Messages in this thread Patch in this message | | | Date | Sun, 17 Jul 2011 21:03:37 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/1] ptrace: fix ptrace_signal() && STOP_DEQUEUED interaction |
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On 07/14, Oleg Nesterov wrote: > > On 07/14, Tejun Heo wrote: > > > > Never mind. I for some reason thought flipping the flag would make > > the extra step in ptrace_signal() unnecessary. We need to clear it > > all the same so it doesn't really improve anything. I think the > > current version should be fine > > Good. > > > (maybe the comment can be beefed up a > > bit?). > > I agree, this is not the best comment... OK, I'll try to make a > better one. Damn this is not easy ;) and I hate the really fat > comments.
OK, how about v2? The patch is the same, only the comment was updated. Not sure it is really better though.
Oleg.
------------------------------------------------------------------------------ [PATCH v2] ptrace: fix ptrace_signal() && STOP_DEQUEUED interaction
Simple test-case,
int main(void) { int pid, status;
pid = fork(); if (!pid) { pause(); assert(0); return 0x23; }
assert(ptrace(PTRACE_ATTACH, pid, 0,0) == 0); assert(wait(&status) == pid); assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);
kill(pid, SIGCONT); // <--- also clears STOP_DEQUEUD
assert(ptrace(PTRACE_CONT, pid, 0,0) == 0); assert(wait(&status) == pid); assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGCONT);
assert(ptrace(PTRACE_CONT, pid, 0, SIGSTOP) == 0); assert(wait(&status) == pid); assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);
kill(pid, SIGKILL); return 0; }
Without the patch it hangs. After the patch SIGSTOP "injected" by the tracer is not ignored and stops the tracee.
Note also that if this test-case uses, say, SIGWINCH instead of SIGCONT, everything works without the patch. This can't be right, and this is confusing.
The problem is that SIGSTOP (or any other sig_kernel_stop() signal) has no effect without JOBCTL_STOP_DEQUEUED. This means it is simply ignored after PTRACE_CONT unless JOBCTL_STOP_DEQUEUED was set "by accident", say it wasn't cleared after initial SIGSTOP sent by PTRACE_ATTACH.
At first glance we could change ptrace_signal() to add STOP_DEQUEUED after return from ptrace_stop(), but this is not right in case when the tracer does not change the reported SIGSTOP and SIGCONT comes in between. This is even more wrong with PT_SEIZED, SIGCONT adds JOBCTL_TRAP_NOTIFY which will be "lost" during the TRAP_STOP | TRAP_NOTIFY report.
So lets add STOP_DEQUEUED _before_ we report the signal. It has no effect unless sig_kernel_stop() == T after the tracer resumes us, and in the latter case the pending STOP_DEQUEUED means no SIGCONT in between, we should stop.
Note also that if SIGCONT was sent, PT_SEIZED tracee will correctly report PTRACE_EVENT_STOP/SIGTRAP and thus the tracer can notice the fact SIGSTOP was cancelled.
Also, move the current->ptrace check from ptrace_signal() to its caller, get_signal_to_deliver(), this looks more natural.
Signed-off-by: Oleg Nesterov <oleg@redhat.com> ---
kernel/signal.c | 17 +++++++++++------ 1 file changed, 11 insertions(+), 6 deletions(-)
--- ptrace/kernel/signal.c~1_ptrace_signal_stop_dequeued 2011-07-17 20:16:36.000000000 +0200 +++ ptrace/kernel/signal.c 2011-07-17 20:57:30.000000000 +0200 @@ -2084,12 +2084,17 @@ static void do_jobctl_trap(void) static int ptrace_signal(int signr, siginfo_t *info, struct pt_regs *regs, void *cookie) { - if (!current->ptrace) - return signr; - ptrace_signal_deliver(regs, cookie); - - /* Let the debugger run. */ + /* + * We do not check sig_kernel_stop(signr) but set this marker + * unconditionally because we do not know whether debugger will + * change signr. This flag has no meaning unless we are going + * to stop after return from ptrace_stop(). In this case it will + * be checked in do_signal_stop(), we should only stop if it was + * not cleared by SIGCONT while we were sleeping. See also the + * comment in dequeue_signal(). + */ + current->jobctl |= JOBCTL_STOP_DEQUEUED; ptrace_stop(signr, CLD_TRAPPED, 0, info); /* We're back. Did the debugger cancel the sig? */ @@ -2193,7 +2198,7 @@ relock: if (!signr) break; /* will return 0 */ - if (signr != SIGKILL) { + if (unlikely(current->ptrace) && signr != SIGKILL) { signr = ptrace_signal(signr, info, regs, cookie); if (!signr)
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