[PATCH 3/6] arm64: compat: Always use sigpage for sigreturn trampoline

[Mark Rutland]

On Tue, Jun 23, 2020 at 09:54:33AM +0100, Will Deacon wrote:
> The 32-bit sigreturn trampoline in the compat sigpage matches the binary
> representation of the arch/arm/ sigpage exactly. This is important for
> debuggers (e.g. GDB) and unwinders (e.g. libunwind) since they rely
> on matching the instruction sequence in order to identify that they are
> unwinding through a signal. The same cannot be said for the sigreturn
> trampoline in the compat vDSO, which defeats the unwinder heuristics and
> instead attempts to use unwind directives for the unwinding. This is in
> contrast to arch/arm/, which never uses the vDSO for sigreturn.
> 
> Ensure compatibility with arch/arm/ and existing unwinders by always
> using the sigpage for the sigreturn trampoline, regardless of the
> presence of the compat vDSO.
> 
> Signed-off-by: Will Deacon <will at kernel.org>

AFAICT this patch doesn't leave any dangling references to the vdso in
the actual signal code, and the diff looks sound to me.

Reviewed-by: Mark Rutland <mark.rutland at arm.com>

Mark.

> ---
>  arch/arm64/kernel/signal32.c | 25 -------------------------
>  1 file changed, 25 deletions(-)
> 
> diff --git a/arch/arm64/kernel/signal32.c b/arch/arm64/kernel/signal32.c
> index 0aa0b33744de..2f507f565c48 100644
> --- a/arch/arm64/kernel/signal32.c
> +++ b/arch/arm64/kernel/signal32.c
> @@ -342,30 +342,6 @@ static void compat_setup_return(struct pt_regs *regs, struct k_sigaction *ka,
>  		retcode = ptr_to_compat(ka->sa.sa_restorer);
>  	} else {
>  		/* Set up sigreturn pointer */
> -#ifdef CONFIG_COMPAT_VDSO
> -		void *vdso_base = current->mm->context.vdso;
> -		void *vdso_trampoline;
> -
> -		if (ka->sa.sa_flags & SA_SIGINFO) {
> -			if (thumb) {
> -				vdso_trampoline = VDSO_SYMBOL(vdso_base,
> -							compat_rt_sigreturn_thumb);
> -			} else {
> -				vdso_trampoline = VDSO_SYMBOL(vdso_base,
> -							compat_rt_sigreturn_arm);
> -			}
> -		} else {
> -			if (thumb) {
> -				vdso_trampoline = VDSO_SYMBOL(vdso_base,
> -							compat_sigreturn_thumb);
> -			} else {
> -				vdso_trampoline = VDSO_SYMBOL(vdso_base,
> -							compat_sigreturn_arm);
> -			}
> -		}
> -
> -		retcode = ptr_to_compat(vdso_trampoline) + thumb;
> -#else
>  		unsigned int idx = thumb << 1;
>  
>  		if (ka->sa.sa_flags & SA_SIGINFO)
> @@ -373,7 +349,6 @@ static void compat_setup_return(struct pt_regs *regs, struct k_sigaction *ka,
>  
>  		retcode = (unsigned long)current->mm->context.sigpage +
>  			  (idx << 2) + thumb;
> -#endif
>  	}
>  
>  	regs->regs[0]	= usig;
> -- 
> 2.27.0.111.gc72c7da667-goog
> 
> 
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