[RFC,PR102768] aarch64: Add compiler support for Shadow Call Stack

  Shadow Call Stack can be used to protect the return address of a
function at runtime, and clang already supports this feature[1].

To enable SCS in user mode, in addition to compiler, other support
is also required (as described in [2]). This patch only adds basic
support for SCS from the compiler side, and provides convenience
for users to enable SCS.

For linux kernel, only the support of the compiler is required.

[1] https://clang.llvm.org/docs/ShadowCallStack.html
[2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102768

gcc/c-family/ChangeLog:

	* c-attribs.c (handle_no_sanitize_shadow_call_stack_attribute):

gcc/ChangeLog:

	* config/aarch64/aarch64-protos.h (aarch64_shadow_call_stack_enabled):
	* config/aarch64/aarch64.c (aarch64_shadow_call_stack_enabled):
	(aarch64_expand_prologue):
	(aarch64_expand_epilogue):
	* config/aarch64/aarch64.h (TARGET_SUPPORT_SHADOW_CALL_STACK):
	* config/aarch64/aarch64.md (scs_push):
	(scs_pop):
	* defaults.h (TARGET_SUPPORT_SHADOW_CALL_STACK):
	* flag-types.h (enum sanitize_code):
	* opts.c (finish_options):

Signed-off-by: Dan Li <ashimida@linux.alibaba.com>
---
 gcc/c-family/c-attribs.c            | 21 +++++++++++++++++++++
 gcc/config/aarch64/aarch64-protos.h |  1 +
 gcc/config/aarch64/aarch64.c        | 27 +++++++++++++++++++++++++++
 gcc/config/aarch64/aarch64.h        |  4 ++++
 gcc/config/aarch64/aarch64.md       | 18 ++++++++++++++++++
 gcc/defaults.h                      |  4 ++++
 gcc/flag-types.h                    |  2 ++
 gcc/opts.c                          |  6 ++++++
 8 files changed, 83 insertions(+)
  

The 11/02/2021 00:06, Dan Li via Gcc-patches wrote:
> Shadow Call Stack can be used to protect the return address of a
> function at runtime, and clang already supports this feature[1].
> 
> To enable SCS in user mode, in addition to compiler, other support
> is also required (as described in [2]). This patch only adds basic
> support for SCS from the compiler side, and provides convenience
> for users to enable SCS.
> 
> For linux kernel, only the support of the compiler is required.
> 
> [1] https://clang.llvm.org/docs/ShadowCallStack.html
> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102768

i'm not a gcc maintainer, but i prefer such feature
to be in upstream gcc instead of in a plugin.

it will require update to the documentation:

which should mention that it depends on -ffixed-x18
(probably that should be enforced too) which is an
important abi issue: functions following the normal
pcs can clobber x18 and break scs.

and that there is no unwinder support.

the abi issue means it is unlikely to be useful in
linux user space (even if libc and unwinder support
is implemented), but it can be still useful in
freestanding code such as the linux kernel.

thanks.

> 
> gcc/c-family/ChangeLog:
> 
> 	* c-attribs.c (handle_no_sanitize_shadow_call_stack_attribute):
> 
> gcc/ChangeLog:
> 
> 	* config/aarch64/aarch64-protos.h (aarch64_shadow_call_stack_enabled):
> 	* config/aarch64/aarch64.c (aarch64_shadow_call_stack_enabled):
> 	(aarch64_expand_prologue):
> 	(aarch64_expand_epilogue):
> 	* config/aarch64/aarch64.h (TARGET_SUPPORT_SHADOW_CALL_STACK):
> 	* config/aarch64/aarch64.md (scs_push):
> 	(scs_pop):
> 	* defaults.h (TARGET_SUPPORT_SHADOW_CALL_STACK):
> 	* flag-types.h (enum sanitize_code):
> 	* opts.c (finish_options):
> 
> Signed-off-by: Dan Li <ashimida@linux.alibaba.com>
> ---
>  gcc/c-family/c-attribs.c            | 21 +++++++++++++++++++++
>  gcc/config/aarch64/aarch64-protos.h |  1 +
>  gcc/config/aarch64/aarch64.c        | 27 +++++++++++++++++++++++++++
>  gcc/config/aarch64/aarch64.h        |  4 ++++
>  gcc/config/aarch64/aarch64.md       | 18 ++++++++++++++++++
>  gcc/defaults.h                      |  4 ++++
>  gcc/flag-types.h                    |  2 ++
>  gcc/opts.c                          |  6 ++++++
>  8 files changed, 83 insertions(+)
> 
> diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
> index 007b928c54b..9b3a35c06bf 100644
> --- a/gcc/c-family/c-attribs.c
> +++ b/gcc/c-family/c-attribs.c
> @@ -56,6 +56,8 @@ static tree handle_cold_attribute (tree *, tree, tree, int, bool *);
>  static tree handle_no_sanitize_attribute (tree *, tree, tree, int, bool *);
>  static tree handle_no_sanitize_address_attribute (tree *, tree, tree,
>  						  int, bool *);
> +static tree handle_no_sanitize_shadow_call_stack_attribute (tree *, tree,
> +						  tree, int, bool *);
>  static tree handle_no_sanitize_thread_attribute (tree *, tree, tree,
>  						 int, bool *);
>  static tree handle_no_address_safety_analysis_attribute (tree *, tree, tree,
> @@ -454,6 +456,10 @@ const struct attribute_spec c_common_attribute_table[] =
>  			      handle_no_sanitize_attribute, NULL },
>    { "no_sanitize_address",    0, 0, true, false, false, false,
>  			      handle_no_sanitize_address_attribute, NULL },
> +  { "no_sanitize_shadow_call_stack",
> +			      0, 0, true, false, false, false,
> +			      handle_no_sanitize_shadow_call_stack_attribute,
> +			      NULL },
>    { "no_sanitize_thread",     0, 0, true, false, false, false,
>  			      handle_no_sanitize_thread_attribute, NULL },
>    { "no_sanitize_undefined",  0, 0, true, false, false, false,
> @@ -1175,6 +1181,21 @@ handle_no_sanitize_address_attribute (tree *node, tree name, tree, int,
>    return NULL_TREE;
>  }
>  
> +/* Handle a "no_sanitize_shadow_call_stack" attribute; arguments as in
> +   struct attribute_spec.handler.  */
> +static tree
> +handle_no_sanitize_shadow_call_stack_attribute (tree *node, tree name,
> +				      tree, int, bool *no_add_attrs)
> +{
> +  *no_add_attrs = true;
> +  if (TREE_CODE (*node) != FUNCTION_DECL)
> +    warning (OPT_Wattributes, "%qE attribute ignored", name);
> +  else
> +    add_no_sanitize_value (*node, SANITIZE_SHADOW_CALL_STACK);
> +
> +  return NULL_TREE;
> +}
> +
>  /* Handle a "no_sanitize_thread" attribute; arguments as in
>     struct attribute_spec.handler.  */
>  
> diff --git a/gcc/config/aarch64/aarch64-protos.h b/gcc/config/aarch64/aarch64-protos.h
> index 768e8fae136..150c015df21 100644
> --- a/gcc/config/aarch64/aarch64-protos.h
> +++ b/gcc/config/aarch64/aarch64-protos.h
> @@ -893,6 +893,7 @@ void aarch64_register_pragmas (void);
>  void aarch64_relayout_simd_types (void);
>  void aarch64_reset_previous_fndecl (void);
>  bool aarch64_return_address_signing_enabled (void);
> +bool aarch64_shadow_call_stack_enabled (void);
>  bool aarch64_bti_enabled (void);
>  void aarch64_save_restore_target_globals (tree);
>  void aarch64_addti_scratch_regs (rtx, rtx, rtx *,
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index 699c105a42a..5a36a459f4e 100644
> --- a/gcc/config/aarch64/aarch64.c
> +++ b/gcc/config/aarch64/aarch64.c
> @@ -79,6 +79,7 @@
>  #include "tree-ssa-loop-niter.h"
>  #include "fractional-cost.h"
>  #include "rtlanal.h"
> +#include "asan.h"
>  
>  /* This file should be included last.  */
>  #include "target-def.h"
> @@ -7799,6 +7800,24 @@ aarch64_return_address_signing_enabled (void)
>  	      && known_ge (cfun->machine->frame.reg_offset[LR_REGNUM], 0)));
>  }
>  
> +/* Return TRUE if shadow call stack should be enabled for the current
> +   function, otherwise return FALSE.  */
> +
> +bool
> +aarch64_shadow_call_stack_enabled (void)
> +{
> +  /* This function should only be called after frame laid out.  */
> +  gcc_assert (cfun->machine->frame.laid_out);
> +
> +  if (crtl->calls_eh_return)
> +    return false;
> +
> +  /* We only deal with a function if its LR is pushed onto stack
> +     and attribute no_sanitize_shadow_call_stack is not specified.  */
> +  return (sanitize_flags_p (SANITIZE_SHADOW_CALL_STACK)
> +	  && known_ge (cfun->machine->frame.reg_offset[LR_REGNUM], 0));
> +}
> +
>  /* Return TRUE if Branch Target Identification Mechanism is enabled.  */
>  bool
>  aarch64_bti_enabled (void)
> @@ -8810,6 +8829,10 @@ aarch64_expand_prologue (void)
>        RTX_FRAME_RELATED_P (insn) = 1;
>      }
>  
> +  /* Push return address to shadow call stack.  */
> +  if (aarch64_shadow_call_stack_enabled ())
> +	emit_insn (gen_scs_push ());
> +
>    if (flag_stack_usage_info)
>      current_function_static_stack_size = constant_lower_bound (frame_size);
>  
> @@ -9066,6 +9089,10 @@ aarch64_expand_epilogue (bool for_sibcall)
>        RTX_FRAME_RELATED_P (insn) = 1;
>      }
>  
> +  /* Pop return address from shadow call stack.  */
> +  if (aarch64_shadow_call_stack_enabled ())
> +	emit_insn (gen_scs_pop ());
> +
>    /* We prefer to emit the combined return/authenticate instruction RETAA,
>       however there are three cases in which we must instead emit an explicit
>       authentication instruction.
> diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
> index 2792bb29adb..1a83875dec8 100644
> --- a/gcc/config/aarch64/aarch64.h
> +++ b/gcc/config/aarch64/aarch64.h
> @@ -100,6 +100,10 @@
>     generating stack clash probes.  */
>  #define STACK_CLASH_MAX_UNROLL_PAGES 4
>  
> +/* This value represents whether the shadow call stack is implemented on
> +   the target platform.  */
> +#define TARGET_SUPPORT_SHADOW_CALL_STACK true
> +
>  /* The architecture reserves all bits of the address for hardware use,
>     so the vbit must go into the delta field of pointers to member
>     functions.  This is the same config as that in the AArch32
> diff --git a/gcc/config/aarch64/aarch64.md b/gcc/config/aarch64/aarch64.md
> index 1a39470a1fe..8e68a6f793d 100644
> --- a/gcc/config/aarch64/aarch64.md
> +++ b/gcc/config/aarch64/aarch64.md
> @@ -6994,6 +6994,24 @@ (define_insn "xpaclri"
>    "hint\t7 // xpaclri"
>  )
>  
> +;; Save X30 in the X18-based POST_INC stack (consistent with clang).
> +(define_insn "scs_push"
> +  [(set (mem:DI (reg:DI R18_REGNUM)) (reg:DI R30_REGNUM))
> +   (set (reg:DI R18_REGNUM) (plus:DI (reg:DI R18_REGNUM) (const_int 8)))]
> +  ""
> +  "str\\tx30, [x18], #8"
> +  [(set_attr "type" "store_8")]
> +)
> +
> +;; Load X30 form the X18-based PRE_DEC stack (consistent with clang).
> +(define_insn "scs_pop"
> +  [(set (reg:DI R18_REGNUM) (minus:DI (reg:DI R18_REGNUM) (const_int 8)))
> +   (set (reg:DI R30_REGNUM) (mem:DI (reg:DI R18_REGNUM)))]
> +  ""
> +  "ldr\\tx30, [x18, #-8]!"
> +  [(set_attr "type" "load_8")]
> +)
> +
>  ;; UNSPEC_VOLATILE is considered to use and clobber all hard registers and
>  ;; all of memory.  This blocks insns from being moved across this point.
>  
> diff --git a/gcc/defaults.h b/gcc/defaults.h
> index bb68d0d1a79..0f1719a3bb5 100644
> --- a/gcc/defaults.h
> +++ b/gcc/defaults.h
> @@ -1172,6 +1172,10 @@ see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
>  #define PCC_BITFIELD_TYPE_MATTERS false
>  #endif
>  
> +#ifndef TARGET_SUPPORT_SHADOW_CALL_STACK
> +#define TARGET_SUPPORT_SHADOW_CALL_STACK false
> +#endif
> +
>  #ifndef INSN_SETS_ARE_DELAYED
>  #define INSN_SETS_ARE_DELAYED(INSN) false
>  #endif
> diff --git a/gcc/flag-types.h b/gcc/flag-types.h
> index a5a637160d7..c22ef35a289 100644
> --- a/gcc/flag-types.h
> +++ b/gcc/flag-types.h
> @@ -321,6 +321,8 @@ enum sanitize_code {
>    SANITIZE_HWADDRESS = 1UL << 28,
>    SANITIZE_USER_HWADDRESS = 1UL << 29,
>    SANITIZE_KERNEL_HWADDRESS = 1UL << 30,
> +  /* Shadow Call Stack.  */
> +  SANITIZE_SHADOW_CALL_STACK = 1UL << 31,
>    SANITIZE_SHIFT = SANITIZE_SHIFT_BASE | SANITIZE_SHIFT_EXPONENT,
>    SANITIZE_UNDEFINED = SANITIZE_SHIFT | SANITIZE_DIVIDE | SANITIZE_UNREACHABLE
>  		       | SANITIZE_VLA | SANITIZE_NULL | SANITIZE_RETURN
> diff --git a/gcc/opts.c b/gcc/opts.c
> index 4472cec1b98..e94f316fb85 100644
> --- a/gcc/opts.c
> +++ b/gcc/opts.c
> @@ -1308,6 +1308,11 @@ finish_options (struct gcc_options *opts, struct gcc_options *opts_set,
>      sorry ("transactional memory is not supported with "
>  	   "%<-fsanitize=kernel-address%>");
>  
> +  if ((opts->x_flag_sanitize & SANITIZE_SHADOW_CALL_STACK)
> +      && !TARGET_SUPPORT_SHADOW_CALL_STACK)
> +    error_at (loc, "%<-fsanitize=shadow-call-stack%> not supported "
> +	      "in current platform");
> +
>    /* Currently live patching is not support for LTO.  */
>    if (opts->x_flag_live_patching && opts->x_flag_lto)
>      sorry ("live patching is not supported with LTO");
> @@ -1994,6 +1999,7 @@ const struct sanitizer_opts_s sanitizer_opts[] =
>    SANITIZER_OPT (vptr, SANITIZE_VPTR, true),
>    SANITIZER_OPT (pointer-overflow, SANITIZE_POINTER_OVERFLOW, true),
>    SANITIZER_OPT (builtin, SANITIZE_BUILTIN, true),
> +  SANITIZER_OPT (shadow-call-stack, SANITIZE_SHADOW_CALL_STACK, false),
>    SANITIZER_OPT (all, ~0U, true),
>  #undef SANITIZER_OPT
>    { NULL, 0U, 0UL, false }
> -- 
> 2.17.1
>
  

On 11/2/21 9:04 PM, Szabolcs Nagy wrote:
> The 11/02/2021 00:06, Dan Li via Gcc-patches wrote:
>> Shadow Call Stack can be used to protect the return address of a
>> function at runtime, and clang already supports this feature[1].
>>
>> To enable SCS in user mode, in addition to compiler, other support
>> is also required (as described in [2]). This patch only adds basic
>> support for SCS from the compiler side, and provides convenience
>> for users to enable SCS.
>>
>> For linux kernel, only the support of the compiler is required.
>>
>> [1] https://clang.llvm.org/docs/ShadowCallStack.html
>> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102768
> 
> i'm not a gcc maintainer, but i prefer such feature
> to be in upstream gcc instead of in a plugin.
> 
> it will require update to the documentation:
> 
> which should mention that it depends on -ffixed-x18
> (probably that should be enforced too) which is an
> important abi issue: functions following the normal
> pcs can clobber x18 and break scs.
> 
Thanks Szabolcs, I will update the documentation in next version.

It sounds reasonable to enforced -ffixed-x18 with scs, but I see
that clang doesn’t do that. Maybe it is better to be consistent
with clang here?

> and that there is no unwinder support.
> 
Ok, let me try to add a support for this.

> the abi issue means it is unlikely to be useful in
> linux user space (even if libc and unwinder support
> is implemented), but it can be still useful in
> freestanding code such as the linux kernel.
> 
> thanks.
>
  

The 11/03/2021 00:24, Dan Li wrote:
> On 11/2/21 9:04 PM, Szabolcs Nagy wrote:
> > The 11/02/2021 00:06, Dan Li via Gcc-patches wrote:
> > > Shadow Call Stack can be used to protect the return address of a
> > > function at runtime, and clang already supports this feature[1].
> > > 
> > > To enable SCS in user mode, in addition to compiler, other support
> > > is also required (as described in [2]). This patch only adds basic
> > > support for SCS from the compiler side, and provides convenience
> > > for users to enable SCS.
> > > 
> > > For linux kernel, only the support of the compiler is required.
> > > 
> > > [1] https://clang.llvm.org/docs/ShadowCallStack.html
> > > [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102768
> > 
> > i'm not a gcc maintainer, but i prefer such feature
> > to be in upstream gcc instead of in a plugin.
> > 
> > it will require update to the documentation:
> > 
> > which should mention that it depends on -ffixed-x18
> > (probably that should be enforced too) which is an
> > important abi issue: functions following the normal
> > pcs can clobber x18 and break scs.
> > 
> Thanks Szabolcs, I will update the documentation in next version.
> 
> It sounds reasonable to enforced -ffixed-x18 with scs, but I see
> that clang doesn’t do that. Maybe it is better to be consistent
> with clang here?

i mean gcc can issue a diagnostic if -ffixed-x18 is not passed.
(it seems clang rejects scs too without -ffixed-x18)

> > and that there is no unwinder support.
> > 
> Ok, let me try to add a support for this.

i assume exception handling info has to change for scs to
work (to pop the shadow stack when transferring control),
so either scs must require -fno-exceptions or the eh info
changes must be implemented.

i think the kernel does not require exceptions and does
not depend on the unwinder runtime in libgcc, so this
is optional for the linux kernel use-case.
  

Hi Szabolcs,

First of all, apologies for my late reply (since I just had a new baby,
I'm quite busy recently and also because I'm not familiar with C++
exception handling, it takes me some time to learn this part).

On 11/3/21 8:00 PM, Szabolcs Nagy wrote:
> The 11/03/2021 00:24, Dan Li wrote:
>> On 11/2/21 9:04 PM, Szabolcs Nagy wrote:
>>> The 11/02/2021 00:06, Dan Li via Gcc-patches wrote:
>>>> Shadow Call Stack can be used to protect the return address of a
>>>> function at runtime, and clang already supports this feature[1].
>>>>
>>>> To enable SCS in user mode, in addition to compiler, other support
>>>> is also required (as described in [2]). This patch only adds basic
>>>> support for SCS from the compiler side, and provides convenience
>>>> for users to enable SCS.
>>>>
>>>> For linux kernel, only the support of the compiler is required.
>>>>
>>>> [1] https://clang.llvm.org/docs/ShadowCallStack.html
>>>> [2] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102768
>>>
>>> i'm not a gcc maintainer, but i prefer such feature
>>> to be in upstream gcc instead of in a plugin.
>>>
>>> it will require update to the documentation:
>>>
>>> which should mention that it depends on -ffixed-x18
>>> (probably that should be enforced too) which is an
>>> important abi issue: functions following the normal
>>> pcs can clobber x18 and break scs.
>>>
>> Thanks Szabolcs, I will update the documentation in next version.
>>
>> It sounds reasonable to enforced -ffixed-x18 with scs, but I see
>> that clang doesn’t do that. Maybe it is better to be consistent
>> with clang here?
> 
> i mean gcc can issue a diagnostic if -ffixed-x18 is not passed.
> (it seems clang rejects scs too without -ffixed-x18)
>
Oh, yes, you are right. Clang rejects scs without -ffixed-x18[1],
I should add a similar check in next version.
>>> and that there is no unwinder support.
>>>
>> Ok, let me try to add a support for this.
> 
> i assume exception handling info has to change for scs to
> work (to pop the shadow stack when transferring control),
> so either scs must require -fno-exceptions or the eh info
> changes must be implemented.
> 
> i think the kernel does not require exceptions and does
> not depend on the unwinder runtime in libgcc, so this
> is optional for the linux kernel use-case.
>
I recompiled a glibc and gcc runtime library with -ffixed-x18 enabled.
As you said, the scs stack needs to be popped at the same time during
exception handling.

I saw that Clang is processed by adding
".cfi_escape 0x16, 0x12, 0x02, 0x82, 0x78"
directive (x18 -= 8;) after each emit of scs push[2].

But this directive has problems when executed in libgcc:
1)context->reg[x] in uw_init_context_1 are all based on cfa, most
   registers have no initial values by default.
2)Address of shadow call stack (x18) cannot(and should not) be calculated
   based on cfa, and I did not yet find a way to assign hardware register
   x18 to context->reg[18].
3)This causes libgcc to crash when parsing .cfi_escape exp because of 0
   address dereference (* x18)
   (execute_stack_op => case DW_OP_breg18: _Unwind_GetGR)
4)uw_install_context_1 does not restore all hardware registers by default
   before eh return, so context->reg[18] can't write directly to hw x18.
   (In clang, __unw_getcontext/__unw_resume will save/restore all hardware
   registers, so this directive works fine in my libunwind test.)

I tried to fix this problem through a patch[3], the exception handling
works fine in my test environment, but I'm not sure if this fix is
ppropriate for two reasons:
1)libgcc does not push/pop all registers by default during exception
   handling. Is this change appropriate?
2)The test case may not be able to test this patch, because the test
   environment requires at least on glibc/gcc runtime compiled with
   -ffixed-x18.

May be it's better to rely on -fno-exceptions for this patch first? and If
the glibc/gcc runtime also supports SCS later, the problem can be fixed
at the same time.

PS:
I'm still not familiar enough with exception handling in libgcc/libunwind,
please correct me if there are any mistakes :)

[1] https://github.com/llvm/llvm-project/commit/f11eb3ebe77729426e562d7d4d7ebb1d5ff2e7c8
[2] https://reviews.llvm.org/D54609
[3] https://gcc.gnu.org/bugzilla/attachment.cgi?id=51854&action=diff
  

The 11/23/2021 16:32, Dan Li wrote:
> On 11/3/21 8:00 PM, Szabolcs Nagy wrote:
> > i assume exception handling info has to change for scs to
> > work (to pop the shadow stack when transferring control),
> > so either scs must require -fno-exceptions or the eh info
> > changes must be implemented.
> > 
> > i think the kernel does not require exceptions and does
> > not depend on the unwinder runtime in libgcc, so this
> > is optional for the linux kernel use-case.
> > 
> I recompiled a glibc and gcc runtime library with -ffixed-x18 enabled.
> As you said, the scs stack needs to be popped at the same time during
> exception handling.
> 
> I saw that Clang is processed by adding
> ".cfi_escape 0x16, 0x12, 0x02, 0x82, 0x78"
> directive (x18 -= 8;) after each emit of scs push[2].
> 
> But this directive has problems when executed in libgcc:
> 1)context->reg[x] in uw_init_context_1 are all based on cfa, most
>   registers have no initial values by default.
> 2)Address of shadow call stack (x18) cannot(and should not) be calculated
>   based on cfa, and I did not yet find a way to assign hardware register
>   x18 to context->reg[18].
> 3)This causes libgcc to crash when parsing .cfi_escape exp because of 0
>   address dereference (* x18)
>   (execute_stack_op => case DW_OP_breg18: _Unwind_GetGR)
> 4)uw_install_context_1 does not restore all hardware registers by default
>   before eh return, so context->reg[18] can't write directly to hw x18.
>   (In clang, __unw_getcontext/__unw_resume will save/restore all hardware
>   registers, so this directive works fine in my libunwind test.)
> 
> I tried to fix this problem through a patch[3], the exception handling
> works fine in my test environment, but I'm not sure if this fix is
> ppropriate for two reasons:
> 1)libgcc does not push/pop all registers by default during exception
>   handling. Is this change appropriate?
> 2)The test case may not be able to test this patch, because the test
>   environment requires at least on glibc/gcc runtime compiled with
>   -ffixed-x18.
> 
> May be it's better to rely on -fno-exceptions for this patch first? and If
> the glibc/gcc runtime also supports SCS later, the problem can be fixed
> at the same time.

i did not look at the exception handling in detail (that's
difficult to understand for me too).

to use scs, non-default abi is required anyway, so not
supporting exceptions sounds fine to me. however it should
be documented and ideally enforced (-fexceptions should
be rejected, just like -fno-fixed-x18).

i assume the linux kernel does not require -fexceptions.

> 
> PS:
> I'm still not familiar enough with exception handling in libgcc/libunwind,
> please correct me if there are any mistakes :)
> 
> [1] https://github.com/llvm/llvm-project/commit/f11eb3ebe77729426e562d7d4d7ebb1d5ff2e7c8
> [2] https://reviews.llvm.org/D54609
> [3] https://gcc.gnu.org/bugzilla/attachment.cgi?id=51854&action=diff
>
  

On 11/23/21 6:51 PM, Szabolcs Nagy wrote:
> The 11/23/2021 16:32, Dan Li wrote:
>> On 11/3/21 8:00 PM, Szabolcs Nagy wrote:
>>> i assume exception handling info has to change for scs to
>>> work (to pop the shadow stack when transferring control),
>>> so either scs must require -fno-exceptions or the eh info
>>> changes must be implemented.
>>>
>>> i think the kernel does not require exceptions and does
>>> not depend on the unwinder runtime in libgcc, so this
>>> is optional for the linux kernel use-case.
>>>
>> I recompiled a glibc and gcc runtime library with -ffixed-x18 enabled.
>> As you said, the scs stack needs to be popped at the same time during
>> exception handling.
>>
>> I saw that Clang is processed by adding
>> ".cfi_escape 0x16, 0x12, 0x02, 0x82, 0x78"
>> directive (x18 -= 8;) after each emit of scs push[2].
>>
>> But this directive has problems when executed in libgcc:
>> 1)context->reg[x] in uw_init_context_1 are all based on cfa, most
>>    registers have no initial values by default.
>> 2)Address of shadow call stack (x18) cannot(and should not) be calculated
>>    based on cfa, and I did not yet find a way to assign hardware register
>>    x18 to context->reg[18].
>> 3)This causes libgcc to crash when parsing .cfi_escape exp because of 0
>>    address dereference (* x18)
>>    (execute_stack_op => case DW_OP_breg18: _Unwind_GetGR)
>> 4)uw_install_context_1 does not restore all hardware registers by default
>>    before eh return, so context->reg[18] can't write directly to hw x18.
>>    (In clang, __unw_getcontext/__unw_resume will save/restore all hardware
>>    registers, so this directive works fine in my libunwind test.)
>>
>> I tried to fix this problem through a patch[3], the exception handling
>> works fine in my test environment, but I'm not sure if this fix is
>> ppropriate for two reasons:
>> 1)libgcc does not push/pop all registers by default during exception
>>    handling. Is this change appropriate?
>> 2)The test case may not be able to test this patch, because the test
>>    environment requires at least on glibc/gcc runtime compiled with
>>    -ffixed-x18.
>>
>> May be it's better to rely on -fno-exceptions for this patch first? and If
>> the glibc/gcc runtime also supports SCS later, the problem can be fixed
>> at the same time.
> 
> i did not look at the exception handling in detail (that's
> difficult to understand for me too).
> 
> to use scs, non-default abi is required anyway, so not
> supporting exceptions sounds fine to me. however it should
> be documented and ideally enforced (-fexceptions should
> be rejected, just like -fno-fixed-x18).
Thanks Szabolcs,

This sounds reasonable to me, and I'll fix it in the next version.
> 
> i assume the linux kernel does not require -fexceptions.
> 
AFAIK, -fexceptions are not used in the linux kernel.
>>
>> PS:
>> I'm still not familiar enough with exception handling in libgcc/libunwind,
>> please correct me if there are any mistakes :)
>>
>> [1] https://github.com/llvm/llvm-project/commit/f11eb3ebe77729426e562d7d4d7ebb1d5ff2e7c8
>> [2] https://reviews.llvm.org/D54609
>> [3] https://gcc.gnu.org/bugzilla/attachment.cgi?id=51854&action=diff
>>