Detect the arm/thumb mode of code SIGRETURN or RT_SIGRETURN returns to

This patch fixes the following regression introduced by commit d0e59a68

step^M
39      } /* handler */^M
1: x/i $pc^M
=> 0x8740 <handler+80>: sub     sp, r11, #0^M
(gdb) step^M
^M
Program received signal SIGSEGV, Segmentation fault.^M
setitimer () at ../sysdeps/unix/syscall-template.S:81^M
81      ../sysdeps/unix/syscall-template.S: No such file or directory.^M
1: x/i $pc^M
=> 0xb6eff9c0 <setitimer>:      push    {r7}^M
(gdb) FAIL: gdb.base/sigstep.exp: continue to handler, si+advance in handler, step from handler: leave handler

in my test setting, program is compiled in arm mode, but the glibc
is built in thumb mode, so when we do 'step' to step over syscall
instruction svc for SIGRETURN, GDB should set breakpoint for arm mode
in the program, even though the current program in glibc is in thumb
mode.  Current GDB doesn't consider the case that the mode of program
SIGRETURN goes to can be different from current program mode.

In fact, GDB has taken care of this arm/thumb mode changes already,
see

/* Copy the value of next pc of sigreturn and rt_sigrturn into PC,
   return 1.  In addition, set IS_THUMB depending on whether we
   will return to ARM or Thumb code.  Return 0 if it is not a
   rt_sigreturn/sigreturn syscall.  */
static int
arm_linux_sigreturn_return_addr (struct frame_info *frame,
				 unsigned long svc_number,
				 CORE_ADDR *pc, int *is_thumb)

but in the commit d0e59a68

> -  arm_linux_sigreturn_return_addr (frame, svc_number, &return_addr, &is_thumb);
> +  if (svc_number == ARM_SIGRETURN || svc_number == ARM_RT_SIGRETURN)
> +    next_pc = arm_linux_sigreturn_next_pc (regcache, svc_number);

the IS_THUMB setting is lost, so it is a regression.

gdb:

2016-01-21  Yao Qi  <yao.qi@linaro.org>

	* arm-linux-tdep.c (arm_linux_sigreturn_next_pc): Add parameter
	is_thumb and set it according to CPSR saved on the stack.
	(arm_linux_get_next_pcs_syscall_next_pc): Pass is_thumb to
	arm_linux_sigreturn_next_pc.

gdb/gdbserver:

2016-01-21  Yao Qi  <yao.qi@linaro.org>

	* linux-arm-low.c (arm_sigreturn_next_pc): Add parameter
	is_thumb and set it according to CPSR saved on the stack.
	(get_next_pcs_syscall_next_pc): Pass is_thumb to
	arm_sigreturn_next_pc.

1 files changed, 15 insertions, 3 deletions

diff --git a/gdb/arm-linux-tdep.c b/gdb/arm-linux-tdep.c
index 2306bda..f6a831a 100644
--- a/gdb/arm-linux-tdep.c
+++ b/gdb/arm-linux-tdep.c
@@ -782,10 +782,12 @@ arm_linux_sigreturn_return_addr (struct frame_info *frame,
 }
 
 /* Find the value of the next PC after a sigreturn or rt_sigreturn syscall
-   based on current processor state.  */
+   based on current processor state.  In addition, set IS_THUMB depending
+   on whether we will return to ARM or Thumb code.  */
+
 static CORE_ADDR
 arm_linux_sigreturn_next_pc (struct regcache *regcache,
-			     unsigned long svc_number)
+			     unsigned long svc_number, int *is_thumb)
 {
   ULONGEST sp;
   unsigned long sp_data;
@@ -794,6 +796,7 @@ arm_linux_sigreturn_next_pc (struct regcache *regcache,
   enum bfd_endian byte_order = gdbarch_byte_order (gdbarch);
   int pc_offset = 0;
   int is_sigreturn = 0;
+  CORE_ADDR cpsr;
 
   gdb_assert (svc_number == ARM_SIGRETURN
 	      || svc_number == ARM_RT_SIGRETURN);
@@ -807,6 +810,10 @@ arm_linux_sigreturn_next_pc (struct regcache *regcache,
 
   next_pc = read_memory_unsigned_integer (sp + pc_offset, 4, byte_order);
 
+  /* Set IS_THUMB according the CPSR saved on the stack.  */
+  cpsr = read_memory_unsigned_integer (sp + pc_offset + 4, 4, byte_order);
+  *is_thumb = ((cpsr & arm_psr_thumb_bit (gdbarch)) != 0);
+
   return next_pc;
 }
 
@@ -899,7 +906,12 @@ arm_linux_get_next_pcs_syscall_next_pc (struct arm_get_next_pcs *self,
     }
 
   if (svc_number == ARM_SIGRETURN || svc_number == ARM_RT_SIGRETURN)
-    next_pc = arm_linux_sigreturn_next_pc (self->regcache, svc_number);
+    {
+      /* SIGRETURN or RT_SIGRETURN may affect the arm thumb mode, so
+	 update IS_THUMB.   */
+      next_pc = arm_linux_sigreturn_next_pc (self->regcache, svc_number,
+					     &is_thumb);
+    }
 
   /* Addresses for calling Thumb functions have the bit 0 set.  */
   if (is_thumb)