No upstream commit, this is a fix to a stable 5.4 specific backport.
The intention of backport commit cac68d12c531 ("io_uring: grab ->fs as part
of async offload") as found in the stable 5.4 tree was to make
io_sq_wq_submit_work() to switch the workqueue task's ->fs over to the
submitting task's one during the IO operation.
However, due to a small logic error, this change turned out to not have any
actual effect. From a high level, the relevant code in
io_sq_wq_submit_work() looks like
old_fs_struct = current->fs;
do {
...
if (req->fs != current->fs && current->fs != old_fs_struct) {
task_lock(current);
if (req->fs)
current->fs = req->fs;
else
current->fs = old_fs_struct;
task_unlock(current);
}
...
} while (req);
The if condition is supposed to cover the case that current->fs doesn't
match what's needed for processing the request, but observe how it fails
to ever evaluate to true due to the second clause:
current->fs != old_fs_struct will be false in the first iteration as per
the initialization of old_fs_struct and because this prevents current->fs
from getting replaced, the same follows inductively for all subsequent
iterations.
Fix said if condition such that
- if req->fs is set and doesn't match current->fs, the latter will be
switched to the former
- or if req->fs is unset, the switch back to the initial old_fs_struct
will be made, if necessary.
While at it, also correct the condition for the ->fs related cleanup right
before the return of io_sq_wq_submit_work(): currently, old_fs_struct is
restored only if it's non-NULL. It is always non-NULL though and thus, the
if-condition is rendundant. Supposedly, the motivation had been to optimize
and avoid switching current->fs back to the initial old_fs_struct in case
it is found to have the desired value already. Make it so.
Cc: stable@xxxxxxxxxxxxxxx # v5.4
Fixes: cac68d12c531 ("io_uring: grab ->fs as part of async offload")
Reviewed-by: Jens Axboe <axboe@xxxxxxxxx>
Signed-off-by: Nicolai Stange <nstange@xxxxxxx>
---
Tested on top of v5.4.90.
fs/io_uring.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
diff --git a/fs/io_uring.c b/fs/io_uring.c
index 4127ea027a14..478df7e10767 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -2226,7 +2226,8 @@ static void io_sq_wq_submit_work(struct work_struct *work)
/* Ensure we clear previously set non-block flag */
req->rw.ki_flags &= ~IOCB_NOWAIT;
- if (req->fs != current->fs && current->fs != old_fs_struct) {
+ if ((req->fs && req->fs != current->fs) ||
+ (!req->fs && current->fs != old_fs_struct)) {
task_lock(current);
if (req->fs)
current->fs = req->fs;
@@ -2351,7 +2352,7 @@ static void io_sq_wq_submit_work(struct work_struct *work)
mmput(cur_mm);
}
revert_creds(old_cred);
- if (old_fs_struct) {
+ if (old_fs_struct != current->fs) {
task_lock(current);
current->fs = old_fs_struct;
task_unlock(current);
--
2.26.2
