work around gdb issues recognizing sigreturn trampoline on x86_64

gdb can only backtrace/unwind across signal handlers if it recognizes
the sa_restorer trampoline. for x86_64, gdb first attempts to
determine the symbol name for the function in which the program
counter resides and match it against "__restore_rt". if no name can be
found (e.g. in the case of a stripped binary), the exact instruction
sequence is matched instead.

when matching the function name, however, gdb's unwind code wrongly
considers the interval [sym,sym+size] rather than [sym,sym+size).
thus, if __restore_rt begins immediately after another function, gdb
wrongly identifies pc as lying within the previous adjacent function.
this patch adds a nop before __restore_rt to preclude that
possibility. it also removes the symbol name __restore and replaces it
with a macro since the stability of whether gdb identifies the
function as __restore_rt or __restore is not clear.

for the no-symbols case, the instruction sequence is changed to use
%rax rather than %eax to match what gdb expects.

based on patch by Szabolcs Nagy, with extended description and
corresponding x32 changes added.

1 files changed, 9 insertions, 0 deletions

diff --git a/arch/x86_64/ksigaction.h b/arch/x86_64/ksigaction.h
new file mode 100644
index 00000000..7743c5c6
--- /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