This is the same issue as ppc64 before, when returning to userland we
shouldn't re-compute the seccomp check or the task could be killed during
sigreturn when orig_eax is overwritten by the sigreturn syscall.  This was
found by Roland.

This was harmless from a security standpoint, but some i686 users reported
failures with auditing enabled system wide (some distro surprisingly makes
it the default) and I reproduced it too by keeping the whole workload under
strace -f.

Patch is tested and works for me under strace -f.

nobody@athlon:~/cpushare> strace -o /tmp/o -f python seccomp_test.py
make: Nothing to be done for `seccomp_test'.
Starting computing some malicious bytecode
init
load
start
stop
receive_data failure
kill
exit_code 0 signal 9
The malicious bytecode has been killed successfully by seccomp
Starting computing some safe bytecode
init
load
start
stop
174 counts
kill
exit_code 0 signal 0
The seccomp_test.py completed successfully, thank you for testing.

(akpm: collaterally cleaned up a bit of do_syscall_trace() too)

Signed-off-by: Andrea Arcangeli <andrea@cpushare.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>