Suppose we have 2 threads, the group-leader L and a sub-theread T,
both parked in ptrace_stop(). Debugger tries to resume both threads
and does
ptrace(PTRACE_CONT, T);
ptrace(PTRACE_CONT, L);
If the sub-thread T execs in between, the 2nd PTRACE_CONT doesn not
resume the old leader L, it resumes the post-exec thread T which was
actually now stopped in PTHREAD_EVENT_EXEC. In this case the
PTHREAD_EVENT_EXEC event is lost, and the tracer can't know that the
tracee changed its pid.
This patch makes ptrace() fail in this case until debugger does wait()
and consumes PTHREAD_EVENT_EXEC which reports old_pid. This affects all
ptrace requests except the "asynchronous" PTRACE_INTERRUPT/KILL.
The patch doesn't add the new PTRACE_ option to not complicate the API,
and I _hope_ this won't cause any noticeable regression:
- If debugger uses PTRACE_O_TRACEEXEC and the thread did an exec
and the tracer does a ptrace request without having consumed
the exec event, it's 100% sure that the thread the ptracer
thinks it is targeting does not exist anymore, or isn't the
same as the one it thinks it is targeting.
- To some degree this patch adds nothing new. In the scenario
above ptrace(L) can fail with -ESRCH if it is called after the
execing sub-thread wakes the leader up and before it "steals"
the leader's pid.
Test-case:
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/ptrace.h>
#include <sys/wait.h>
#include <errno.h>
#include <pthread.h>
#include <assert.h>
void *tf(void *arg)
{
execve("/usr/bin/true", NULL, NULL);
assert(0);
return NULL;
}
int main(void)
{
int leader = fork();
if (!leader) {
kill(getpid(), SIGSTOP);
pthread_t th;
pthread_create(&th, NULL, tf, NULL);
for (;;)
pause();
return 0;
}
waitpid(leader, NULL, WSTOPPED);
ptrace(PTRACE_SEIZE, leader, 0,
PTRACE_O_TRACECLONE | PTRACE_O_TRACEEXEC);
waitpid(leader, NULL, 0);
ptrace(PTRACE_CONT, leader, 0,0);
waitpid(leader, NULL, 0);
int status, thread = waitpid(-1, &status, 0);
assert(thread > 0 && thread != leader);
assert(status == 0x80137f);
ptrace(PTRACE_CONT, thread, 0,0);
/*
* waitid() because waitpid(leader, &status, WNOWAIT) does not
* report status. Why ????
*
* Why WEXITED? because we have another kernel problem connected
* to mt-exec.
*/
siginfo_t info;
assert(waitid(P_PID, leader, &info, WSTOPPED|WEXITED|WNOWAIT)
== 0);
assert(info.si_pid == leader && info.si_status == 0x0405);
/* OK, it sleeps in ptrace(PTRACE_EVENT_EXEC == 0x04) */
assert(ptrace(PTRACE_CONT, leader, 0,0) == -1);
assert(errno == ESRCH);
assert(leader == waitpid(leader, &status, WNOHANG));
assert(status == 0x04057f);
assert(ptrace(PTRACE_CONT, leader, 0,0) == 0);
return 0;
}
Signed-off-by: Oleg Nesterov <o...@redhat.com>
---
kernel/ptrace.c | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)
diff --git a/kernel/ptrace.c b/kernel/ptrace.c
index 43d6179508d6..1037251ae4a5 100644
--- a/kernel/ptrace.c
+++ b/kernel/ptrace.c
@@ -169,6 +169,27 @@ void __ptrace_unlink(struct task_struct *child)
spin_unlock(&child->sighand->siglock);
}
+static bool looks_like_a_spurious_pid(struct task_struct *task)
+{
+ int pid;
+
+ if (task->exit_code != ((PTRACE_EVENT_EXEC << 8) | SIGTRAP))
+ return false;
+
+ rcu_read_lock();
+ pid = task_pid_nr_ns(task, task_active_pid_ns(task->parent));
+ rcu_read_unlock();
+
+ if (pid == task->ptrace_message)
+ return false;
+ /*
+ * The tracee changed its pid but the PTRACE_EVENT_EXEC event
+ * was not wait()'ed, most probably debugger targets the old
+ * leader which was destroyed in de_thread().
+ */
+ return true;
+}
+
/* Ensure that nothing can wake it up, even SIGKILL */
static bool ptrace_freeze_traced(struct task_struct *task)
{
@@ -179,7 +200,8 @@ static bool ptrace_freeze_traced(struct task_struct *task)
return ret;
spin_lock_irq(&task->sighand->siglock);
- if (task_is_traced(task) && !__fatal_signal_pending(task)) {
+ if (task_is_traced(task) && !looks_like_a_spurious_pid(task) &&
+ !__fatal_signal_pending(task)) {
task->state = __TASK_TRACED;
ret = true;
}
--
2.25.1.362.g51ebf55
