i386: fix infinite loop with singlestep int80 syscalls

The commit 635cf99a80f4ebee59d70eb64bb85ce829e4591f introduced a
regression.  Executing a ptrace single step after certain int80
accesses will infinitely loop and never advance the PC.

The TIF_SINGLESTEP check should be done on the return from the syscall
and not before it.

The new test case is below:

/* Test whether singlestep through an int80 syscall works.
 */
#define _GNU_SOURCE
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <sys/mman.h>
#include <asm/user.h>
#include <string.h>

static int child, status;
static struct user_regs_struct regs;

static void do_child()
{
	char str[80] = "child: int80 test\n";

	ptrace(PTRACE_TRACEME, 0, 0, 0);
	kill(getpid(), SIGUSR1);
	write(fileno(stdout),str,strlen(str));
	asm ("int $0x80" : : "a" (20)); /* getpid */
}

static void do_parent()
{
	unsigned long eip, expected = 0;
again:
	waitpid(child, &status, 0);
	if (WIFEXITED(status) || WIFSIGNALED(status))
		return;

	if (WIFSTOPPED(status)) {
		ptrace(PTRACE_GETREGS, child, 0, &regs);
		eip = regs.eip;
		if (expected)
			fprintf(stderr, "child stop @ %08lx, expected %08lx %s\n",
					eip, expected,
					eip == expected ? "" : " <== ERROR");

		if (*(unsigned short *)eip == 0x80cd) {
			fprintf(stderr, "int 0x80 at %08x\n", (unsigned int)eip);
			expected = eip + 2;
		} else
			expected = 0;

		ptrace(PTRACE_SINGLESTEP, child, NULL, NULL);
	}
	goto again;
}

int main(int argc, char * const argv[])
{
	child = fork();
	if (child)
		do_parent();
	else
		do_child();
	return 0;
}

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Cc: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Chuck Ebbert <cebbert@redhat.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/arch/i386/kernel/entry.S b/arch/i386/kernel/entry.S
index 18bddcb8e9e82757042d4813568120257b9bb25f..cb1f16c5a04535a4eb8fd1a9e0090bc1e526cfa1 100644 (file)
--- a/arch/i386/kernel/entry.S
+++ b/arch/i386/kernel/entry.S
@@ -371,10 +371,6 @@ ENTRY(system_call)
        CFI_ADJUST_CFA_OFFSET 4
        SAVE_ALL
        GET_THREAD_INFO(%ebp)
-       testl $TF_MASK,PT_EFLAGS(%esp)
-       jz no_singlestep
-       orl $_TIF_SINGLESTEP,TI_flags(%ebp)
-no_singlestep:
                                        # system call tracing in operation / emulation
        /* Note, _TIF_SECCOMP is bit number 8, and so it needs testw and not testb */
        testw $(_TIF_SYSCALL_EMU|_TIF_SYSCALL_TRACE|_TIF_SECCOMP|_TIF_SYSCALL_AUDIT),TI_flags(%ebp)
@@ -389,6 +385,10 @@ syscall_exit:
                                        # setting need_resched or sigpending
                                        # between sampling and the iret
        TRACE_IRQS_OFF
+       testl $TF_MASK,PT_EFLAGS(%esp)  # If tracing set singlestep flag on exit
+       jz no_singlestep
+       orl $_TIF_SINGLESTEP,TI_flags(%ebp)
+no_singlestep:
        movl TI_flags(%ebp), %ecx
        testw $_TIF_ALLWORK_MASK, %cx   # current->work
        jne syscall_exit_work