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Subject[PATCH 04/11] ptrace: implement PTRACE_INTERRUPT
Date
Currently, there's no way to trap a running ptracee short of sending a
signal which has various side effects. This patch implements
PTRACE_INTERRUPT which traps ptracee without any signal or job control
related side effect.

The implementation is almost trivial. It uses the same trap site and
event as PTRACE_SEIZE. A new trap flag JOBCTL_TRAP_INTERRUPT is
added, which is set on PTRACE_INTERRUPT and cleared when tracee
commits to INTERRUPT trap. As INTERRUPT should be useable regardless
of the current state of tracee, task_is_traced() test in
ptrace_check_attach() is skipped for INTERRUPT.

PTRACE_INTERRUPT is available iff tracee is attached with
PTRACE_SEIZE.

Test program follows.

#define PTRACE_SEIZE 0x4206
#define PTRACE_INTERRUPT 0x4207

#define PTRACE_SEIZE_DEVEL 0x80000000
static const struct timespec ts100ms = { .tv_nsec = 100000000 };
static const struct timespec ts1s = { .tv_sec = 1 };
static const struct timespec ts3s = { .tv_sec = 3 };
int main(int argc, char **argv)
{
pid_t tracee;
tracee = fork();
if (tracee == 0) {
nanosleep(&ts100ms, NULL);
while (1) {
printf("tracee: alive pid=%d\n", getpid());
nanosleep(&ts1s, NULL);
}
}
if (argc > 1)
kill(tracee, SIGSTOP);
nanosleep(&ts100ms, NULL);

ptrace(PTRACE_SEIZE, tracee, NULL,
(void *)(unsigned long)PTRACE_SEIZE_DEVEL);
waitid(P_PID, tracee, NULL, WSTOPPED);
ptrace(PTRACE_CONT, tracee, NULL, NULL);
nanosleep(&ts3s, NULL);

printf("tracer: INTERRUPT and DETACH\n");
ptrace(PTRACE_INTERRUPT, tracee, NULL, NULL);
waitid(P_PID, tracee, NULL, WSTOPPED);
ptrace(PTRACE_DETACH, tracee, NULL, NULL);
nanosleep(&ts3s, NULL);

printf("tracer: exiting\n");
kill(tracee, SIGKILL);
return 0;
}
When called without argument, tracee is seized from running state,
continued, interrupted and then detached back to running state.

# ./test-interrupt
tracee: alive pid=4546
tracee: alive pid=4546
tracee: alive pid=4546
tracer: INTERRUPT and DETACH
tracee: alive pid=4546
tracee: alive pid=4546
tracee: alive pid=4546
tracer: exiting

When called with argument, it's the same but tracee is detached back
to stopped state.

# ./test-interrupt 1
tracee: alive pid=4548
tracee: alive pid=4548
tracee: alive pid=4548
tracer: INTERRUPT and DETACH
tracer: exiting

Before PTRACE_INTERRUPT, once the tracee was continued, there was no
easy way to do PTRACE_DETACH without causing side effect as tracee
couldn't be trapped without side effect.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
include/linux/ptrace.h | 1 +
include/linux/sched.h | 3 ++-
kernel/ptrace.c | 23 +++++++++++++++++++++--
kernel/signal.c | 4 ++++
4 files changed, 28 insertions(+), 3 deletions(-)
diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 8de301a..5b6128b 100644
--- a/include/linux/ptrace.h
+++ b/include/linux/ptrace.h
@@ -48,6 +48,7 @@
#define PTRACE_SETREGSET 0x4205

#define PTRACE_SEIZE 0x4206
+#define PTRACE_INTERRUPT 0x4207

/* flags in @data for PTRACE_SEIZE */
#define PTRACE_SEIZE_DEVEL 0x80000000 /* temp flag for development */
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2f383eb..221ab51 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1785,9 +1785,10 @@ extern void thread_group_times(struct task_struct *p, cputime_t *ut, cputime_t *
#define JOBCTL_STOP_PENDING (1 << 17) /* task should stop for group stop */
#define JOBCTL_STOP_CONSUME (1 << 18) /* consume group stop count */
#define JOBCTL_TRAP_SEIZE (1 << 19) /* trap for seize */
+#define JOBCTL_TRAP_INTERRUPT (1 << 20) /* trap for interrupt */
#define JOBCTL_TRAPPING (1 << 22) /* switching to TRACED */

-#define JOBCTL_TRAP_MASK JOBCTL_TRAP_SEIZE
+#define JOBCTL_TRAP_MASK (JOBCTL_TRAP_SEIZE | JOBCTL_TRAP_INTERRUPT)
#define JOBCTL_PENDING_MASK (JOBCTL_STOP_PENDING | JOBCTL_TRAP_MASK)

extern void task_clear_jobctl_stop_pending(struct task_struct *task);
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 0f0121a..1262a36 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -693,6 +693,23 @@ int ptrace_request(struct task_struct *child, long request,
ret = ptrace_setsiginfo(child, &siginfo);
break;

+ case PTRACE_INTERRUPT:
+ if (!likely(child->ptrace & PT_SEIZED))
+ break;
+ /*
+ * Stop tracee without any side-effect on signal or job
+ * control. If @child is already trapped, the current trap
+ * is not disturbed and INTERRUPT trap will happen after
+ * the current trap is ended with PTRACE_CONT. Note that
+ * other traps may happen before the scheduled INTERRUPT.
+ */
+ spin_lock(&child->sighand->siglock);
+ child->jobctl |= JOBCTL_TRAP_INTERRUPT;
+ signal_wake_up(child, 0);
+ spin_unlock(&child->sighand->siglock);
+ ret = 0;
+ break;
+
case PTRACE_DETACH: /* detach a process that was attached. */
ret = ptrace_detach(child, data);
break;
@@ -818,7 +835,8 @@ SYSCALL_DEFINE4(ptrace, long, request, long, pid, unsigned long, addr,
goto out_put_task_struct;
}

- ret = ptrace_check_attach(child, request == PTRACE_KILL);
+ ret = ptrace_check_attach(child, request == PTRACE_KILL ||
+ request == PTRACE_INTERRUPT);
if (ret < 0)
goto out_put_task_struct;

@@ -960,7 +978,8 @@ asmlinkage long compat_sys_ptrace(compat_long_t request, compat_long_t pid,
goto out_put_task_struct;
}

- ret = ptrace_check_attach(child, request == PTRACE_KILL);
+ ret = ptrace_check_attach(child, request == PTRACE_KILL ||
+ request == PTRACE_INTERRUPT);
if (!ret)
ret = compat_arch_ptrace(child, request, addr, data);

diff --git a/kernel/signal.c b/kernel/signal.c
index 9249230..7add912 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -1711,6 +1711,7 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
__releases(&current->sighand->siglock)
__acquires(&current->sighand->siglock)
{
+ bool is_intr = exit_code == (SIGTRAP | (PTRACE_EVENT_INTERRUPT << 8));
bool gstop_done = false;

if (arch_ptrace_stop_needed(exit_code, info)) {
@@ -1760,6 +1761,9 @@ static void ptrace_stop(int exit_code, int why, int clear_code, siginfo_t *info)
task_clear_jobctl_trapping(current);
current->jobctl &= ~JOBCTL_TRAP_SEIZE;

+ if (is_intr)
+ current->jobctl &= ~JOBCTL_TRAP_INTERRUPT;
+
spin_unlock_irq(&current->sighand->siglock);
read_lock(&tasklist_lock);
if (may_ptrace_stop()) {
--
1.7.1


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