From: Oleg Nesterov
Date: Sat Apr 11 2015 - 09:33:12 EST

On 04/10, Andy Lutomirski wrote:
> Hi all-
> AFAICS there are several things wrong with our magical do_debug
> handling of single-stepping through the kernel. They boil down to two
> issues:
> 1. do_debug seems to be overly permissive in terms of what faults in
> kernel space it thinks are okay. This isn't obviously a problem
> except that it obfuscates what's going on. AFAICT the *only*
> acceptable case is TF set on sysenter. All the mentions of syscalls
> are garbage -- both int80 and syscall clear TF.
> 2. I think this is wrong:
> set_tsk_thread_flag(tsk, TIF_SINGLESTEP);
> regs->flags &= ~X86_EFLAGS_TF;
> TIF_SINGLESTEP doesn't mean "set TF in this sysenter's saved flags and
> then clear TIF_SINGLESTEP". It means something complicated.

Yes, plus TIF_FORCED_TF adds more confusion...

And to me another problem is that these flags are not cleared in/after

> The upshot AFAICT is that the attached program blows up if you build
> it with -m32 and run it on an Intel machine.

I thinks the patch below should help, but most probably there are other
isssues. Actually I am sure there are other issues, but I forgot the
problems I found when I tried to understand this logic in details some
time ago.

The patch is already in -mm. I'll try to check if it actually helps
when I have the access to my testing machine (I need it to compile
with -m32, my user-space environment is broken ;).


Subject: [PATCH 2nd RESEND] ptrace/x86: fix the TIF_FORCED_TF logic in handle_signal()

When the TIF_SINGLESTEP tracee dequeues a signal, handle_signal()
clears TIF_FORCED_TF and X86_EFLAGS_TF but leaves TIF_SINGLESTEP set.

If the tracer does PTRACE_SINGLESTEP again, enable_single_step() sets
X86_EFLAGS_TF but not TIF_FORCED_TF. This means that the subsequent
PTRACE_CONT doesn't not clear X86_EFLAGS_TF, and the tracee gets the
wrong SIGTRAP.

Test-case (needs -O2 to avoid prologue insns in signal handler):

#include <unistd.h>
#include <stdio.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <sys/user.h>
#include <assert.h>
#include <stddef.h>

void handler(int n)

int child(void)
assert(ptrace(PTRACE_TRACEME, 0,0,0) == 0);
signal(SIGALRM, handler);
kill(getpid(), SIGALRM);
return 0x23;

void *getip(int pid)
return (void*)ptrace(PTRACE_PEEKUSER, pid,
offsetof(struct user,, 0);

int main(void)
int pid, status;

pid = fork();
if (!pid)
return child();

assert(wait(&status) == pid);
assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGALRM);

assert(ptrace(PTRACE_SINGLESTEP, pid, 0, SIGALRM) == 0);
assert(wait(&status) == pid);
assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
assert((getip(pid) - (void*)handler) == 0);

assert(ptrace(PTRACE_SINGLESTEP, pid, 0, SIGALRM) == 0);
assert(wait(&status) == pid);
assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
assert((getip(pid) - (void*)handler) == 1);

assert(ptrace(PTRACE_CONT, pid, 0,0) == 0);
assert(wait(&status) == pid);
assert(WIFEXITED(status) && WEXITSTATUS(status) == 0x23);

return 0;

The last assert() fails because PTRACE_CONT wrongly triggers another
single-step and X86_EFLAGS_TF can't be cleared by debugger until the
tracee does sys_rt_sigreturn().

Change handle_signal() to do user_disable_single_step() if stepping,
we do not need to preserve TIF_SINGLESTEP because we are going to do
ptrace_notify(), and it is simply wrong to leak this bit.

While at it, change the comment to explain why we also need to clear
TF unconditionally after setup_rt_frame().

Note: in the longer term we should probably change setup_sigcontext()
to use get_flags() and then just remove this user_disable_single_step().
And, the state of TIF_FORCED_TF can be wrong after restore_sigcontext()
which can set/clear TF, this needs another fix.

Reported-by: Evan Teran <eteran@xxxxxxxxxxxx>
Reported-by: Pedro Alves <palves@xxxxxxxxxx>
Tested-By: Andres Freund <andres@xxxxxxxxxxx>
Signed-off-by: Oleg Nesterov <oleg@xxxxxxxxxx>
arch/x86/kernel/signal.c | 22 +++++++++++-----------
1 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/arch/x86/kernel/signal.c b/arch/x86/kernel/signal.c
index ed37a76..9d3a15b 100644
--- a/arch/x86/kernel/signal.c
+++ b/arch/x86/kernel/signal.c
@@ -629,7 +629,8 @@ setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs)
static void
handle_signal(struct ksignal *ksig, struct pt_regs *regs)
- bool failed;
+ bool stepping, failed;
/* Are we from a system call? */
if (syscall_get_nr(current, regs) >= 0) {
/* If so, check system call restarting.. */
@@ -653,12 +654,13 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)

- * If TF is set due to a debugger (TIF_FORCED_TF), clear the TF
- * flag so that register information in the sigcontext is correct.
+ * If TF is set due to a debugger (TIF_FORCED_TF), clear TF now
+ * so that register information in the sigcontext is correct and
+ * then notify the tracer before entering the signal handler.
- if (unlikely(regs->flags & X86_EFLAGS_TF) &&
- likely(test_and_clear_thread_flag(TIF_FORCED_TF)))
- regs->flags &= ~X86_EFLAGS_TF;
+ stepping = test_thread_flag(TIF_SINGLESTEP);
+ if (stepping)
+ user_disable_single_step(current);

failed = (setup_rt_frame(ksig, regs) < 0);
if (!failed) {
@@ -669,10 +671,8 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
* it might disable possible debug exception from the
* signal handler.
- * Clear TF when entering the signal handler, but
- * notify any tracer that was single-stepping it.
- * The tracer may want to single-step inside the
- * handler too.
+ * Clear TF for the case when it wasn't set by debugger to
+ * avoid the recursive send_sigtrap() in SIGTRAP handler.
regs->flags &= ~(X86_EFLAGS_DF|X86_EFLAGS_RF|X86_EFLAGS_TF);
@@ -681,7 +681,7 @@ handle_signal(struct ksignal *ksig, struct pt_regs *regs)
if (used_math())
- signal_setup_done(failed, ksig, test_thread_flag(TIF_SINGLESTEP));
+ signal_setup_done(failed, ksig, stepping);

#ifdef CONFIG_X86_32

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