[COMMITTED] x86: Fix THREAD_SELF definition to avoid ld.so crash (bug 27004)

  I reviewed and pushed this patch for Jakub.  Tested with GCC 11
(x86-64), GCC 10 (x86-64), GCC 9 (i686.

Thanks,
Florian
8<------------------------------------------------------------------8<
From: Jakub Jelinek <jakub@redhat.com>

The previous definition of THREAD_SELF did not tell the compiler
that %fs (or %gs) usage is invalid for the !DL_LOOKUP_GSCOPE_LOCK
case in _dl_lookup_symbol_x.  As a result, ld.so could try to use the
TCB before it was initialized.

As the comment in tls.h explains, asm volatile is undesirable here.
Using the __seg_fs (or __seg_gs) namespace does not interfere with
optimization, and expresses that THREAD_SELF is potentially trapping.

---
 sysdeps/i386/nptl/tls.h   | 7 ++++++-
 sysdeps/x86_64/nptl/tls.h | 7 ++++++-
 2 files changed, 12 insertions(+), 2 deletions(-)
  

On 03/12/2020 09:51, Florian Weimer via Libc-alpha wrote:
> I reviewed and pushed this patch for Jakub.  Tested with GCC 11
> (x86-64), GCC 10 (x86-64), GCC 9 (i686.
> 
> Thanks,
> Florian
> 8<------------------------------------------------------------------8<
> From: Jakub Jelinek <jakub@redhat.com>
> 
> The previous definition of THREAD_SELF did not tell the compiler
> that %fs (or %gs) usage is invalid for the !DL_LOOKUP_GSCOPE_LOCK
> case in _dl_lookup_symbol_x.  As a result, ld.so could try to use the
> TCB before it was initialized.
> 
> As the comment in tls.h explains, asm volatile is undesirable here.
> Using the __seg_fs (or __seg_gs) namespace does not interfere with
> optimization, and expresses that THREAD_SELF is potentially trapping.
> 
> ---
>  sysdeps/i386/nptl/tls.h   | 7 ++++++-
>  sysdeps/x86_64/nptl/tls.h | 7 ++++++-
>  2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/sysdeps/i386/nptl/tls.h b/sysdeps/i386/nptl/tls.h
> index b74347bacd..ab98c24a02 100644
> --- a/sysdeps/i386/nptl/tls.h
> +++ b/sysdeps/i386/nptl/tls.h
> @@ -234,11 +234,16 @@ tls_fill_user_desc (union user_desc_init *desc,
>     assignments like
>  	pthread_descr self = thread_self();
>     do not get optimized away.  */
> -# define THREAD_SELF \
> +# if __GNUC_PREREQ (6, 0)
> +#  define THREAD_SELF \
> +  (*(struct pthread *__seg_gs *) offsetof (struct pthread, header.self))
> +# else
> +#  define THREAD_SELF \
>    ({ struct pthread *__self;						      \
>       asm ("movl %%gs:%c1,%0" : "=r" (__self)				      \
>  	  : "i" (offsetof (struct pthread, header.self)));		      \
>       __self;})
> +# endif
>  
>  /* Magic for libthread_db to know how to do THREAD_SELF.  */
>  # define DB_THREAD_SELF \

Do wee need to __GLIBC_PREREQ here? This is an internal header and the minimum
required gcc should be suffice here.

> diff --git a/sysdeps/x86_64/nptl/tls.h b/sysdeps/x86_64/nptl/tls.h
> index a08bf972de..ccb5f24d92 100644
> --- a/sysdeps/x86_64/nptl/tls.h
> +++ b/sysdeps/x86_64/nptl/tls.h
> @@ -180,11 +180,16 @@ _Static_assert (offsetof (tcbhead_t, __glibc_unused2) == 0x80,
>     assignments like
>  	pthread_descr self = thread_self();
>     do not get optimized away.  */
> -# define THREAD_SELF \
> +# if __GNUC_PREREQ (6, 0)
> +#  define THREAD_SELF \
> +  (*(struct pthread *__seg_fs *) offsetof (struct pthread, header.self))
> +# else
> +#  define THREAD_SELF \
>    ({ struct pthread *__self;						      \
>       asm ("mov %%fs:%c1,%0" : "=r" (__self)				      \
>  	  : "i" (offsetof (struct pthread, header.self)));	 	      \
>       __self;})
> +# endif
>  
>  /* Magic for libthread_db to know how to do THREAD_SELF.  */
>  # define DB_THREAD_SELF_INCLUDE  <sys/reg.h> /* For the FS constant.  */
>
  

* Adhemerval Zanella via Libc-alpha:

>> diff --git a/sysdeps/i386/nptl/tls.h b/sysdeps/i386/nptl/tls.h
>> index b74347bacd..ab98c24a02 100644
>> --- a/sysdeps/i386/nptl/tls.h
>> +++ b/sysdeps/i386/nptl/tls.h
>> @@ -234,11 +234,16 @@ tls_fill_user_desc (union user_desc_init *desc,
>>     assignments like
>>  	pthread_descr self = thread_self();
>>     do not get optimized away.  */
>> -# define THREAD_SELF \
>> +# if __GNUC_PREREQ (6, 0)
>> +#  define THREAD_SELF \
>> +  (*(struct pthread *__seg_gs *) offsetof (struct pthread, header.self))
>> +# else
>> +#  define THREAD_SELF \
>>    ({ struct pthread *__self;						      \
>>       asm ("movl %%gs:%c1,%0" : "=r" (__self)				      \
>>  	  : "i" (offsetof (struct pthread, header.self)));		      \
>>       __self;})
>> +# endif
>>  
>>  /* Magic for libthread_db to know how to do THREAD_SELF.  */
>>  # define DB_THREAD_SELF \
>
> Do wee need to __GLIBC_PREREQ here? This is an internal header and the minimum
> required gcc should be suffice here.

I think we can rework the header to use __seg_fs/__seg_gs throughout,
yes.  I assumed we were still supporting GCC 5, which I see is not the
case.

Thanks,
Florian