[f2fs-dev] [PATCH v3 1/2] f2fs: avoid NULL checkpoint thread access in sysfs

checkpoint_merge can be enabled even when no checkpoint merge thread is
running. A read-only mount is one case: f2fs does not start
f2fs_issue_ckpt there, but ckpt_thread_ioprio is still writable through
sysfs.

The ckpt_thread_ioprio store path updates the saved ioprio value and,
when checkpoint_merge is enabled, calls set_task_ioprio() for the
checkpoint thread. If cprc->f2fs_issue_ckpt is NULL, that dereferences a
NULL task pointer.

Protect ckpt_thread_ioprio sysfs writes with s_umount as well, so the
checkpoint thread cannot disappear under the store path while updating
its ioprio.

Fixes: e65920661708 ("f2fs: add ckpt_thread_ioprio sysfs node")
Cc: [email protected]
Signed-off-by: Wenjie Qi <[email protected]>
---
v3:
- make patch 1 self-contained by protecting ckpt_thread_ioprio sysfs writes
  with s_umount as well
- split critical_task_priority handling out into patch 2

 fs/f2fs/sysfs.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/fs/f2fs/sysfs.c b/fs/f2fs/sysfs.c
index be92c05a5420..0729b3670415 100644
--- a/fs/f2fs/sysfs.c
+++ b/fs/f2fs/sysfs.c
@@ -557,7 +557,7 @@ static ssize_t __sbi_store(struct f2fs_attr *a,
 			return -EINVAL;
 
 		cprc->ckpt_thread_ioprio = IOPRIO_PRIO_VALUE(class, level);
-		if (test_opt(sbi, MERGE_CHECKPOINT)) {
+		if (cprc->f2fs_issue_ckpt) {
 			ret = set_task_ioprio(cprc->f2fs_issue_ckpt,
 					cprc->ckpt_thread_ioprio);
 			if (ret)
@@ -1007,13 +1007,14 @@ static ssize_t f2fs_sbi_store(struct f2fs_attr *a,
 	ssize_t ret;
 	bool gc_entry = (!strcmp(a->attr.name, "gc_urgent") ||
 					a->struct_type == GC_THREAD);
+	bool thread_entry = !strcmp(a->attr.name, "ckpt_thread_ioprio");
 
-	if (gc_entry) {
+	if (gc_entry || thread_entry) {
 		if (!down_read_trylock(&sbi->sb->s_umount))
 			return -EAGAIN;
 	}
 	ret = __sbi_store(a, sbi, buf, count);
-	if (gc_entry)
+	if (gc_entry || thread_entry)
 		up_read(&sbi->sb->s_umount);
 
 	return ret;
-- 
2.43.0


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