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.y...@nifty.ne.jp>
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