Re: [PATCH v2] f2fs: Reduce the scope of setting fsck tag when de->name_len is zero

From: Chao Yu
Date: Tue Jul 20 2021 - 00:11:18 EST


On 2021/7/19 18:14, Yangtao Li wrote:
I recently found a case where de->name_len is 0 in f2fs_fill_dentries() easily reproduced,
and finally set the fsck flag.

Thread A Thread B

f2fs_readdir
f2fs_read_inline_dir
ctx->pos = d.max
f2fs_add_dentry
f2fs_add_inline_entry
do_convert_inline_dir
f2fs_add_regular_entry
f2fs_readdir
f2fs_fill_dentries
set_sbi_flag(sbi, SBI_NEED_FSCK)

Process A opens the folder, and has been reading without closing it. During this period,
Process B created a file under the folder (occupying multiple f2fs_dir_entry, exceeding
the d.max of the inline dir). After creation, process A uses the d.max of inline dir to
read it again, and it will read that de->name_len is 0.

And Chao pointed out that w/o inline conversion, the race condition still can happen as below

dir_entry1: A
dir_entry2: B
dir_entry3: C
free slot: _

ctx->pos: ^


Before:
AAAABBBB___
^

AAAABBBB___
^

Thread B delete dir_entry2, and create dir_entry3.

After:
AAAACCCCC__
^

AAAACCCCC__
^


In these scenarios, the file system is not damaged, and it's hard to avoid it. But we can bypass
tagging FSCK flag if:
a) bit_pos (:= ctx->pos % d->max) is non-zero & b) before bit_pos moves to first
valid dir_entry.

Signed-off-by: Yangtao Li <frank.li@xxxxxxxx>
---
fs/f2fs/dir.c | 14 +++++++++-----
1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index 456651682daf..3e2a61a3600c 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -1000,6 +1000,7 @@ int f2fs_fill_dentries(struct dir_context *ctx, struct f2fs_dentry_ptr *d,
struct f2fs_sb_info *sbi = F2FS_I_SB(d->inode);
struct blk_plug plug;
bool readdir_ra = sbi->readdir_ra == 1;
+ bool find_valid_dentry = false;

found_valid_dirent?

int err = 0;
bit_pos = ((unsigned long)ctx->pos % d->max);
@@ -1014,13 +1015,15 @@ int f2fs_fill_dentries(struct dir_context *ctx, struct f2fs_dentry_ptr *d,
de = &d->dentry[bit_pos];
if (de->name_len == 0) {
+ if (!(ctx->pos % d->max != 0 && find_valid_dentry == false)) {

if (found_valid_dirent || !bit_pos)) will be more clean?


+ printk_ratelimited(
+ "%sF2FS-fs (%s): invalid namelen(0), ino:%u, run fsck to fix.",
+ KERN_WARNING, sbi->sb->s_id,
+ le32_to_cpu(de->ino));
+ set_sbi_flag(sbi, SBI_NEED_FSCK);
+ }
bit_pos++;
ctx->pos = start_pos + bit_pos;
- printk_ratelimited(
- "%sF2FS-fs (%s): invalid namelen(0), ino:%u, run fsck to fix.",
- KERN_WARNING, sbi->sb->s_id,
- le32_to_cpu(de->ino));
- set_sbi_flag(sbi, SBI_NEED_FSCK);
continue;
}
@@ -1063,6 +1066,7 @@ int f2fs_fill_dentries(struct dir_context *ctx, struct f2fs_dentry_ptr *d,
f2fs_ra_node_page(sbi, le32_to_cpu(de->ino));
ctx->pos = start_pos + bit_pos;
+ find_valid_dentry = true;

found_valid_dirent = true;

Thanks,

}
out:
if (readdir_ra)