STATUS_HEAP_CORRUPTION if signal arrives when x86 direction flag is set

[Takashi Yano]

On Sun, 23 Mar 2025 12:54:36 +0100
Christian Franke wrote:
> 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
> 

Thanks for the report. I'll submit a patch to fix that.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>