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Date: Mon, 31 Jan 2022 17:00:00 +0000
From: Richard Sandiford <richard.sandiford@....com>
To: Dan Li <ashimida@...ux.alibaba.com>
Cc: gcc-patches@....gnu.org, richard.earnshaw@....com,
marcus.shawcroft@....com, kyrylo.tkachov@....com, hp@....gnu.org,
ndesaulniers@...gle.com, nsz@....gnu.org, pageexec@...il.com,
qinzhao@....gnu.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] [PATCH,v3,1/1,AARCH64][PR102768] aarch64: Add compiler support for Shadow Call Stack
Dan Li <ashimida@...ux.alibaba.com> writes:
> 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 discussed 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
>
> Signed-off-by: Dan Li <ashimida@...ux.alibaba.com>
>
> gcc/ChangeLog:
>
> * config/aarch64/aarch64.c (aarch64_layout_frame):
> Change callee_adjust when scs is enabled.
> (aarch64_restore_callee_saves): Avoid pop x30 twice
> when scs is enabled.
> (aarch64_expand_prologue): Push x30 onto SCS before it's
> pushed onto stack.
> (aarch64_expand_epilogue): Pop x30 frome SCS, while
> preventing it from being popped from the regular stack again.
> (aarch64_override_options_internal): Add SCS compile option check.
> (TARGET_HAVE_SHADOW_CALL_STACK): New hook.
> * config/aarch64/aarch64.h (struct GTY): Add is_scs_enabled.
> * config/aarch64/aarch64.md (scs_push): New template.
> (scs_pop): Likewise.
> * doc/invoke.texi: Document -fsanitize=shadow-call-stack.
> * doc/tm.texi: Regenerate.
> * doc/tm.texi.in: Add hook have_shadow_call_stack.
> * flag-types.h (enum sanitize_code):
> Add SANITIZE_SHADOW_CALL_STACK.
> * opts.c: Add shadow-call-stack.
> * target.def: New hook.
> * toplev.c (process_options): Add SCS compile option check.
>
> gcc/testsuite/ChangeLog:
>
> * gcc.target/aarch64/shadow_call_stack_1.c: New test.
> * gcc.target/aarch64/shadow_call_stack_2.c: New test.
> * gcc.target/aarch64/shadow_call_stack_3.c: New test.
> * gcc.target/aarch64/shadow_call_stack_4.c: New test.
> * gcc.target/aarch64/shadow_call_stack_5.c: New test.
> * gcc.target/aarch64/shadow_call_stack_6.c: New test.
> * gcc.target/aarch64/shadow_call_stack_7.c: New test.
> * gcc.target/aarch64/shadow_call_stack_8.c: New test.
> ---
> V3:
> - Change scs_push/pop to standard move patterns.
> - Optimize scs_pop to avoid pop x30 twice when shadow stack is enabled.
>
> gcc/config/aarch64/aarch64.c | 66 +++++++++++++++++--
> gcc/config/aarch64/aarch64.h | 4 ++
> gcc/config/aarch64/aarch64.md | 10 +++
> gcc/doc/invoke.texi | 30 +++++++++
> gcc/doc/tm.texi | 5 ++
> gcc/doc/tm.texi.in | 2 +
> gcc/flag-types.h | 2 +
> gcc/opts.c | 1 +
> gcc/target.def | 8 +++
> .../gcc.target/aarch64/shadow_call_stack_1.c | 6 ++
> .../gcc.target/aarch64/shadow_call_stack_2.c | 6 ++
> .../gcc.target/aarch64/shadow_call_stack_3.c | 45 +++++++++++++
> .../gcc.target/aarch64/shadow_call_stack_4.c | 20 ++++++
> .../gcc.target/aarch64/shadow_call_stack_5.c | 18 +++++
> .../gcc.target/aarch64/shadow_call_stack_6.c | 18 +++++
> .../gcc.target/aarch64/shadow_call_stack_7.c | 18 +++++
> .../gcc.target/aarch64/shadow_call_stack_8.c | 24 +++++++
> gcc/toplev.c | 10 +++
> 18 files changed, 289 insertions(+), 4 deletions(-)
> create mode 100644 gcc/testsuite/gcc.target/aarch64/shadow_call_stack_1.c
> create mode 100644 gcc/testsuite/gcc.target/aarch64/shadow_call_stack_2.c
> create mode 100644 gcc/testsuite/gcc.target/aarch64/shadow_call_stack_3.c
> create mode 100644 gcc/testsuite/gcc.target/aarch64/shadow_call_stack_4.c
> create mode 100644 gcc/testsuite/gcc.target/aarch64/shadow_call_stack_5.c
> create mode 100644 gcc/testsuite/gcc.target/aarch64/shadow_call_stack_6.c
> create mode 100644 gcc/testsuite/gcc.target/aarch64/shadow_call_stack_7.c
> create mode 100644 gcc/testsuite/gcc.target/aarch64/shadow_call_stack_8.c
>
> diff --git a/gcc/config/aarch64/aarch64.c b/gcc/config/aarch64/aarch64.c
> index 699c105a42a..461c205010e 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"
> @@ -7478,10 +7479,31 @@ aarch64_layout_frame (void)
> frame.sve_callee_adjust = 0;
> frame.callee_offset = 0;
>
> + /* Shadow call stack only deal with functions where the LR is pushed
Typo: s/deal/deals/
> + onto the stack and without specifying the "no_sanitize" attribute
> + with the argument "shadow-call-stack". */
> + frame.is_scs_enabled
> + = (!crtl->calls_eh_return
> + && (sanitize_flags_p (SANITIZE_SHADOW_CALL_STACK)
> + && known_ge (cfun->machine->frame.reg_offset[LR_REGNUM], 0)));
Nit, but normal GCC style would be to use a single chain of &&s here:
frame.is_scs_enabled
= (!crtl->calls_eh_return
&& sanitize_flags_p (SANITIZE_SHADOW_CALL_STACK)
&& known_ge (cfun->machine->frame.reg_offset[LR_REGNUM], 0));
> +
> + /* When shadow call stack is enabled, the scs_pop in the epilogue will
> + restore x30, and we don't need to pop x30 again in the traditional
> + way. At this time, if candidate2 is x30, we need to adjust
> + max_push_offset to 256 to ensure that the offset meets the requirements
> + of emit_move_insn. Similarly, if candidate1 is x30, we need to set
> + max_push_offset to 0, because x30 is not popped up at this time, so
> + callee_adjust cannot be adjusted. */
> HOST_WIDE_INT max_push_offset = 0;
> if (frame.wb_candidate2 != INVALID_REGNUM)
> - max_push_offset = 512;
> - else if (frame.wb_candidate1 != INVALID_REGNUM)
> + {
> + if (frame.is_scs_enabled && frame.wb_candidate2 == R30_REGNUM)
> + max_push_offset = 256;
> + else
> + max_push_offset = 512;
> + }
> + else if ((frame.wb_candidate1 != INVALID_REGNUM)
> + && !(frame.is_scs_enabled && frame.wb_candidate1 == R30_REGNUM))
> max_push_offset = 256;
> HOST_WIDE_INT const_size, const_outgoing_args_size, const_fp_offset;
Maybe we should instead add separate fields for wb_push_candidate[12] and
wb_pop_candidate[12]. The pop candidates would start out the same as the
push candidates, but R30_REGNUM would get replaced with INVALID_REGNUM
for SCS.
Admittedly, suppressing the restore of x30 is turning out to be a bit
more difficult than I'd realised :-/
> […]
> diff --git a/gcc/config/aarch64/aarch64.h b/gcc/config/aarch64/aarch64.h
> index 2792bb29adb..1610a4fd74c 100644
> --- a/gcc/config/aarch64/aarch64.h
> +++ b/gcc/config/aarch64/aarch64.h
> @@ -916,6 +916,10 @@ struct GTY (()) aarch64_frame
> unsigned spare_pred_reg;
>
> bool laid_out;
> +
> + /* Nonzero if shadow call stack should be enabled for the current
> + function, otherwise return FALSE. */
“True” seems better than “Nonzero” given that this is a bool.
(A lot of GCC bools were originally ints, which is why “nonzero”
still appears in non-obvious places.)
I think we can just drop “otherwise return FALSE”: “return” doesn't
seem appropriate here, given that it's a variable.
Looks great otherwise. Thanks especially for testing the corner cases. :-)
One minor thing:
> +/* { dg-final { scan-assembler-times "str\tx30, \\\[x18\\\], \[#|$\]?8" 2 } } */
> +/* { dg-final { scan-assembler-times "ldr\tx30, \\\[x18, \[#|$\]?-8\\\]!" 2 } } */
This sort of regexp can be easier to write if you quote them using {…}
rather than "…", since it reduces the number of backslashes needed. E.g.:
/* { dg-final { scan-assembler-times {str\tx30, \[x18\], [#|$]?8} 2 } } */
The current version is fine too though, and is widely used. Just mentioning
it in case it's useful in future.
Also, [#|$]? can be written #?.
Thanks,
Richard
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