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SubjectRe: [PATCH 06/11] ptrace: make group stop state visible via PTRACE_GETSIGINFO
On 05/08, Tejun Heo wrote:
>
> Ptracer can detect tracee entering group stop by watching for the
> group stop trap; however, there is no reliable way to find out when
> group stop ends - SIGCONT may be processed by another thread and
> signal delivery is blocked while tracee is trapped.

Confused.

> This patch adds siginfo.si_pt_flags and uses PTRACE_SI_STOPPED flag to
> indicate whether group stop is in effect or not. While tracee is
> trapped for anything other than signal delivery and group stop itself,
> tracer can use PTRACE_GETSIGINFO to access this information. Note
> that it's only available if tracee was seized.

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.

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?

> Later patches will deal with
> notification and trap transition.

OK, probably I'll understand the intent later.

> __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.

> int main(int argc, char **argv)
> {
> pid_t tracee, tracer;
> int i;
>
> tracee = fork();
> if (!tracee)
> while (1)
> nanosleep(&ts1s, NULL);
>
> tracer = fork();
> if (!tracer) {
> int last_stopped = 0, stopped;
> siginfo_t si;
>
> ptrace(PTRACE_SEIZE, tracee, NULL,
> (void *)(unsigned long)PTRACE_SEIZE_DEVEL);
> repeat:
> waitid(P_PID, tracee, NULL, WSTOPPED);
>
> 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?

> @@ -540,6 +542,17 @@ static int ptrace_getsiginfo(struct task_struct *child, siginfo_t *info)
>
> error = 0;
> *info = *child->last_siginfo;
> +
> + /*
> + * If reporting ptrace trap for a seized tracee, enable reporting
> + * of info->si_pt_flags.
> + */
> + 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 ?

> + /* 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 ;)

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.
Oleg.



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