Re: [PATCH] Fix a race in pid generation that causes pids to be reused immediately.

Michel Lespinasse
2010-06-09T11:49:13Z

On Tue, Jun 08, 2010 at 11:24:38PM -0700, Salman wrote:
> A program that repeatedly forks and waits is susceptible to having the
> same pid repeated, especially when it competes with another instance of the
> same program. This is really bad for bash implementation. Furthermore, many shell
> scripts assume that pid numbers will not be used for some length of time.
>
> Thanks to Ted Tso for the key ideas of this implementation.
>
> Signed-off-by: Salman Qazi <sqazi@google.com>
> ---
> kernel/pid.c | 11 ++++++++++-
> 1 files changed, 10 insertions(+), 1 deletions(-)
>
> diff --git a/kernel/pid.c b/kernel/pid.c
> index e9fd8c1..8cedeab 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -153,8 +153,17 @@ static int alloc_pidmap(struct pid_namespace *pid_ns)
> if (likely(atomic_read(&map->nr_free))) {
> do {
> if (!test_and_set_bit(offset, map->page)) {
> + int prev;
> atomic_dec(&map->nr_free);
> - pid_ns->last_pid = pid;
> +
> + do {
> + prev = last;
> + last = cmpxchg(&pid_ns->last_pid,
> + prev, pid);
> + if (last >= pid)
> + break;

You should make sure to handle pid wrap-around for this last/pid comparison.
I think proper way to do that would be:

/* last is the pid we started scanning at
* last_read is the last observed value of pid_ns->last_pid
*/
last_read = last;
do {
prev = last_read;
last_read = cmpxchg(&pid_ns->last_pid, prev, pid);
/* Exit if one of these conditions is true:
* - cmpxchg succeeded
* - last <= pid <= last_read (other thread already bumped last_pid)
* - last_read <= last <= pid (same with wraparound)
* - pid <= last_read <= last (same with different wraparound)
*/
} while (last_read != prev &&
(last > pid || pid > last_read) &&
(last_read > last || last > pid) &&
(pid > last_read || last_read > last));

The last_read == pid case is also interesting - it means another thread found
the same pid, forked a child with that pid, and the child exited already
(since the bitmap was cleared). However we don't need to handle that case -
first, that race is much less likely to happen, and second, the duplicate
pid would be returned in two separate tasks - so this would not cause problems
in bash as in your example.

--
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.