klibc - Jan 2016 - [klibc:master] ppc64: ELFv2: Load TOC value in system call stub

Commit-ID:  6e2976b9d103fb37a6a06e5bccb323fa5ab1c53b
Gitweb:    
http://git.kernel.org/?p=libs/klibc/klibc.git;a=commit;h=6e2976b9d103fb37a6a06e5bccb323fa5ab1c53b
Author:     Mauricio Faria de Oliveira <mauricfo at linux.vnet.ibm.com>
AuthorDate: Wed, 6 Jan 2016 00:43:10 +0000
Committer:  H. Peter Anvin <hpa at linux.intel.com>
CommitDate: Tue, 5 Jan 2016 17:45:31 -0800

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

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>
Signed-off-by: Ben Hutchings <ben at decadent.org.uk>
Signed-off-by: H. Peter Anvin <hpa at linux.intel.com>

---
 usr/klibc/arch/ppc64/sysstub.ph | 3 +++
 1 file changed, 3 insertions(+)

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