STATUS_HEAP_CORRUPTION if signal arrives when x86 direction flag is set

[Christian Franke]

Found because 'stress-ng --memcpy ...' and other tests report segfaults:

An exception 0xc0000374 (STATUS_HEAP_CORRUPTION) occurs if a signal 
arrives during a memmove() which copies backwards due to overlap.

The related snippet[s] from winsup/cygwin/x86_64/bcopy.S:
         std
         rep
         movs[qb]
         cld

The testcase below shows that a set DF arrives at the signal handler. 
This violates the ABI, AFAIK. After return, the process aborts 
regardless of a "cld" in the signal handler.

$ uname -r # also reproducible with 3.5.7-1
3.6.0-1.x86_64

$ cat dflagsig.c
#include <signal.h>
#include <unistd.h>

static volatile sig_atomic_t sigcnt;

static void sighandler(int sig)
{
   (void)sig;
   // asm volatile ("cld"); // <== does not prevent crash
   if (__builtin_ia32_readeflags_u64() & 0x0400)
     write(1, "[DF=1]\n", 7);
   else
     write(1, "[DF=0]\n", 7);
   ++sigcnt;
}

int main()
{
   signal(SIGINT, sighandler);
   int std = 0, cnt;
   while ((cnt = sigcnt) < 5) {
     if (cnt == 2 && !std) {
       asm volatile ("std");
       std = 1;
     }
     else if (cnt > 2 && std) {
       asm volatile ("cld");
       std = 0;
     }
   }
   return 42;
}

$ gcc -o dflagsig dflagsig.c

$ ./dflagsig # ... and press 3x ^C
[DF=0]
[DF=0]
[DF=1]

$ echo $? # Hmm... "silent" crash!
0

$  strace ./dflagsig # ... and run 3x 'kill -INT 1288' from other window
...
    48 14882485 [main] dflagsig 1288 set_signal_mask: setmask 2, newmask 
0, mask_bits 2
863030 15745515 [sig] dflagsig 1288 sigpacket::process: signal 2 processing
...
    55 15746773 [sig] dflagsig 1288 _cygtls::interrupt_setup: armed 
signal_arrived 0x0, signal 2
    70 15746843 [sig] dflagsig 1288 sigpacket::setup_handler: signal 2 
delivered
--- Process 12736 (pid: 1288), exception c0000374 at 00007ffe342dcba9
...
--- Process 12736 exited with status 0xc0000374

-- 
Regards,
Christian