linux-kernel - [PATCH v3] f2fs: Reduce the scope of setting fsck tag when de->name

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Date:   Tue, 20 Jul 2021 15:06:33 +0800
From:   Yangtao Li <frank.li@...o.com>
To:     jaegeuk@...nel.org, chao@...nel.org
Cc:     linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, Yangtao Li <frank.li@...o.com>
Subject: [PATCH v3] f2fs: Reduce the scope of setting fsck tag when de->name_len is zero

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___
	 ^
Thread B delete dir_entry2, and create dir_entry3.

After:
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@...o.com>
---
 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..bfe942733b5e 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 found_valid_dirent  = false;
 	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 (found_valid_dirent || !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);
+			}
 			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;
+		found_valid_dirent = true;
 	}
 out:
 	if (readdir_ra)
-- 
2.32.0