Fix 32-bit arm unwinding through signal frames.

gdb was already okay; libgcc and libunwind need a little extra help.

Bug: 17436734

(cherry picked from commit 148dff3ec6)

Change-Id: I2cc997017acc57c930284af5264f353656b98c7b

libc/arch-arm/arm.mk

@@ -55,7 +55,6 @@ libc_bionic_src_files_arm += \
     arch-arm/bionic/_exit_with_stack_teardown.S \
     arch-arm/bionic/libgcc_compat.c \
     arch-arm/bionic/memcmp.S \
-    arch-arm/bionic/__restore_rt.S \
     arch-arm/bionic/__restore.S \
     arch-arm/bionic/_setjmp.S \
     arch-arm/bionic/setjmp.S \

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)

libc/arch-arm/bionic/__restore_rt.S

@@ -1,35 +0,0 @@
-/*
- * Copyright (C) 2014 The Android Open Source Project
- * All rights reserved.
- *
- * Redistribution and use in source and binary forms, with or without
- * modification, are permitted provided that the following conditions
- * are met:
- *  * Redistributions of source code must retain the above copyright
- *    notice, this list of conditions and the following disclaimer.
- *  * Redistributions in binary form must reproduce the above copyright
- *    notice, this list of conditions and the following disclaimer in
- *    the documentation and/or other materials provided with the
- *    distribution.
- *
- * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
- * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
- * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
- * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
- * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
- * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
- * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
- * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
- * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
- * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
- * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
- * SUCH DAMAGE.
- */
-
-#include <private/bionic_asm.h>
-
-// This function must have exactly this instruction sequence for gdb and libunwind.
-ENTRY_PRIVATE(__restore_rt)
-  mov r7, #__NR_rt_sigreturn
-  swi #0
-END(__restore_rt)