A task can end up indefinitely sleeping in do_select() ->
poll_schedule_timeout() when the following race happens:
TASK1 (thread1) TASK2 TASK1 (thread2)
do_select()
setup poll_wqueues table
with 'fd'
write data to 'fd'
pollwake()
table->triggered = 1
closes 'fd' thread1 is
waiting for
poll_schedule_timeout()
- sees table->triggered
table->triggered = 0
return -EINTR
loop back in do_select() but fdget() in the setup of poll_wqueues
fails now so we never find 'fd' is ready for reading and sleep in
poll_schedule_timeout() indefinitely.
Treat fd that got closed as a fd on which some event happened. This
makes sure cannot block indefinitely in do_select().
Another option would be to return -EBADF in this case but that has a
potential of subtly breaking applications that excercise this behavior
and it happens to work for them. So returning fd as active seems like a
safer choice.
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
CC: stable@xxxxxxxxxxxxxxx
Signed-off-by: Jan Kara <jack@xxxxxxx>
---
fs/select.c | 63 ++++++++++++++++++++++++++++-------------------------
1 file changed, 33 insertions(+), 30 deletions(-)
diff --git a/fs/select.c b/fs/select.c
index 945896d0ac9e..5edffee1162c 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -458,9 +458,11 @@ static int max_select_fd(unsigned long n, fd_set_bits *fds)
return max;
}
-#define POLLIN_SET (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN | EPOLLHUP | EPOLLERR)
-#define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT | EPOLLERR)
-#define POLLEX_SET (EPOLLPRI)
+#define POLLIN_SET (EPOLLRDNORM | EPOLLRDBAND | EPOLLIN | EPOLLHUP | EPOLLERR |\
+ EPOLLNVAL)
+#define POLLOUT_SET (EPOLLWRBAND | EPOLLWRNORM | EPOLLOUT | EPOLLERR |\
+ EPOLLNVAL)
+#define POLLEX_SET (EPOLLPRI | EPOLLNVAL)
static inline void wait_key_set(poll_table *wait, unsigned long in,
unsigned long out, unsigned long bit,
@@ -527,6 +529,7 @@ static int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
break;
if (!(bit & all_bits))
continue;
+ mask = EPOLLNVAL;
f = fdget(i);
if (f.file) {
wait_key_set(wait, in, out, bit,
@@ -534,34 +537,34 @@ static int do_select(int n, fd_set_bits *fds, struct timespec64 *end_time)
mask = vfs_poll(f.file, wait);
fdput(f);
- if ((mask & POLLIN_SET) && (in & bit)) {
- res_in |= bit;
- retval++;
- wait->_qproc = NULL;
- }
- if ((mask & POLLOUT_SET) && (out & bit)) {
- res_out |= bit;
- retval++;
- wait->_qproc = NULL;
- }
- if ((mask & POLLEX_SET) && (ex & bit)) {
- res_ex |= bit;
- retval++;
- wait->_qproc = NULL;
- }
- /* got something, stop busy polling */
- if (retval) {
- can_busy_loop = false;
- busy_flag = 0;
-
- /*
- * only remember a returned
- * POLL_BUSY_LOOP if we asked for it
- */
- } else if (busy_flag & mask)
- can_busy_loop = true;
-
}
+ if ((mask & POLLIN_SET) && (in & bit)) {
+ res_in |= bit;
+ retval++;
+ wait->_qproc = NULL;
+ }
+ if ((mask & POLLOUT_SET) && (out & bit)) {
+ res_out |= bit;
+ retval++;
+ wait->_qproc = NULL;
+ }
+ if ((mask & POLLEX_SET) && (ex & bit)) {
+ res_ex |= bit;
+ retval++;
+ wait->_qproc = NULL;
+ }
+ /* got something, stop busy polling */
+ if (retval) {
+ can_busy_loop = false;
+ busy_flag = 0;
+
+ /*
+ * only remember a returned
+ * POLL_BUSY_LOOP if we asked for it
+ */
+ } else if (busy_flag & mask)
+ can_busy_loop = true;
+
}
if (res_in)
*rinp = res_in;
--
2.31.1
