The following trivial patch fixes a bug in signal handling on x86-64: the 
kernel currently fails to save and restore the CS and SS segment registers on 
user-mode signal handler dispatch/return, which makes it impossible for 
64-bit applications to catch and handle signals properly that occur while 
running 32-bit code fragments in compatibility mode.

The proposed patch doesn't affect any performance-critical paths (e.g., 
syscall or interrupt entry/exit), and merely involves a couple more moves 
to/from user space on signal frame setup and sigreturn.  It also doesn't 
affect the size or shape of the sigcontext at all, since there already was an
(unused) slot for CS, and I've assigned the convenient __pad0 field as a slot 
for SS.  The existing, unused slots for FS and GS remain unused for now, and 
I don't see any urgent need to change that.  The only way this might break an 
existing app is if the app tries to cons up its own signal frame (not 
generated by the kernel) and pass it to sigreturn, but this is presumably a 
no-no anyway.

The patch is against linux-2.6.13.3.
Author: Bryan Ford, baford@mit.edu
No copyright claimed; public domain.