Fix 32-bit arm unwinding through signal frames.
gdb was already okay; libgcc and libunwind need a little extra help.
Bug: 17436734
Change-Id: I1f5108f9877373be078554a1ea4938aa4436204e
diff --git a/libc/arch-arm/bionic/__restore.S b/libc/arch-arm/bionic/__restore.S
index e76628e..9898125 100644
--- a/libc/arch-arm/bionic/__restore.S
+++ b/libc/arch-arm/bionic/__restore.S
@@ -28,8 +28,34 @@
#include <private/bionic_asm.h>
-// This function must have exactly this instruction sequence for gdb and libunwind.
+// gdb is smart enough to unwind through signal frames with just the regular
+// CFI information but libgcc and libunwind both need extra help. We do this
+// by using .fnstart/.fnend and inserting a nop before both __restore and
+// __restore_rt (but covered by the .fnstart/.fnend) so that although they're
+// not inside the functions from objdump's point of view, an unwinder that
+// blindly looks at the previous instruction (but is then smart enough to check
+// the DWARF information to find out where it landed) gets the right answer.
+
+// We need to place .fnstart ourselves (but we may as well keep the free .fnend).
+#undef __bionic_asm_custom_entry
+#define __bionic_asm_custom_entry(f)
+
+ .fnstart
+ .save {r0-r15}
+ .pad #32
+ nop
ENTRY_PRIVATE(__restore)
+ // This function must have exactly this instruction sequence.
mov r7, #__NR_sigreturn
swi #0
END(__restore)
+
+ .fnstart
+ .save {r0-r15}
+ .pad #160
+ nop
+ENTRY_PRIVATE(__restore_rt)
+ // This function must have exactly this instruction sequence.
+ mov r7, #__NR_rt_sigreturn
+ swi #0
+END(__restore_rt)