On 6/18/26 18:05, Wenjie Qi wrote:
f2fs_write_end_io() decrements the writeback page counter and then reads
it again with get_pages() to decide whether the last F2FS_WB_CP_DATA
completion should wake cp_wait.
That second read can miss the zero transition as below:
Looks comments of v7 patch is quite different from the one of v1 patch?
Quoted from v1:
"f2fs_write_end_io() currently decrements the writeback page counter before
waking sbi->cp_wait for the last F2FS_WB_CP_DATA completion.
That decrement can drop the F2FS_WB_CP_DATA count to zero. It can unblock
a concurrent unmount path waiting in f2fs_wait_on_all_pages(). Unmount can
continue through f2fs_put_super() and eventually free sbi while the end_io
callback is still about to evaluate wq_has_sleeper() and wake_up() on
sbi->cp_wait.
Commit 2d9c4a4ed4ee ("f2fs: fix UAF caused by decrementing sbi->nr_pages[]
in f2fs_write_end_io()") fixed one post-decrement sbi access by moving the
warm-node-list handling before dec_page_count(). The compressed writeback
path follows the same rule and documents that dec_page_count() must be the
last access to sbi when it can drop F2FS_WB_CP_DATA to zero.
Apply the same ordering rule to the cp_wait wakeup. Check whether this is
the last F2FS_WB_CP_DATA completion and wake the waiter before the counter
decrement. Then the callback no longer dereferences sbi->cp_wait after the
lifetime boundary. A waiter that runs before the decrement may observe old
count and sleep until the one-jiffy timeout, but correctness no longer
depends on touching sbi after the counter reaches zero."
I may found something interesting: v7 codes try to fix UAF bug described in
v1 comment, however v7 comment tries to explain what v2 codes want to do.
I suspect your LLM goes another direction after prompted w/ my comments on
patch v1? Let me know I'm wrong. :P
Thanks,
checkpoint end_io A submitter B
- f2fs_wait_on_all_pages
- get_pages() > 0
- prepare_to_wait(cp_wait)
- io_schedule_timeout
- f2fs_write_end_io
- dec_page_count
: count 1 -> 0
- f2fs_submit_page_write
- inc_page_count
: count 0 -> 1
- get_pages() > 0
: skip wake_up(cp_wait)
The checkpoint thread can then keep sleeping until
DEFAULT_SCHEDULE_TIMEOUT, even though end_io A completed the old last
F2FS_WB_CP_DATA page.
Use the post-decrement value for F2FS_WB_CP_DATA completions so the wakeup
decision is tied to this completion. Keep the existing dec_page_count()
path for other writeback counters.
Fixes: e234088758fc ("f2fs: avoid wait if IO end up when do_checkpoint for better
performance")
Fixes: ce2739e482bc ("f2fs: fix to avoid UAF in f2fs_write_end_io()")
Cc: [email protected]
Signed-off-by: Wenjie Qi <[email protected]>
---
fs/f2fs/data.c | 12 +++++++-----
fs/f2fs/f2fs.h | 6 ++++++
2 files changed, 13 insertions(+), 5 deletions(-)
diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index d83a21998ec2..2afdcd209d54 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -392,15 +392,17 @@ static void f2fs_write_end_io(struct bio *bio)
if (f2fs_in_warm_node_list(folio))
f2fs_del_fsync_node_entry(sbi, folio);
- dec_page_count(sbi, type);
-
/*
* we should access sbi before folio_end_writeback() to
* avoid racing w/ kill_f2fs_super()
*/
- if (type == F2FS_WB_CP_DATA && !get_pages(sbi, type) &&
- wq_has_sleeper(&sbi->cp_wait))
- wake_up(&sbi->cp_wait);
+ if (type == F2FS_WB_CP_DATA) {
+ if (!dec_page_count_return(sbi, type) &&
+ wq_has_sleeper(&sbi->cp_wait))
+ wake_up(&sbi->cp_wait);
+ } else {
+ dec_page_count(sbi, type);
+ }
folio_clear_f2fs_gcing(folio);
folio_end_writeback(folio);
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 9f24287de4c3..db750cef371d 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -2776,6 +2776,12 @@ static inline void dec_page_count(struct f2fs_sb_info
*sbi, int count_type)
atomic_dec(&sbi->nr_pages[count_type]);
}
+static inline int dec_page_count_return(struct f2fs_sb_info *sbi,
+ int count_type)
+{
+ return atomic_dec_return(&sbi->nr_pages[count_type]);
+}
+
static inline void inode_dec_dirty_pages(struct inode *inode)
{
if (!S_ISDIR(inode->i_mode) && !S_ISREG(inode->i_mode) &&
base-commit: c0b65f6129c7fbb526e921dd60261650f1b2bef9
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