Takashi Yano wrote:
x86_64 ABI requires the direction flag in CPU flags register cleared.
https://learn.microsoft.com/en-us/cpp/build/x64-software-conventions
However, currently that flag is not maintained in signal handler.
Therefore, if the signal handler is called when that flag is set, it
destroys the data and may crash if rep instruction is used in the
signal handler. With this patch, the direction flag is cleared in
sigdelayed() by adding cld instruction.
Addresses: https://cygwin.com/pipermail/cygwin/2025-March/257704.html
Fixes: 1fd5e000ace5 ("import winsup-2000-02-17 snapshot")
Reported-by: Christian Franke <christian.fra...@t-online.de>
Reviewed-by:
Signed-off-by: Takashi Yano <takashi.y...@nifty.ne.jp>
---
winsup/cygwin/scripts/gendef | 1 +
1 file changed, 1 insertion(+)
diff --git a/winsup/cygwin/scripts/gendef b/winsup/cygwin/scripts/gendef
index a2f0392bc..861a2405b 100755
--- a/winsup/cygwin/scripts/gendef
+++ b/winsup/cygwin/scripts/gendef
@@ -179,6 +179,7 @@ sigdelayed:
movq %rsp,%rbp
pushf
.seh_pushreg %rax # fake, there's no .seh_pushreg
for the flags
+ cld # x86_64 ABI requires direction
flag cleared
# stack is aligned or unaligned on entry!
# make sure it is aligned from here on
# We could be called from an interrupted thread which doesn't know
Works as expected:
- the testcase no longer aborts.
- a version with modified main loop does not detect DF modification by
the signal:
while ((cnt = sigcnt) < 1000) {
if (!(__builtin_ia32_readeflags_u64() & 0x0400) != !std)
return 13;
if ((cnt & 1) && !std) {
asm volatile ("std");
std = 1;
}
else if (!(cnt & 1) && std) {
asm volatile ("cld");
std = 0;
}
}
- The related stress-ng testcases no longer report segfaults:
$ n=0
$ while
stress-ng --parallel 2 --with memcpy,tree --memcpy-method libc
--tree-method btree -t 2;
do echo OK $((++n)); done
...
OK 500
...
--
Thanks,
Christian
