From: Wenjie Qi <qiwenjie@xiaomi.com>

The address-space write_begin callback is also used by internal callers
that pass a NULL kiocb, including page_symlink().

Check iocb before propagating IOCB_DONTCACHE into the F2FS-specific folio
lookup flags.

Fixes: cd5bc4fbb900 ("f2fs: enable buffered RWF_DONTCACHE")
Reported-by: syzbot+cid5582b8164122eda@syzkaller.appspotmail.com
Closes: https://ci.syzbot.org/series/cbb9ded8-1388-4a7b-bfa7-44082b04025b
Signed-off-by: Wenjie Qi <qiwenjie@xiaomi.com>
---
#syz test

 fs/f2fs/data.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
index 6e816c9349a..63625ab3f1e 100644
--- a/fs/f2fs/data.c
+++ b/fs/f2fs/data.c
@@ -3890,7 +3890,7 @@ static int f2fs_write_begin(const struct kiocb *iocb,
 	block_t blkaddr = NULL_ADDR;
 	int err = 0;
 
-	if (iocb->ki_flags & IOCB_DONTCACHE)
+	if (iocb && iocb->ki_flags & IOCB_DONTCACHE)
 		fgp_flags |= FGP_DONTCACHE;
 
 	trace_f2fs_write_begin(inode, pos, len);
-- 
2.43.0

syzbot ci has tested the suggested fix patch on top of the following series:

[v4] f2fs: enable buffered RWF_DONTCACHE
https://lore.kernel.org/all/cover.1788438786.git.qiwenjie@xiaomi.com

Patch: https://ci.syzbot.org/jobs/279baf88-89a3-4cb8-93c2-4bd86aa5092e/patch
Testing results:
* [build 0] Build Patched: passed
* [build 0] Boot test: Patched: passed
* [build 0] Previous reproducers: passed
  - general protection fault in f2fs_write_begin (patched) - passed

Full report is available here:
https://ci.syzbot.org/session/7e05d54c-8b9e-4e22-a746-48153ea42a60

---
This report is generated by a bot. It may contain errors.
syzbot ci engineers can be reached at syzkaller@googlegroups.com.