[PATCH] ptrace/x86: introduce TS_COMPAT_RESTART to fix get_nr_restart_syscall()

[Oleg Nesterov <oleg@redhat.com>]

The comment in get_nr_restart_syscall() says:

	 * The problem is that we can get here when ptrace pokes
	 * syscall-like values into regs even if we're not in a syscall
	 * at all.

Yes. but if we are not in syscall then the

	status & (TS_COMPAT|TS_I386_REGS_POKED)

check below can't really help:

	- TS_COMPAT can't be set

	- TS_I386_REGS_POKED is only set if regs->orig_ax was changed by
	  32bit debugger; and even in this case get_nr_restart_syscall()
	  is only correct if the tracee is 32bit too.

Suppose that 64bit debugger plays with 32bit tracee and

	* Tracee calls sleep(2)	// TS_COMPAT is set
	* User interrupts the tracee by CTRL-C after 1 sec and does
	  "(gdb) call func()"
	* gdb saves the regs by PTRACE_GETREGS
	* does PTRACE_SETREGS to set %rip='func' and %orig_rax=-1
	* PTRACE_CONT		// TS_COMPAT is cleared
	* func() hits int3.
	* Debugger catches SIGTRAP.
	* Restore original regs by PTRACE_SETREGS.
	* PTRACE_CONT

get_nr_restart_syscall() wrongly returns __NR_restart_syscall==219, the
tracee calls ia32_sys_call_table[219] == sys_madvise.

This patch adds the sticky TS_COMPAT_RESTART flag which survives after
return to user mode, hopefully it allows us to kill TS_I386_REGS_POKED
but this needs another patch.

Test-case:

  $ cvs -d :pserver:anoncvs:anoncvs@sourceware.org:/cvs/systemtap co ptrace-tests
  $ gcc -o erestartsys-trap-debuggee ptrace-tests/tests/erestartsys-trap-debuggee.c --m32
  $ gcc -o erestartsys-trap-debugger ptrace-tests/tests/erestartsys-trap-debugger.c -lutil
  $ ./erestartsys-trap-debugger
  Unexpected: retval 1, errno 22
  erestartsys-trap-debugger: ptrace-tests/tests/erestartsys-trap-debugger.c:421

Reported-by: Jan Kratochvil <jan.kratochvil@redhat.com>
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 arch/x86/entry/common.c            | 9 +++++++++
 arch/x86/include/asm/thread_info.h | 1 +
 arch/x86/kernel/signal.c           | 6 ++----
 3 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 3f8e226..1e2c2ca 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -263,6 +263,15 @@ __visible inline void syscall_return_slowpath(struct pt_regs *regs)
 
 	rseq_syscall(regs);
 
+#ifdef CONFIG_IA32_EMULATION
+	if (regs->ax == -ERESTART_RESTARTBLOCK) {
+		struct thread_info *ti = current_thread_info();
+		if (ti->status & TS_COMPAT)
+			ti->status |= TS_COMPAT_RESTART;
+		else
+			ti->status &= ~TS_COMPAT_RESTART;
+	}
+#endif
 	/*
 	 * First do one-time work.  If these work items are enabled, we
 	 * want to run them exactly once per syscall exit with IRQs on.
diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index f945353..0784591 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -223,6 +223,7 @@ static inline int arch_within_stack_frames(const void * const stack,
 
 #ifdef CONFIG_COMPAT
 #define TS_I386_REGS_POKED	0x0004	/* regs poked by 32-bit ptracer */
+#define TS_COMPAT_RESTART	0x0008
 #endif
 #ifndef __ASSEMBLY__
 
diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index 8eb7193..1054f1c 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -788,12 +788,10 @@ static inline unsigned long get_nr_restart_syscall(const struct pt_regs *regs)
 	 * For now, we maintain historical behavior and guess based on
 	 * stored state.  We could do better by saving the actual
 	 * syscall arch in restart_block or (with caveats on x32) by
-	 * checking if regs->ip points to 'int $0x80'.  The current
-	 * behavior is incorrect if a tracer has a different bitness
-	 * than the tracee.
+	 * checking if regs->ip points to 'int $0x80'.
 	 */
 #ifdef CONFIG_IA32_EMULATION
-	if (current_thread_info()->status & (TS_COMPAT|TS_I386_REGS_POKED))
+	if (current_thread_info()->status & TS_COMPAT_RESTART)
 		return __NR_ia32_restart_syscall;
 #endif
 #ifdef CONFIG_X86_X32_ABI
-- 
2.5.0