[PATCH 0/2] x86_64,signal: Fix the set of saved segment registers

From: Andy Lutomirski
Date: Fri Jul 11 2014 - 12:29:37 EST

The x86_64 signal code claims to save and restore CS, FS, and GS,
and it further claims that this is the minimal set that's needed.

Neither of these claims is true. The code does not, and AFAICT
never has, saved or restored FS and GS, nor does it need to. On the
other hand, all 64-bit syscalls (and rt_sigreturn in particular)
clobber SS, making it impossible for signal handlers to correctly
restore SS without using a trampoline or ptracing themselves.

This patchset saves and restores SS in the __pad0 slot and renames
the FS and GS slots __pad1 and __pad2 to more accurately document
their purpose.

I discovered this while writing a test for espfix64.

Andy Lutomirski (2):
x86_64,signal: Save and restore SS in signal frames
x86_64,signal: Remove 'fs' and 'gs' from sigcontext

arch/x86/include/asm/sigcontext.h | 6 +++---
arch/x86/include/uapi/asm/sigcontext.h | 6 +++---
arch/x86/kernel/signal.c | 12 +++---------
3 files changed, 9 insertions(+), 15 deletions(-)


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