linux-kernel - Re: [PATCH] arm64: Fix off-by-one vdso trampoline return value

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Date:   Thu, 12 Nov 2020 10:12:05 +0000
From:   Will Deacon <will@...nel.org>
To:     Will McVicker <willmcvicker@...gle.com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Nathan Chancellor <natechancellor@...il.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Andrei Vagin <avagin@...il.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com, kernel-team@...roid.com
Subject: Re: [PATCH] arm64: Fix off-by-one vdso trampoline return value

On Thu, Nov 12, 2020 at 12:14:22AM +0000, Will McVicker wrote:
> Depending on your host nm version, the generated header
> `include/generated/vdso32-offsets.h` may have the bottom bit set for the
> thumb vdso offset addresses (as observed when using llvm-nm). This
> results in an additional +1 for thumb vdso trampoline return values
> since compat_setup_return() already includes `vdso_trampoline + thumb`.
> As a result, I see a SIGBUS error when running the LTP test
> syscalls.rt_sigaction01. To fix this, let's clear the bottom bit of the
> vdso_offset in the VDSO_SYMBOL macro.
> 
> Test: LTP test syscalls.rt_sigaction01
> Fixes: f01703b3d2e6 ("arm64: compat: Get sigreturn trampolines from vDSO")
> Signed-off-by: Will McVicker <willmcvicker@...gle.com>
> ---
>  arch/arm64/include/asm/vdso.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/vdso.h b/arch/arm64/include/asm/vdso.h
> index f99dcb94b438..a7384379e8e1 100644
> --- a/arch/arm64/include/asm/vdso.h
> +++ b/arch/arm64/include/asm/vdso.h
> @@ -23,7 +23,7 @@
>  
>  #define VDSO_SYMBOL(base, name)						   \
>  ({									   \
> -	(void *)(vdso_offset_##name - VDSO_LBASE + (unsigned long)(base)); \
> +	(void *)((vdso_offset_##name & ~1UL) - VDSO_LBASE + (unsigned long)(base)); \

I don't think we need this in mainline, because the sigreturn trampoline
is just a bunch of .byte directives and I removed the sigreturn code from
the compat vdso in 2d071968a405 ("arm64: compat: Remove 32-bit sigreturn code
from the vDSO").

Might be needed in some stable kernels though (or we just backport the
patch I mentioned above)

Will