[PATCH v2] Fix get ERESTARTSYS with m32 in x86_64 when debug by GDB

[Hui Zhu <hui at codesourcery dot com>]

cat gdb.base/interrupt.c
#include <errno.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

#ifdef SIGNALS
#include <signal.h>

static void
sigint_handler (int signo)
{
}
#endif

int
main ()
{
  char x;
  int nbytes;
#ifdef SIGNALS
  signal (SIGINT, sigint_handler);
#endif
  printf ("talk to me baby\n");
  while (1)
    {
      nbytes = read (0, &x, 1);
      if (nbytes < 0)
{
#ifdef EINTR
 if (errno != EINTR)
#endif
   {
     perror ("");
     return 1;
   }
}
      else if (nbytes == 0)
{
 printf ("end of file\n");
 exit (0);
}
      else
write (1, &x, 1);
    }
  return 0;
}

int
func1 ()
{
  return 4;
}
gcc -g -m32 gdb.base/interrupt.c
gdb ./a.out
(gdb) r
Starting program: /home/teawater/gdb/binutils-gdb/gdb/testsuite/a.out
talk to me baby
data
data
^C
Program received signal SIGINT, Interrupt.
0xf7ffd430 in __kernel_vsyscall ()
(gdb) c
Continuing.
^C
Program received signal SIGINT, Interrupt.
0xf7ffd430 in __kernel_vsyscall ()
(gdb) p func1()
$1 = 4
(gdb) c
Continuing.
Unknown error 512
[Inferior 1 (process 7953) exited with code 01]

The root cause is:
When inferior call 32 bits syscall "read", Linux kernel function
"ia32_cstar_target" will set TS_COMPAT to current_thread_info->status.

syscall read is interrupt by ctrl-c.   Then the $rax will be set to
errno -512 in 64 bits.
And the inferior will be stopped by Linux kernel function ptrace_stop,
the call trace is:
#0  freezable_schedule () at include/linux/freezer.h:172
#1  ptrace_stop (exit_code=exit_code@entry=5, why=why@entry=262148,
    clear_code=clear_code@entry=0, info=info@entry=0xffff88001d833e78)
    at kernel/signal.c:1920
#2  0xffffffff8107ec33 in ptrace_signal (info=0xffff88001d833e78, signr=5)
    at kernel/signal.c:2157
#3  get_signal_to_deliver (info=info@entry=0xffff88001d833e78,
    return_ka=return_ka@entry=0xffff88001d833e58, regs=<optimized out>,
    cookie=cookie@entry=0x0 <irq_stack_union>) at kernel/signal.c:2269
#4  0xffffffff81013438 in do_signal (regs=regs@entry=0xffff88001d833f58)
    at arch/x86/kernel/signal.c:696
#5  0xffffffff81013a40 in do_notify_resume (regs=0xffff88001d833f58,
    unused=<optimized out>, thread_info_flags=4) at arch/x86/kernel/signal.c:747
#6  <signal handler called>
#7  0x0000000000000000 in irq_stack_union ()

After that, GDB can control the stopped inferior.
To call function "func1()" of inferior, GDB need:
Step 1, save current values of registers ($rax 0xfffffffffffffe00(64 bits -512)
is cut to 0xfffffe00(32 bits -512) because inferior is a 32 bits program).
Step 2, change the values of registers.
Step 3, Push a dummy frame to stack.
Step 4, set a breakpint in the return address.

When GDB resume the inferior, it will keep execut from ptrace_stop
with new values of registers that set by GDB.
And TS_COMPAT inside current_thread_info->status will be cleared when
inferior switch back to user space.

When function "func1()" return, inferior will be stoped by breakpoint
inferior will be stopped by Linux kernel function "ptrace_stop" again.
current_thread_info->status will not set TS_COMPAT when inferior swith
from user space to kernel space because breakpoint handler "int3" doesn't
has code for that.

GDB begin to set saved values of registers back to inferior that use
function "amd64_collect_native_gregset".  Because this function just
zero-extend each 32 bits value to 64 bits value before put them to inferior.
$rax's value is set to 0xfffffe00(32 bits -512) but not
0xfffffffffffffe00(64 bits -512).

When GDB continue syscall "read" that is interrupted by "ctrl-c", it will
keep execute from ptrace_stop without "TS_COMPAT".
Then in Linux kernel function "syscall_get_error", current_thread_info->status
doesn't have TS_COMPAT and $rax is 0xfffffe00(32 bits -512).  Then in
function do_signal will not handle this -ERESTARTSYS.

-ERESTARTSYS will be return back to inferior, that is why inferior got a
errno -ERESTARTSYS.

According to comments from Mark.  I make a new version that add following
code to putreg():
	case offsetof(struct user_regs_struct, orig_ax):
		/*
		 * A 64-bit debugger setting orig_ax of a 32-bit inferior
		 * means to restore the state of the task restarting a
		 * 32-bit syscall.
		 * Make sure we interpret the -ERESTART* codes correctly
		 * in case the task is not actually still sitting at the
		 * exit from a 32-bit syscall with TS_COMPAT still set.
		 */
		if (test_ti_thread_flag(task_thread_info(child), TIF_IA32)) {
			struct pt_regs *regs = task_pt_regs(child);
			regs->orig_ax = value;
			if (syscall_get_nr(child, regs) >= 0)
				task_thread_info(child)->status |= TS_COMPAT;
			return 0;
		}
		break;

Signed-off-by: Hui Zhu <hui@codesourcery.com>
---
--- a/arch/x86/kernel/ptrace.c
+++ b/arch/x86/kernel/ptrace.c
@@ -452,6 +452,23 @@ static int putreg(struct task_struct *ch
 		if (child->thread.gs != value)
 			return do_arch_prctl(child, ARCH_SET_GS, value);
 		return 0;
+	case offsetof(struct user_regs_struct, orig_ax):
+		/*
+		 * A 64-bit debugger setting orig_ax of a 32-bit inferior
+		 * means to restore the state of the task restarting a
+		 * 32-bit syscall.
+		 * Make sure we interpret the -ERESTART* codes correctly
+		 * in case the task is not actually still sitting at the
+		 * exit from a 32-bit syscall with TS_COMPAT still set.
+		 */
+		if (test_ti_thread_flag(task_thread_info(child), TIF_IA32)) {
+			struct pt_regs *regs = task_pt_regs(child);
+			regs->orig_ax = value;
+			if (syscall_get_nr(child, regs) >= 0)
+				task_thread_info(child)->status |= TS_COMPAT;
+			return 0;
+		}
+		break;
 #endif
 	}