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Subject[RFC PATCH 3/3] ptrace: dont send SIGSTOP on auto-attach if PT_SEIZED
The fake SIGSTOP during attach has numerous problems. PTRACE_SEIZE
is already fine, but we have basically the same problems is SIGSTOP
is sent on auto-attach, the tracer can't know if this signal signal
should be cancelled or not.

Change ptrace_event() to set JOBCTL_TRAP_STOP if the new child is
PT_SEIZED, this triggers the PTRACE_EVENT_STOP report.

Thereafter a PT_SEIZED task can never report the bogus SIGSTOP.

Test-case:

#define PTRACE_SEIZE 0x4206
#define PTRACE_SEIZE_DEVEL 0x80000000
#define PTRACE_EVENT_STOP 7
#define WEVENT(s) ((s & 0xFF0000) >> 16)
int main(void)
{
int child, grand_child, status;
long message;
child = fork();
if (!child) {
kill(getpid(), SIGSTOP);
fork();
assert(0);
return 0x23;
}
assert(ptrace(PTRACE_SEIZE, child, 0,PTRACE_SEIZE_DEVEL) == 0);
assert(wait(&status) == child);
assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGSTOP);

assert(ptrace(PTRACE_SETOPTIONS, child, 0, PTRACE_O_TRACEFORK) == 0);

assert(ptrace(PTRACE_CONT, child, 0,0) == 0);
assert(waitpid(child, &status, 0) == child);
assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
assert(WEVENT(status) == PTRACE_EVENT_FORK);

assert(ptrace(PTRACE_GETEVENTMSG, child, 0, &message) == 0);
grand_child = message;

assert(waitpid(grand_child, &status, 0) == grand_child);
assert(WIFSTOPPED(status) && WSTOPSIG(status) == SIGTRAP);
assert(WEVENT(status) == PTRACE_EVENT_STOP);

kill(child, SIGKILL);
kill(grand_child, SIGKILL);
return 0;
}
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
include/linux/ptrace.h | 13 +++++++------
1 file changed, 7 insertions(+), 6 deletions(-)
--- ptrace/include/linux/ptrace.h~5_clone_no_stop_if_seized 2011-07-08 18:32:05.000000000 +0200
+++ ptrace/include/linux/ptrace.h 2011-07-08 18:32:53.000000000 +0200
@@ -228,7 +228,11 @@ static inline void ptrace_init_task(stru
child->ptrace = current->ptrace;
__ptrace_link(child, current->parent);

- sigaddset(&child->pending.signal, SIGSTOP);
+ if (child->ptrace & PT_SEIZED)
+ task_set_jobctl_pending(child, JOBCTL_TRAP_STOP);
+ else
+ sigaddset(&child->pending.signal, SIGSTOP);
+
set_tsk_thread_flag(child, TIF_SIGPENDING);
}
}


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