x32 is broken in 4.9-rc1 due to "x86/signal: Add SA_{X32,IA32}_ABI sa_flags"

[Mikulas Patocka]

Hi

In the kernel 4.9-rc1, the x32 support is seriously broken, a x32 process 
is killed with SIGKILL after returning from any signal handler.

I use Debian sid x64-64 distribution with x32 architecture added from 
debian-ports.

I bisected the bug and found out that it is caused by the patch 
6846351052e685c2d1428e80ead2d7ca3d7ed913 ("x86/signal: Add 
SA_{X32,IA32}_ABI sa_flags").

example (strace of a process after receiving the SIGWINCH signal):

epoll_wait(10, 0xef6890, 32, -1)        = -1 EINTR (Interrupted system call)
--- SIGWINCH {si_signo=SIGWINCH, si_code=SI_USER, si_pid=1772, si_uid=0} ---
poll([{fd=4, events=POLLOUT}], 1, 0)    = 1 ([{fd=4, revents=POLLOUT}])
write(4, "\0", 1)                       = 1
rt_sigreturn({mask=[INT QUIT ILL TRAP BUS KILL SEGV USR2 PIPE ALRM STKFLT TSTP TTOU URG XCPU XFSZ VTALRM IO PWR SYS RTMIN]}) = 0
--- SIGSEGV {si_signo=SIGSEGV, si_code=SI_KERNEL, si_addr=NULL} ---
+++ killed by SIGSEGV +++
Neoprávnìný pøístup do pamìti (SIGSEGV)

Mikulas