[RFA] Darwin: single step through sigreturn

[Tristan Gingold <gingold@adacore.com>]

Hi,

as other platform, single stepping through the sigreturn syscall is  
special: changing the cpu state
is useless as the context is restored.  The trace bit must be set in  
the to be restored context.

Tristan.

2008-12-04  Tristan Gingold  <gingold@adacore.com>

	* i386-darwin-nat.c (i386_darwin_sstep_at_sigreturn): New function.
	(amd64_darwin_sstep_at_sigreturn): New function.
	(darwin_set_sstep): The sigreturn is a special case: the trace flag
	must be set in the mcontext structure.

diff -c -p -r1.1 i386-darwin-nat.c
*** i386-darwin-nat.c	27 Nov 2008 09:23:01 -0000	1.1
--- i386-darwin-nat.c	4 Dec 2008 15:27:37 -0000
***************
*** 34,39 ****
--- 34,40 ----
   #include "i387-tdep.h"
   #include "gdbarch.h"
   #include "arch-utils.h"
+ #include "gdbcore.h"

   #include "darwin-nat.h"
   #include "i386-darwin-tdep.h"
*************** darwin_check_osabi (darwin_inferior *inf
*** 433,438 ****
--- 434,503 ----

   #define X86_EFLAGS_T 0x100UL

+ /* Returning from a signal trampoline is done by calling a
+    special system call (sigreturn).  This system call
+    restores the registers that were saved when the signal was
+    raised, including %eflags/%rflags.  That means that single-stepping
+    won't work.  Instead, we'll have to modify the signal context
+    that's about to be restored, and set the trace flag there.  */
+
+ static int
+ i386_darwin_sstep_at_sigreturn (x86_thread_state_t *regs)
+ {
+   static const gdb_byte darwin_syscall[] = { 0xcd, 0x80 }; /* int  
0x80 */
+   gdb_byte buf[sizeof (darwin_syscall)];
+
+   /* Check if PC is at a sigreturn system call.  */
+   if (target_read_memory (regs->uts.ts32.__eip, buf, sizeof (buf))  
== 0
+       && memcmp (buf, darwin_syscall, sizeof (darwin_syscall)) == 0
+       && regs->uts.ts32.__eax == 0xb8 /* SYS_sigreturn */)
+     {
+       ULONGEST uctx_addr;
+       ULONGEST mctx_addr;
+       ULONGEST flags_addr;
+       unsigned int eflags;
+
+       uctx_addr = read_memory_unsigned_integer (regs->uts.ts32.__esp  
+ 4, 4);
+       mctx_addr = read_memory_unsigned_integer (uctx_addr + 28, 4);
+
+       flags_addr = mctx_addr + 12 + 9 * 4;
+       read_memory (flags_addr, (gdb_byte *) &eflags, 4);
+       eflags |= X86_EFLAGS_T;
+       write_memory (flags_addr, (gdb_byte *) &eflags, 4);
+
+       return 1;
+     }
+   return 0;
+ }
+
+ static int
+ amd64_darwin_sstep_at_sigreturn (x86_thread_state_t *regs)
+ {
+   static const gdb_byte darwin_syscall[] = { 0x0f, 0x05 }; /*  
syscall */
+   gdb_byte buf[sizeof (darwin_syscall)];
+
+   /* Check if PC is at a sigreturn system call.  */
+   if (target_read_memory (regs->uts.ts64.__rip, buf, sizeof (buf))  
== 0
+       && memcmp (buf, darwin_syscall, sizeof (darwin_syscall)) == 0
+       && (regs->uts.ts64.__rax & 0xffffffff) == 0x20000b8 /*  
SYS_sigreturn */)
+     {
+       ULONGEST mctx_addr;
+       ULONGEST flags_addr;
+       unsigned int rflags;
+
+       mctx_addr = read_memory_unsigned_integer (regs->uts.ts64.__rdi  
+ 48, 8);
+       flags_addr = mctx_addr + 16 + 17 * 8;
+
+       /* AMD64 is little endian.  */
+       read_memory (flags_addr, (gdb_byte *) &rflags, 4);
+       rflags |= X86_EFLAGS_T;
+       write_memory (flags_addr, (gdb_byte *) &rflags, 4);
+
+       return 1;
+     }
+   return 0;
+ }
+
   void
   darwin_set_sstep (thread_t thread, int enable)
   {
*************** darwin_set_sstep (thread_t thread, int e
*** 448,459 ****
--- 513,527 ----
   			 kret, thread);
         return;
       }
+
     switch (regs.tsh.flavor)
       {
       case x86_THREAD_STATE32:
         {
   	__uint32_t bit = enable ? X86_EFLAGS_T : 0;
   	
+ 	if (enable && i386_darwin_sstep_at_sigreturn (&regs))
+ 	  return;
   	if ((regs.uts.ts32.__eflags & X86_EFLAGS_T) == bit)
   	  return;
   	regs.uts.ts32.__eflags = (regs.uts.ts32.__eflags & ~X86_EFLAGS_T)  
| bit;
*************** darwin_set_sstep (thread_t thread, int e
*** 466,471 ****
--- 534,541 ----
         {
   	__uint64_t bit = enable ? X86_EFLAGS_T : 0;

+ 	if (enable && amd64_darwin_sstep_at_sigreturn (&regs))
+ 	  return;
   	if ((regs.uts.ts64.__rflags & X86_EFLAGS_T) == bit)
   	  return;
   	regs.uts.ts64.__rflags = (regs.uts.ts64.__rflags & ~X86_EFLAGS_T)  
| bit;