Re: [PATCH v2 1/1] ARM: kprobes: Explicitly reserve r7 for local variables

[Nathan Chancellor]

On Wed, Nov 15, 2023 at 05:58:30PM +0800, Maria Yu wrote:
> Registers r7 is removed in clobber list, so compiler may choose r7 for
> local variables usage, while r7 will be actually updated by the inline asm
> code. This caused the runtime behavior wrong.
> While those kind of reserved registers cannot be set to clobber list
> because of error like "inline asm clobber list contains reserved
> registers".
> Explicitly reserve r7 by adding attribute no-omit-frame-pointer for needed
> function, then in T32 asm code r7 is used as a frame pointer and is not
> available for use as a general-purpose register.
> Note that "no-omit-frame-pointer" will make the code size a little bigger
> to store the stack frame pointer. So limited to needed functions can have
> the less impact than the full source file.
> 
> Fixes: dd12e97f3c72 ("ARM: kprobes: treat R7 as the frame pointer register in Thumb2 builds")
> Signed-off-by: Maria Yu <quic_aiquny@xxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx

This causes warnings with clang:

  arch/arm/probes/kprobes/actions-thumb.c:441:47: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
    441 | static unsigned long __kprobes __attribute__((optimize("no-omit-frame-pointer")))
        |                                               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  arch/arm/probes/kprobes/actions-thumb.c:524:38: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
    524 | static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
        |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  arch/arm/probes/kprobes/actions-thumb.c:560:38: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
    560 | static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
        |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  arch/arm/probes/kprobes/actions-thumb.c:579:38: warning: unknown attribute 'optimize' ignored [-Wunknown-attributes]
    579 | static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
        |                                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  4 warnings generated.

Furthermore, as far as I am aware, the optimize attribute has other issues so
its use is discouraged, see commits 080b6f407635 ("bpf: Don't rely on GCC
__attribute__((optimize)) to disable GCSE") and a7223f5bfcae ("powerpc: Avoid
broken GCC __attribute__((optimize))").

Cheers,
Nathan

> ---
>  arch/arm/probes/kprobes/actions-thumb.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm/probes/kprobes/actions-thumb.c b/arch/arm/probes/kprobes/actions-thumb.c
> index 51624fc263fc..c2fdaf9f6dba 100644
> --- a/arch/arm/probes/kprobes/actions-thumb.c
> +++ b/arch/arm/probes/kprobes/actions-thumb.c
> @@ -438,7 +438,7 @@ t16_simulate_branch(probes_opcode_t insn,
>  	regs->ARM_pc = pc + (offset * 2);
>  }
>  
> -static unsigned long __kprobes
> +static unsigned long __kprobes __attribute__((optimize("no-omit-frame-pointer")))
>  t16_emulate_loregs(probes_opcode_t insn,
>  		   struct arch_probes_insn *asi, struct pt_regs *regs)
>  {
> @@ -521,7 +521,7 @@ t16_decode_hiregs(probes_opcode_t insn, struct arch_probes_insn *asi,
>  	return INSN_GOOD;
>  }
>  
> -static void __kprobes
> +static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
>  t16_emulate_push(probes_opcode_t insn,
>  		struct arch_probes_insn *asi, struct pt_regs *regs)
>  {
> @@ -557,7 +557,7 @@ t16_decode_push(probes_opcode_t insn, struct arch_probes_insn *asi,
>  	return INSN_GOOD;
>  }
>  
> -static void __kprobes
> +static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
>  t16_emulate_pop_nopc(probes_opcode_t insn,
>  		struct arch_probes_insn *asi, struct pt_regs *regs)
>  {
> @@ -576,7 +576,7 @@ t16_emulate_pop_nopc(probes_opcode_t insn,
>  		);
>  }
>  
> -static void __kprobes
> +static void __kprobes __attribute__((optimize("no-omit-frame-pointer")))
>  t16_emulate_pop_pc(probes_opcode_t insn,
>  		struct arch_probes_insn *asi, struct pt_regs *regs)
>  {
> 
> base-commit: 9bacdd8996c77c42ca004440be610692275ff9d0
> -- 
> 2.17.1
> 
> 
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