lkml.org 
[lkml]   [2011]   [May]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 06/11] ptrace: make group stop state visible via PTRACE_GETSIGINFO
Hello,

On Tue, May 10, 2011 at 06:55:45PM +0200, Oleg Nesterov wrote:
> IOW, if the tracee reports via ptrace_notify*, the tracee can look at
> si_pt_flags == stop-in-effect. If the tracer reports a signal, the
> tracer obviously lacks this info, hmm.

Which indicates tracee is in group stop trap.

> Probably I need more time to get used to this... But at first glance
> this looks a bit unnatural. Say, can't we simply implement
> PTRACE_GET_GROUP_STOP_STATUS request which returns this (and probably
> more) info?

I don't know. PTRACE_GETSIGINFO seemed to already fit the bill and I
want to avoid introducing a new request if at all possible. It sure
is a bit quirky but doesn't compromisea functionality.

> > __SI_TRAP is defined to implement copying of
> > the new field to userland.
>
> Heh. I am shy to admit, I didn't know copy_siginfo_to_user() trims
> si_code, that is why your change is correct but I spent a lot of time
> before I was able to understand this.

Oh, don't be shy. I scratched my head for quite a while trying to
figure out why the hell the new flag field isn't getting out to
userland. It's an ugly piece of sh*t. :-)

> > if (!ptrace(PTRACE_GETSIGINFO, tracee, NULL, &si)) {
> > if (si.si_code) {
> > stopped = !!si.si_status;
>
> In this case this "si_code != 0" check is correct, but how can the
> tracer detect this case in general?

This was quick hack. Proper test would look like,

si.si_code && (si.si_pt_flags & PTRACE_SI_STOPPED)

> > @@ -540,6 +542,17 @@ static int ptrace_getsiginfo(struct task_struct *child, siginfo_t *info)
> > + if ((child->ptrace & PT_SEIZED) &&
> > + (info->si_code & (0x7f | ~0xffff)) == (__SI_TRAP | SIGTRAP)) {
>
> Can't we simply check (from->si_code & __SI_MASK) == __SI_TRAP ?

Right, I originally lifted the test from ptrace_notify() before adding
__SI_TRAP and forgot to update it later. Will change.

> > + /* report whether group stop is in effect w/ SI_STOPPED */
> > + if (sig->group_stop_count || (sig->flags & SIGNAL_STOP_STOPPED))
>
> We have more and more "group_stop_count || SIGNAL_STOP_STOPPED" checks,
> perhaps we should make a helper. Or at least invent the short name to
> denote the group-stopped-or-in-progress to simplify the discussions ;)

Yeah, how about group_stop_in_effect()?

> Still, this is strange. With this change ptrace_getsiginfo() reports
> the extra "volatile" info which wasn't reported by the tracee itself.
> If the tracer does PTRACE_SETSIGINFO twice in a row, it can see the
> different si_pt_flags's.

(answering to both get/setsiginfo concerns)

* I think we better block PTRACE_SETSIGINFO for non signal delivery
traps. It doesn't make any sense. Let's just fail that with
-EINVAL if PT_SEIZED.

* I don't think PTRACE_GETSIGINFO returning volatile information to be
problematic. The information is generated on the fly on trap
anyway. For non signal delivery traps, PTRACE_GETSIGINFO is
basically (ab)using siginfo as a container for debugging
information. It might have been better if something else was used
from the beginning but the damage is already done and I don't see
too much benefit in making things pretty at this point.

Thank you.

--
tejun


\
 
 \ /
  Last update: 2011-05-11 17:39    [W:1.430 / U:0.036 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site