_buf
> is no less than offset of shadow stack pointer + shadow stack pointer
> size.
> 
> Since functions, like LIBC_START_MAIN, START_THREAD_DEFN as well as
> these with thread cancellation, call setjmp, but never return after
> __libc_unwind_longjmp, __libc_unwind_longjmp, which is defined as
> __libc_longjmp on x86, doesn't need to restore shadow stack register.
> __libc_longjmp, which is a private interface for thread cancellation
> implementation in libpthread, is changed to call __longjmp_cancel,
> instead of __longjmp.  __longjmp_cancel is a new internal function
> in libc, which is similar to __longjmp, but doesn't restore shadow
> stack register.
> 
> The compatibility longjmp and siglongjmp in libpthread.so are changed
> to call __libc_siglongjmp, instead of __libc_longjmp, so that they will
> restore shadow stack register.
> 
> Tested with build-many-glibcs.py.
> 
> 	* nptl/pthread_create.c (START_THREAD_DEFN): Clear previous
> 	handlers after setjmp.
> 	* setjmp/longjmp.c (__libc_longjmp): Don't define alias if
> 	defined.
> 	* sysdeps/unix/sysv/linux/x86/setjmpP.h: Include
> 	<libc-pointer-arith.h>.
> 	(_JUMP_BUF_SIGSET_BITS_PER_WORD): New.
> 	(_JUMP_BUF_SIGSET_NSIG): Changed to 96.
> 	(_JUMP_BUF_SIGSET_NWORDS): Changed to use ALIGN_UP and
> 	_JUMP_BUF_SIGSET_BITS_PER_WORD.
> 	* sysdeps/x86/Makefile (sysdep_routines): Add __longjmp_cancel.
> 	* sysdeps/x86/__longjmp_cancel.S: New file.
> 	* sysdeps/x86/longjmp.c: Likewise.
> 	* sysdeps/x86/nptl/pt-longjmp.c: Likewise.
> ---
>  nptl/pthread_create.c                 | 18 +++++--
>  setjmp/longjmp.c                      |  2 +
>  sysdeps/unix/sysv/linux/x86/setjmpP.h | 12 +++--
>  sysdeps/x86/Makefile                  |  4 ++
>  sysdeps/x86/__longjmp_cancel.S        | 20 ++++++++
>  sysdeps/x86/longjmp.c                 | 45 ++++++++++++++++
>  sysdeps/x86/nptl/pt-longjmp.c         | 97 +++++++++++++++++++++++++++++++++++
>  7 files changed, 191 insertions(+), 7 deletions(-)
>  create mode 100644 sysdeps/x86/__longjmp_cancel.S
>  create mode 100644 sysdeps/x86/longjmp.c
>  create mode 100644 sysdeps/x86/nptl/pt-longjmp.c
> 
> diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
> index caaf07c134..8b1f06599d 100644
> --- a/nptl/pthread_create.c
> +++ b/nptl/pthread_create.c
> @@ -427,12 +427,24 @@ START_THREAD_DEFN
>       compilers without that support we do use setjmp.  */
>    struct pthread_unwind_buf unwind_buf;
>  
> -  /* No previous handlers.  */
> +  int not_first_call;
> +  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
> +
> +  /* No previous handlers.  NB: This must be done after setjmp since
> +     the private space in the unwind jump buffer may overlap space
> +     used by setjmp to store extra architecture-specific information
> +     which is never be used by the cancellation-specific

s/be//g

> +     __libc_unwind_longjmp.
> +
> +     The private space is allowed to overlap because the unwinder never
> +     has to return through any of the jumped-to call frames, and thus
> +     only a minimum amount of saved data need be stored, and for example,
> +     need not include the process signal mask information. This is all
> +     an optimization to reduce stack usage when pushing cancellation
> +     handlers.  */
>    unwind_buf.priv.data.prev = NULL;
>    unwind_buf.priv.data.cleanup = NULL;
>  
> -  int not_first_call;
> -  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);

OK.

>    if (__glibc_likely (! not_first_call))
>      {
>        /* Store the new cleanup handler info.  */
> diff --git a/setjmp/longjmp.c b/setjmp/longjmp.c
> index a2a7065a85..453889e103 100644
> --- a/setjmp/longjmp.c
> +++ b/setjmp/longjmp.c
> @@ -40,9 +40,11 @@ __libc_siglongjmp (sigjmp_buf env, int val)
>  }
>  
>  #ifndef __libc_siglongjmp
> +# ifndef __libc_longjmp
>  /* __libc_longjmp is a private interface for cancellation implementation
>     in libpthread.  */
>  strong_alias (__libc_siglongjmp, __libc_longjmp)
> +# endif

OK.

>  weak_alias (__libc_siglongjmp, _longjmp)
>  weak_alias (__libc_siglongjmp, longjmp)
>  weak_alias (__libc_siglongjmp, siglongjmp)
> diff --git a/sysdeps/unix/sysv/linux/x86/setjmpP.h b/sysdeps/unix/sysv/linux/x86/setjmpP.h
> index c0ed767a0d..24f87da204 100644
> --- a/sysdeps/unix/sysv/linux/x86/setjmpP.h
> +++ b/sysdeps/unix/sysv/linux/x86/setjmpP.h
> @@ -20,13 +20,17 @@
>  #define	_SETJMPP_H	1
>  
>  #include <bits/types/__sigset_t.h>
> +#include <libc-pointer-arith.h>
>  
> -/* The biggest signal number + 1.  As of kernel 4.14, x86 _NSIG is 64.
> -   Define it to 513 to leave some rooms for future use.  */
> -#define _JUMP_BUF_SIGSET_NSIG	513
> +/* Number of bits per long.  */
> +#define _JUMP_BUF_SIGSET_BITS_PER_WORD (8 * sizeof (unsigned long int))
> +/* The biggest signal number.  As of kernel 4.14, x86 _NSIG is 64.
> +   Define it to 96 to leave some rooms for future use.  */

Why 96?

Sure on x86_64 the cancel jump buf has 4 x void*'s and each is 64-bits,
so you have 128 signals worth of space and then the shadow stack pointer.

Does this work on x32?

On x32 you have only 4 void*'s in the private pad.

Your presently structured sigset_t looks like this:

typedef union
  {
    __sigset_t __saved_mask_compat;
    struct
      {
        __jmp_buf_sigset_t __saved_mask;
        /* Used for shadow stack pointer.  */
        unsigned long int __shadow_stack_pointer;
      } __saved;
  } __jmpbuf_arch_t;

Which means you have a sigset_t *before* the __shadow_stack_pointer.

On x32 to save a 64-bit shadow stack pointer, you'll only have 2 x 32-bit
words left? That's only space for 64 signals?

Are you counting on one additional 32-bit word of padding between the
__mask_was_saved and the rest of the long arguments?

If so, then this still needs spelling out in an a comment why 96 signals
works on both i686, x32, and x86_64.

Also it should be explained if 96 is the *maximum* we can do with the current
layout, or if more is possible. In which case picking 96 is *not* arbitrary.

> +#define _JUMP_BUF_SIGSET_NSIG	96
>  /* Number of longs to hold all signals.  */
>  #define _JUMP_BUF_SIGSET_NWORDS \
> -  ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
> +  (ALIGN_UP (_JUMP_BUF_SIGSET_NSIG, _JUMP_BUF_SIGSET_BITS_PER_WORD) \
> +   / _JUMP_BUF_SIGSET_BITS_PER_WORD)
>  
>  typedef struct
>    {
> diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
> index 0d0326c21a..d25d6f0ae4 100644
> --- a/sysdeps/x86/Makefile
> +++ b/sysdeps/x86/Makefile
> @@ -8,3 +8,7 @@ sysdep-dl-routines += dl-get-cpu-features
>  tests += tst-get-cpu-features
>  tests-static += tst-get-cpu-features-static
>  endif
> +
> +ifeq ($(subdir),setjmp)
> +sysdep_routines += __longjmp_cancel
> +endif
> diff --git a/sysdeps/x86/__longjmp_cancel.S b/sysdeps/x86/__longjmp_cancel.S
> new file mode 100644
> index 0000000000..b57dbfa376
> --- /dev/null
> +++ b/sysdeps/x86/__longjmp_cancel.S
> @@ -0,0 +1,20 @@
> +/* __longjmp_cancel for x86.
> +   Copyright (C) 2018 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#define __longjmp __longjmp_cancel
> +#include <__longjmp.S>

OK.

> diff --git a/sysdeps/x86/longjmp.c b/sysdeps/x86/longjmp.c
> new file mode 100644
> index 0000000000..a53f31e1dd
> --- /dev/null
> +++ b/sysdeps/x86/longjmp.c
> @@ -0,0 +1,45 @@
> +/* __libc_siglongjmp for x86.
> +   Copyright (C) 2018 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +#define __libc_longjmp __redirect___libc_longjmp
> +#include <setjmp/longjmp.c>
> +#undef __libc_longjmp
> +
> +extern void __longjmp_cancel (__jmp_buf __env, int __val)
> +     __attribute__ ((__noreturn__)) attribute_hidden;
> +
> +/* Since __libc_longjmp is a private interface for cancellation
> +   implementation in libpthread, there is no need to restore shadow
> +   stack register.  */
> +
> +void
> +__libc_longjmp (sigjmp_buf env, int val)
> +{
> +  /* Perform any cleanups needed by the frames being unwound.  */
> +  _longjmp_unwind (env, val);
> +
> +  if (env[0].__mask_was_saved)
> +    /* Restore the saved signal mask.  */
> +    (void) __sigprocmask (SIG_SETMASK,
> +			  (sigset_t *) &env[0].__saved_mask,
> +			  (sigset_t *) NULL);
> +
> +  /* Call the machine-dependent function to restore machine state
> +     without shadow stack.  */
> +  __longjmp_cancel (env[0].__jmpbuf, val ?: 1);
> +}

OK.

> diff --git a/sysdeps/x86/nptl/pt-longjmp.c b/sysdeps/x86/nptl/pt-longjmp.c
> new file mode 100644
> index 0000000000..7eb8651cfe
> --- /dev/null
> +++ b/sysdeps/x86/nptl/pt-longjmp.c
> @@ -0,0 +1,97 @@
> +/* ABI compatibility for 'longjmp' and 'siglongjmp' symbols in libpthread ABI.
> +   X86 version.
> +   Copyright (C) 18 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <http://www.gnu.org/licenses/>.  */
> +
> +/* <nptl/descr.h> has
> +
> +struct pthread_unwind_buf
> +{
> +  struct
> +  {
> +    __jmp_buf jmp_buf;
> +    int mask_was_saved;
> +  } cancel_jmp_buf[1];
> +
> +  union
> +  {
> +    void *pad[4];
> +    struct
> +    {
> +      struct pthread_unwind_buf *prev;
> +      struct _pthread_cleanup_buffer *cleanup;
> +      int canceltype;
> +    } data;
> +  } priv;
> +};
> +
> +  The pad array in struct pthread_unwind_buf is used by setjmp to save
> +  shadow stack register.  Assert that size of struct pthread_unwind_buf
> +  is no less than offset of shadow stack pointer plus shadow stack
> +  pointer size.
> +
> +  NB: setjmp is called in libpthread to save shadow stack register.  But
> +  __libc_unwind_longjmp doesn't restore shadow stack register since they
> +  never return after longjmp.  */

Suggest:
~~~
The pad array in struct pthread_unwind_buf is used by setjmp to save
the shadow stack register.  Assert that the size of struct pthread_unwind_buf
is no less than the offset of the shadow stack pointer plus the shadow stack
pointer size.

NB: We use setjmp in thread cancellation and this saves the shadow stack
register, but __libc_unwind_longjmp doesn't restore the shadow stack register
since cancellation never returns after longjmp.
~~~

> +
> +#include <pthreadP.h>
> +#include <jmp_buf-ssp.h>
> +
> +#ifdef __x86_64__
> +# define SHADOW_STACK_POINTER_SIZE 8
> +#else
> +# define SHADOW_STACK_POINTER_SIZE 4
> +#endif
> +
> +_Static_assert ((sizeof (struct pthread_unwind_buf)
> +		 >= (SHADOW_STACK_POINTER_OFFSET
> +		     + SHADOW_STACK_POINTER_SIZE)),
> +		"size of struct pthread_unwind_buf < "
> +		"(SHADOW_STACK_POINTER_OFFSET + SHADOW_STACK_POINTER_SIZE)");

This assertion is too loose.

The assertion you need is that the shadow stack pointer itself falls within
the range of the private padding. This would have caught the previous bug
with the rounded up size.

Please adjust the assertion to be as tight as possible or add new assertions.

Completely untested, but just to show what I'm thinking:

_Static_assert ((offsetof (struct pthread_unwind_buf, cancel_jump_buf[0].mask_was_saved)
		 + sizeof (int) < SHADOW_STACK_POINTER_OFFSET
		 && (offsetof (struct pthread_unwind_buf, priv.pad[4])
		    > (SHADOW_STACK_POINTER_OFFSET + SHADOW_STACK_POINTER_SIZE),
		"Shadow stack pointer is not within private storage of pthread_unwind_buf.");

> +
> +#include <shlib-compat.h>
> +
> +/* libpthread once had its own longjmp (and siglongjmp alias), though there
> +   was no apparent reason for it.  There is no use in having a separate
> +   symbol in libpthread, but the historical ABI requires it.  For static
> +   linking, there is no need to provide anything here--the libc version
> +   will be linked in.  For shared library ABI compatibility, there must be
> +   longjmp and siglongjmp symbols in libpthread.so.
> +
> +   With an IFUNC resolver, it would be possible to avoid the indirection,
> +   but the IFUNC resolver might run before the __libc_longjmp symbol has
> +   been relocated, in which case the IFUNC resolver would not be able to
> +   provide the correct address.  */
> +
> +#if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_22)
> +
> +static void __attribute__ ((noreturn, used))
> +longjmp_compat (jmp_buf env, int val)
> +{
> +  /* NB: We call __libc_siglongjmp,  instead of __libc_longjmp, since
> +     __libc_longjmp is a private interface for cancellation which
> +     doesn't restore shadow stack register.  */
> +  __libc_siglongjmp (env, val);
> +}
> +
> +strong_alias (longjmp_compat, longjmp_alias)
> +compat_symbol (libpthread, longjmp_alias, longjmp, GLIBC_2_0);
> +
> +strong_alias (longjmp_alias, siglongjmp_alias)
> +compat_symbol (libpthread, siglongjmp_alias, siglongjmp, GLIBC_2_0);
> +
> +#endif
> -- 2.14.3

Looking forward to a v3.

Patch

From 1ce2b4f199070a63d9a60bd9b7ca9e33013d4208 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Sat, 24 Feb 2018 17:23:54 -0800
Subject: [PATCH] x86: Use pad in pthread_unwind_buf to preserve shadow stack
 register

The pad array in struct pthread_unwind_buf is used by setjmp to save
shadow stack register.  We assert that size of struct pthread_unwind_buf
is no less than offset of shadow stack pointer + shadow stack pointer
size.

Since functions, like LIBC_START_MAIN, START_THREAD_DEFN as well as
these with thread cancellation, call setjmp, but never return after
__libc_unwind_longjmp, __libc_unwind_longjmp, which is defined as
__libc_longjmp on x86, doesn't need to restore shadow stack register.
__libc_longjmp, which is a private interface for thread cancellation
implementation in libpthread, is changed to call __longjmp_cancel,
instead of __longjmp.  __longjmp_cancel is a new internal function
in libc, which is similar to __longjmp, but doesn't restore shadow
stack register.

The compatibility longjmp and siglongjmp in libpthread.so are changed
to call __libc_siglongjmp, instead of __libc_longjmp, so that they will
restore shadow stack register.

Tested with build-many-glibcs.py.

	* nptl/pthread_create.c (START_THREAD_DEFN): Clear previous
	handlers after setjmp.
	* setjmp/longjmp.c (__libc_longjmp): Don't define alias if
	defined.
	* sysdeps/unix/sysv/linux/x86/setjmpP.h: Include
	<libc-pointer-arith.h>.
	(_JUMP_BUF_SIGSET_BITS_PER_WORD): New.
	(_JUMP_BUF_SIGSET_NSIG): Changed to 96.
	(_JUMP_BUF_SIGSET_NWORDS): Changed to use ALIGN_UP and
	_JUMP_BUF_SIGSET_BITS_PER_WORD.
	* sysdeps/x86/Makefile (sysdep_routines): Add __longjmp_cancel.
	* sysdeps/x86/__longjmp_cancel.S: New file.
	* sysdeps/x86/longjmp.c: Likewise.
	* sysdeps/x86/nptl/pt-longjmp.c: Likewise.
---
 nptl/pthread_create.c                 | 18 +++++--
 setjmp/longjmp.c                      |  2 +
 sysdeps/unix/sysv/linux/x86/setjmpP.h | 12 +++--
 sysdeps/x86/Makefile                  |  4 ++
 sysdeps/x86/__longjmp_cancel.S        | 20 ++++++++
 sysdeps/x86/longjmp.c                 | 45 ++++++++++++++++
 sysdeps/x86/nptl/pt-longjmp.c         | 97 +++++++++++++++++++++++++++++++++++
 7 files changed, 191 insertions(+), 7 deletions(-)
 create mode 100644 sysdeps/x86/__longjmp_cancel.S
 create mode 100644 sysdeps/x86/longjmp.c
 create mode 100644 sysdeps/x86/nptl/pt-longjmp.c

diff --git a/nptl/pthread_create.c b/nptl/pthread_create.c
index caaf07c134..8b1f06599d 100644
--- a/nptl/pthread_create.c
+++ b/nptl/pthread_create.c
@@ -427,12 +427,24 @@  START_THREAD_DEFN
      compilers without that support we do use setjmp.  */
   struct pthread_unwind_buf unwind_buf;
 
-  /* No previous handlers.  */
+  int not_first_call;
+  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
+
+  /* No previous handlers.  NB: This must be done after setjmp since
+     the private space in the unwind jump buffer may overlap space
+     used by setjmp to store extra architecture-specific information
+     which is never be used by the cancellation-specific
+     __libc_unwind_longjmp.
+
+     The private space is allowed to overlap because the unwinder never
+     has to return through any of the jumped-to call frames, and thus
+     only a minimum amount of saved data need be stored, and for example,
+     need not include the process signal mask information. This is all
+     an optimization to reduce stack usage when pushing cancellation
+     handlers.  */
   unwind_buf.priv.data.prev = NULL;
   unwind_buf.priv.data.cleanup = NULL;
 
-  int not_first_call;
-  not_first_call = setjmp ((struct __jmp_buf_tag *) unwind_buf.cancel_jmp_buf);
   if (__glibc_likely (! not_first_call))
     {
       /* Store the new cleanup handler info.  */
diff --git a/setjmp/longjmp.c b/setjmp/longjmp.c
index a2a7065a85..453889e103 100644
--- a/setjmp/longjmp.c
+++ b/setjmp/longjmp.c
@@ -40,9 +40,11 @@  __libc_siglongjmp (sigjmp_buf env, int val)
 }
 
 #ifndef __libc_siglongjmp
+# ifndef __libc_longjmp
 /* __libc_longjmp is a private interface for cancellation implementation
    in libpthread.  */
 strong_alias (__libc_siglongjmp, __libc_longjmp)
+# endif
 weak_alias (__libc_siglongjmp, _longjmp)
 weak_alias (__libc_siglongjmp, longjmp)
 weak_alias (__libc_siglongjmp, siglongjmp)
diff --git a/sysdeps/unix/sysv/linux/x86/setjmpP.h b/sysdeps/unix/sysv/linux/x86/setjmpP.h
index c0ed767a0d..24f87da204 100644
--- a/sysdeps/unix/sysv/linux/x86/setjmpP.h
+++ b/sysdeps/unix/sysv/linux/x86/setjmpP.h
@@ -20,13 +20,17 @@ 
 #define	_SETJMPP_H	1
 
 #include <bits/types/__sigset_t.h>
+#include <libc-pointer-arith.h>
 
-/* The biggest signal number + 1.  As of kernel 4.14, x86 _NSIG is 64.
-   Define it to 513 to leave some rooms for future use.  */
-#define _JUMP_BUF_SIGSET_NSIG	513
+/* Number of bits per long.  */
+#define _JUMP_BUF_SIGSET_BITS_PER_WORD (8 * sizeof (unsigned long int))
+/* The biggest signal number.  As of kernel 4.14, x86 _NSIG is 64.
+   Define it to 96 to leave some rooms for future use.  */
+#define _JUMP_BUF_SIGSET_NSIG	96
 /* Number of longs to hold all signals.  */
 #define _JUMP_BUF_SIGSET_NWORDS \
-  ((_JUMP_BUF_SIGSET_NSIG - 1 + 7) / (8 * sizeof (unsigned long int)))
+  (ALIGN_UP (_JUMP_BUF_SIGSET_NSIG, _JUMP_BUF_SIGSET_BITS_PER_WORD) \
+   / _JUMP_BUF_SIGSET_BITS_PER_WORD)
 
 typedef struct
   {
diff --git a/sysdeps/x86/Makefile b/sysdeps/x86/Makefile
index 0d0326c21a..d25d6f0ae4 100644
--- a/sysdeps/x86/Makefile
+++ b/sysdeps/x86/Makefile
@@ -8,3 +8,7 @@  sysdep-dl-routines += dl-get-cpu-features
 tests += tst-get-cpu-features
 tests-static += tst-get-cpu-features-static
 endif
+
+ifeq ($(subdir),setjmp)
+sysdep_routines += __longjmp_cancel
+endif
diff --git a/sysdeps/x86/__longjmp_cancel.S b/sysdeps/x86/__longjmp_cancel.S
new file mode 100644
index 0000000000..b57dbfa376
--- /dev/null
+++ b/sysdeps/x86/__longjmp_cancel.S
@@ -0,0 +1,20 @@ 
+/* __longjmp_cancel for x86.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#define __longjmp __longjmp_cancel
+#include <__longjmp.S>
diff --git a/sysdeps/x86/longjmp.c b/sysdeps/x86/longjmp.c
new file mode 100644
index 0000000000..a53f31e1dd
--- /dev/null
+++ b/sysdeps/x86/longjmp.c
@@ -0,0 +1,45 @@ 
+/* __libc_siglongjmp for x86.
+   Copyright (C) 2018 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#define __libc_longjmp __redirect___libc_longjmp
+#include <setjmp/longjmp.c>
+#undef __libc_longjmp
+
+extern void __longjmp_cancel (__jmp_buf __env, int __val)
+     __attribute__ ((__noreturn__)) attribute_hidden;
+
+/* Since __libc_longjmp is a private interface for cancellation
+   implementation in libpthread, there is no need to restore shadow
+   stack register.  */
+
+void
+__libc_longjmp (sigjmp_buf env, int val)
+{
+  /* Perform any cleanups needed by the frames being unwound.  */
+  _longjmp_unwind (env, val);
+
+  if (env[0].__mask_was_saved)
+    /* Restore the saved signal mask.  */
+    (void) __sigprocmask (SIG_SETMASK,
+			  (sigset_t *) &env[0].__saved_mask,
+			  (sigset_t *) NULL);
+
+  /* Call the machine-dependent function to restore machine state
+     without shadow stack.  */
+  __longjmp_cancel (env[0].__jmpbuf, val ?: 1);
+}
diff --git a/sysdeps/x86/nptl/pt-longjmp.c b/sysdeps/x86/nptl/pt-longjmp.c
new file mode 100644
index 0000000000..7eb8651cfe
--- /dev/null
+++ b/sysdeps/x86/nptl/pt-longjmp.c
@@ -0,0 +1,97 @@ 
+/* ABI compatibility for 'longjmp' and 'siglongjmp' symbols in libpthread ABI.
+   X86 version.
+   Copyright (C) 18 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+/* <nptl/descr.h> has
+
+struct pthread_unwind_buf
+{
+  struct
+  {
+    __jmp_buf jmp_buf;
+    int mask_was_saved;
+  } cancel_jmp_buf[1];
+
+  union
+  {
+    void *pad[4];
+    struct
+    {
+      struct pthread_unwind_buf *prev;
+      struct _pthread_cleanup_buffer *cleanup;
+      int canceltype;
+    } data;
+  } priv;
+};
+
+  The pad array in struct pthread_unwind_buf is used by setjmp to save
+  shadow stack register.  Assert that size of struct pthread_unwind_buf
+  is no less than offset of shadow stack pointer plus shadow stack
+  pointer size.
+
+  NB: setjmp is called in libpthread to save shadow stack register.  But
+  __libc_unwind_longjmp doesn't restore shadow stack register since they
+  never return after longjmp.  */
+
+#include <pthreadP.h>
+#include <jmp_buf-ssp.h>
+
+#ifdef __x86_64__
+# define SHADOW_STACK_POINTER_SIZE 8
+#else
+# define SHADOW_STACK_POINTER_SIZE 4
+#endif
+
+_Static_assert ((sizeof (struct pthread_unwind_buf)
+		 >= (SHADOW_STACK_POINTER_OFFSET
+		     + SHADOW_STACK_POINTER_SIZE)),
+		"size of struct pthread_unwind_buf < "
+		"(SHADOW_STACK_POINTER_OFFSET + SHADOW_STACK_POINTER_SIZE)");
+
+#include <shlib-compat.h>
+
+/* libpthread once had its own longjmp (and siglongjmp alias), though there
+   was no apparent reason for it.  There is no use in having a separate
+   symbol in libpthread, but the historical ABI requires it.  For static
+   linking, there is no need to provide anything here--the libc version
+   will be linked in.  For shared library ABI compatibility, there must be
+   longjmp and siglongjmp symbols in libpthread.so.
+
+   With an IFUNC resolver, it would be possible to avoid the indirection,
+   but the IFUNC resolver might run before the __libc_longjmp symbol has
+   been relocated, in which case the IFUNC resolver would not be able to
+   provide the correct address.  */
+
+#if SHLIB_COMPAT (libpthread, GLIBC_2_0, GLIBC_2_22)
+
+static void __attribute__ ((noreturn, used))
+longjmp_compat (jmp_buf env, int val)
+{
+  /* NB: We call __libc_siglongjmp,  instead of __libc_longjmp, since
+     __libc_longjmp is a private interface for cancellation which
+     doesn't restore shadow stack register.  */
+  __libc_siglongjmp (env, val);
+}
+
+strong_alias (longjmp_compat, longjmp_alias)
+compat_symbol (libpthread, longjmp_alias, longjmp, GLIBC_2_0);
+
+strong_alias (longjmp_alias, siglongjmp_alias)
+compat_symbol (libpthread, siglongjmp_alias, siglongjmp, GLIBC_2_0);
+
+#endif
-- 
2.14.3