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-rw-r--r--libc/sysdeps/linux/x86_64/sigaction.c10
1 files changed, 9 insertions, 1 deletions
diff --git a/libc/sysdeps/linux/x86_64/sigaction.c b/libc/sysdeps/linux/x86_64/sigaction.c
index 703b24c51..a4042a9c8 100644
--- a/libc/sysdeps/linux/x86_64/sigaction.c
+++ b/libc/sysdeps/linux/x86_64/sigaction.c
@@ -111,11 +111,19 @@ libc_hidden_weak(sigaction)
signal handlers work right. Important are both the names
(__restore_rt) and the exact instruction sequence.
If you ever feel the need to make any changes, please notify the
- appropriate GDB maintainer. */
+ appropriate GDB maintainer.
+
+ The unwind information starts a byte before __restore_rt, so that
+ it is found when unwinding, to get an address the unwinder assumes
+ will be in the middle of a call instruction. See the Linux kernel
+ (the i386 vsyscall, in particular) for an explanation of the complex
+ unwind information used here in order to get the traditional CFA.
+ */
#define RESTORE(name, syscall) RESTORE2(name, syscall)
#define RESTORE2(name, syscall) \
__asm__ ( \
+ "nop\n" \
".text\n" \
"__" #name ":\n" \
" movq $" #syscall ", %rax\n" \