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Subject[PATCH 3/5] 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 group stop trap -
SIGTRAP | PTRACE_EVENT_STOP << 8. A new trap flag
JOBCTL_TRAP_INTERRUPT is added, which is set on PTRACE_INTERRUPT and
cleared when any trap happens. 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);
if (argc > 1) {
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,
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, tracee is seized from stopped state,
continued, interrupted and then 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 running, there was no way
to trap tracee and do PTRACE_DETACH without causing side effect.

-v2: Updated to use task_set_jobctl_pending() so that it doesn't end
up scheduling TRAP_STOP if child is dying which may make the
child unkillable. Spotted by Oleg.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
---
include/linux/ptrace.h | 1 +
kernel/ptrace.c | 29 +++++++++++++++++++++++++++--
2 files changed, 28 insertions(+), 2 deletions(-)
diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
index 67ad3f1..ad754d1 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/kernel/ptrace.c b/kernel/ptrace.c
index dcf9f97..6852c0f 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -658,10 +658,12 @@ static int ptrace_regset(struct task_struct *task, int req, unsigned int type,
int ptrace_request(struct task_struct *child, long request,
unsigned long addr, unsigned long data)
{
+ bool seized = child->ptrace & PT_SEIZED;
int ret = -EIO;
siginfo_t siginfo;
void __user *datavp = (void __user *) data;
unsigned long __user *datalp = datavp;
+ unsigned long flags;

switch (request) {
case PTRACE_PEEKTEXT:
@@ -694,6 +696,27 @@ int ptrace_request(struct task_struct *child, long request,
ret = ptrace_setsiginfo(child, &siginfo);
break;

+ case PTRACE_INTERRUPT:
+ /*
+ * Stop tracee without any side-effect on signal or job
+ * control. At least one trap is guaranteed to happen
+ * after this request. If @child is already trapped, the
+ * current trap is not disturbed and another trap will
+ * happen after the current trap is ended with PTRACE_CONT.
+ *
+ * The actual trap might not be PTRACE_EVENT_STOP trap but
+ * the pending condition is cleared regardless.
+ */
+ if (unlikely(!seized || !lock_task_sighand(child, &flags)))
+ break;
+
+ if (likely(task_set_jobctl_pending(child, JOBCTL_TRAP_STOP)))
+ signal_wake_up(child, 0);
+
+ unlock_task_sighand(child, &flags);
+ ret = 0;
+ break;
+
case PTRACE_DETACH: /* detach a process that was attached. */
ret = ptrace_detach(child, data);
break;
@@ -819,7 +842,8 @@ 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;

@@ -961,7 +985,8 @@ 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);

--
1.7.5.2


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