linux-kernel - Re: [PATCH] arm64: vdso: Fix ABI regression in compat vdso

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Message-ID: <20190710132532.r27yryvt25ex76xk@willie-the-truck>
Date:   Wed, 10 Jul 2019 14:25:33 +0100
From:   Will Deacon <will@...nel.org>
To:     Vincenzo Frascino <vincenzo.frascino@....com>
Cc:     linux-arch@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org, linux-mips@...r.kernel.org,
        linux-kselftest@...r.kernel.org, catalin.marinas@....com,
        will.deacon@....com, arnd@...db.de, linux@...linux.org.uk,
        ralf@...ux-mips.org, paul.burton@...s.com,
        daniel.lezcano@...aro.org, tglx@...utronix.de, salyzyn@...roid.com,
        pcc@...gle.com, shuah@...nel.org, 0x7f454c46@...il.com,
        linux@...musvillemoes.dk, huw@...eweavers.com,
        sthotton@...vell.com, andre.przywara@....com, luto@...nel.org,
        john.stultz@...aro.org
Subject: Re: [PATCH] arm64: vdso: Fix ABI regression in compat vdso

On Wed, Jul 10, 2019 at 02:04:52PM +0100, Vincenzo Frascino wrote:
> Prior to the introduction of Unified vDSO support and compat layer for
> vDSO on arm64, AT_SYSINFO_EHDR was not defined for compat tasks.
> In the current implementation, AT_SYSINFO_EHDR is defined even if the
> compat vdso layer is not built and this causes a regression in the
> expected behavior of the ABI.
> 
> Restore the ABI behavior making sure that AT_SYSINFO_EHDR for compat
> tasks is defined only when CONFIG_GENERIC_COMPAT_VDSO and
> CONFIG_COMPAT_VDSO are enabled.

I think you could do a better job in the changelog of explaining what's
actually going on here. The problem seems to be that you're advertising
the presence of a non-existent vDSO to userspace.

> Reported-by: John Stultz <john.stultz@...aro.org>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@....com>
> ---
>  arch/arm64/include/asm/elf.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/elf.h b/arch/arm64/include/asm/elf.h
> index 3c7037c6ba9b..b7992bb9d414 100644
> --- a/arch/arm64/include/asm/elf.h
> +++ b/arch/arm64/include/asm/elf.h
> @@ -202,7 +202,7 @@ typedef compat_elf_greg_t		compat_elf_gregset_t[COMPAT_ELF_NGREG];
>  ({									\
>  	set_thread_flag(TIF_32BIT);					\
>   })
> -#ifdef CONFIG_GENERIC_COMPAT_VDSO
> +#if defined(CONFIG_COMPAT_VDSO) && defined(CONFIG_GENERIC_COMPAT_VDSO)

Can't this just be #ifdef CONFIG_COMPAT_VDSO ?

John -- can you give this a whirl, please?

Cheers,

Will