[klibc] [PATCH] ppc64: ELFv2: Load TOC value in system call stub

[Aurelien Jarno]

On Tue, Sep 09, 2014 at 07:17:19PM -0300, Mauricio Faria de Oliveira wrote:
> This fixes a segmentation fault in the system call's error handling path with
> dynamically-linked binaries on PowerPC64 little endian.  The system call stub
> wasn't loading up r2 with the appropriate TOC value in its global entry point.
> 
> The r2 setup code comes from the FUNC_START macro in gcc [1] and an equivalent
> one can also be found in the LOCALENTRY macro in glibc [2].
> 
> On the ELFv2 ABI (see [1]):
>  - The global entry point is expected to load up r2 with the appropriate TOC
>    value for this function.
>  - The local entry point expects r2 to be set up to the current TOC.
> 
> The problem happened with dynamically-linked binaries because:
>  - the system call is an indirect call (via global entry point) from the binary
>    to the shared library, landing in the syscall stub  (which didn't load up r2
>    with the TOC of the shared library)
>  - its branch to __syscall_error is a direct call (via local entry point) within
>    the shared library, landing in the function (which expects r2 to be set up to
>    that TOC)
>  - when the function attempts to store errno (in an address relative to the TOC),
>    that address incorrectly pointed to a read-only segment -- segmentation fault.
> 
> The problem didn't happen with statically-linked binaries because the TOC value
> wasn't different on that case.
> 
> Thanks and credits to Alan Modra and Ulrich Weigand, for helping with this and
> pointing out the solution.
> 
> [1] https://gcc.gnu.org/ml/gcc-patches/2013-11/msg01141.html
> [2] https://www.sourceware.org/ml/libc-alpha/2013-11/msg00315.html
> 
> Signed-off-by: Mauricio Faria de Oliveira <mauricfo at linux.vnet.ibm.com>
> ---
>  usr/klibc/arch/ppc64/sysstub.ph |    3 +++
>  1 files changed, 3 insertions(+), 0 deletions(-)
> 
> diff --git a/usr/klibc/arch/ppc64/sysstub.ph b/usr/klibc/arch/ppc64/sysstub.ph
> index b3f6e38..a0c6d41 100644
> --- a/usr/klibc/arch/ppc64/sysstub.ph
> +++ b/usr/klibc/arch/ppc64/sysstub.ph
> @@ -18,6 +18,9 @@ sub make_sysstub($$$$$@) {
>  #if _CALL_ELF == 2
>  	.type ${fname},\@function
>  ${fname}:
> +0:	addis	2,12,(.TOC.-0b)\@ha
> +	addi	2,2,(.TOC.-0b)\@l
> +	.localentry ${fname},.-${fname}
>  #else
>  	.section ".opd","aw"
>  	.balign 8

Thanks for the patch, I confirm it fixes the problem.

-- 
Aurelien Jarno                          GPG: 4096R/1DDD8C9B
aurelien at aurel32.net                 http://www.aurel32.net