On Tue, 06 Oct 2009 15:10:10 +0200, Oleg Nesterov wrote:
> On 10/06, Jan Kratochvil wrote:
> > On Mon, 05 Oct 2009 21:00:37 +0200, Oleg Nesterov wrote:
> > > On 10/05, Jan Kratochvil wrote:
> > > > Naive programs expect the first signal after PTRACE_ATTACH will be
> > > > SIGSTOP.
> > >
> > > They should not, this is just wrong.
> >
> > That may be a right point but such programs are in use out there. Sure if
> > it
> > would be a real difficulty one can keep it as-is as GDB-7.0 soon to be
> > released has it already fixed, strace works with it. Still ltrace crashes
> > the
> > inferior in such case.
>
> Confused. Do you mean we should fix the kernel to match this
> expectation?
Yes, I was thinking it would be a good idea.
> This was never true.
I agree.
> You attached the test-case which sends SIGALRM to itself in a loop.
> If the tracer attaches to this program, it is very possible that
> SIGALRM will be reported, not SIGSTOP.
Yes. And the real world tracers do not expect so.
> > > This test-case also does:
> > >
> > > /* detach with SIGPIPE/attach. This should kill tracee */
> > > ptrace (PTRACE_DETACH, child, (void *) 1, (void *) SIGPIPE);
> > >
> > > ptrace (PTRACE_ATTACH, child, (void *) 0, (void *) 0);
> > >
> > > waitpid (child, &status, 0);
> > > assert (WIFSIGNALED (status) && WTERMSIG (status) == SIGPIPE);
> > >
> > > It fails if the second PTRACE_ATTACH sees SIGPIPE. This is what
> > > I can't understand.
Second keyword "PTRACE_ATTACH" is on line 167.
First keyword "SIGPIPE" is on line 199.
Line 167 cannot see anything from line 199.
Assuming you did mean "third PTRACE_ATTACH".
Line 222: if (WIFSIGNALED (status) && WTERMSIG (status) == SIGPIPE)
Line 227: assert (WIFSTOPPED (status));
Line 228: assert (WSTOPSIG (status) == SIGSTOP);
Yes, I agree with the current general behavior of ptrace there is missing:
if (WIFSTOPPED (status) && WSTOPSIG (status) == SIGPIPE)
> It fails on ptrace-over-utrace.
Attached, I do not have kernel.rpm with ptrace-over-utrace ready, please
verify it and check it in (or ping me or so).
> Once again. Suppose that the tracer does ptrace(PTRACE_DETACH, SIGXXX).
> Currently, if the next thacer attaches right after this detach it has no
> way to intercept SIGXXX, it will be never reported via ptrace_signal().
No matter if it gets reported to the new tracer still SIGXXX should never get
lost. If it is not reported to the new tracer then it will be always
processed by the tracee, is it right?
> Is this really important? Do you know any application which can be
> broken if we change this behaviour? With the current utrace-ptrace
> implementation SIGXXX can be reported to the new tracer.
I think there is no application which would handle non-SIGSTOP as the first
signal after PTRACE_ATTACH while it would get confused by getting non-SIGSTOP
signal as the first one after PTRACE_ATTACH after PTRACE_DETACH.
> OK, this relates to "the first signal should be SIGSTOP" but this
> is wrong anyway, and the case above is very unlikely.
If "the first signal should be SIGSTOP" is not satisfied (current state)
I think you can freely change this behavior whether SIGXXX will be reported to
the new tracer and we should apply the attached ptrace-testsuite patch.
If "the first signal should be SIGSTOP" gets fixed/satisfied (proposed state)
I think it is clear SIGXXX from PTRACE_DETACH must not be lost and it must not
be visible as the first signal after PTRACE_ATTACH. In such case the
ptrace-testsuite testcase attach-into-signal should be simplified a lot to
always just require SIGSTOP as the first signal after PTRACE_ATTACH and the
attached change gets irrelevant in such case.
Thanks,
Jan
--- attach-into-signal.c 31 Jan 2009 21:11:40 -0000 1.5
+++ attach-into-signal.c 6 Oct 2009 14:27:08 -0000
@@ -224,6 +224,18 @@ static void reproduce (void)
child = 0;
return;
}
+ /* SIGPIPE was still pending and it has not been yet delivered. */
+ if (WIFSTOPPED (status) && WSTOPSIG (status) == SIGPIPE)
+ {
+ /* Deliver it and get the queued SIGSTOP. */
+ errno = 0;
+ ptrace (PTRACE_CONT, child, (void *) 1, (void *) SIGPIPE);
+ assert_perror (errno);
+
+ errno = 0;
+ pid = waitpid (child, &status, 0);
+ assert (pid == child);
+ }
assert (WIFSTOPPED (status));
assert (WSTOPSIG (status) == SIGSTOP);
/* let tracee run. it must be killed very soon by SIGPIPE */
