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

Will Deacon
2019-07-10T13:25:41Z

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@linaro.org>
> Signed-off-by: Vincenzo Frascino <vincenzo.frascino@arm.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