[PATCH v2] fix x86_64 sigreturn trampoline so gdb recognizes it

[Szabolcs Nagy <nsz@port70.net>]

gdb can only unwind across signal handlers if it recognizes the trampoline.
on x86_64 this means the pc must be in a function called __restore_rt or
if gdb cannot determine the symbol name the instruction sequence must match
to what gdb expects.

a nop is added before __restore_rt, the __restore alias is removed and
size annotation is added to make sure gdb maps any pc value within
__restore_rt unambigously to the __restore_rt symbol name.

and the instruction sequence got fixed to match what gdb expects.
---
 arch/x86_64/ksigaction.h    | 9 +++++++++
 src/signal/x86_64/restore.s | 7 +++----
 2 files changed, 12 insertions(+), 4 deletions(-)
 create mode 100644 arch/x86_64/ksigaction.h

diff --git a/arch/x86_64/ksigaction.h b/arch/x86_64/ksigaction.h
new file mode 100644
index 0000000..7743c5c
--- /dev/null
+++ b/arch/x86_64/ksigaction.h
@@ -0,0 +1,9 @@
+struct k_sigaction {
+	void (*handler)(int);
+	unsigned long flags;
+	void (*restorer)(void);
+	unsigned mask[2];
+};
+
+void __restore_rt();
+#define __restore __restore_rt
diff --git a/src/signal/x86_64/restore.s b/src/signal/x86_64/restore.s
index 682af2d..b5c6e37 100644
--- a/src/signal/x86_64/restore.s
+++ b/src/signal/x86_64/restore.s
@@ -1,8 +1,7 @@
+	nop
 .global __restore_rt
-.global __restore
 .type __restore_rt,@function
-.type __restore,@function
 __restore_rt:
-__restore:
-	movl $15, %eax
+	mov $15, %rax
 	syscall
+.size __restore_rt,.-__restore_rt
-- 
2.10.2