linux-kernel - Re: [PATCH v3] hfs: remove BUG() from hfs_release_folio()/hfs_test_inode()/hfs_write

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Message-ID: <650d29da-4f3a-4cfe-b633-ea3b1f27de96@I-love.SAKURA.ne.jp>
Date: Fri, 25 Jul 2025 13:16:10 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: Viacheslav Dubeyko <Slava.Dubeyko@....com>,
        "willy@...radead.org" <willy@...radead.org>
Cc: "glaubitz@...sik.fu-berlin.de" <glaubitz@...sik.fu-berlin.de>,
        "frank.li@...o.com" <frank.li@...o.com>,
        "slava@...eyko.com" <slava@...eyko.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3] hfs: remove BUG() from
 hfs_release_folio()/hfs_test_inode()/hfs_write_inode()

On 2025/07/25 8:20, Tetsuo Handa wrote:
> On 2025/07/25 7:05, Tetsuo Handa wrote:
>>>> But I can't be convinced that above change is sufficient, for if I do
>>>>
>>>> +	            static u8 serial;
>>>> +               if (inode->i_ino < HFS_FIRSTUSER_CNID && ((1U << inode->i_ino) & bad_cnid_list))
>>>> +                       inode->i_ino = (serial++) % 16;
>>>
>>> I don't see the point in flags introduction. It makes logic very complicated.
>>
>> The point of this change is to excecise inode->i_ino for all values between 0 and 15.
>> Some of values between 0 and 15 must be valid as inode->i_ino , doesn't these? Then,
> 
> Background: I assume that the value of rec->dir.DirID comes from the hfs filesystem image in the
> reproducer (i.e. memfd file associated with /dev/loop0 ). But since I don't know the offset to modify
> the value if I want the reproducer to pass rec->dir.DirID == 1...15 instead of rec->dir.DirID == 0,
> I am modifying inode->i_ino here when rec->dir.DirID == 0.
> 

I managed to find the offset of rec->dir.DirID in the filesystem image used by
the reproducer, and confirmed that any 0...15 values except 2..4 shall hit BUG()
in hfs_write_inode().

Also, a legitimate filesystem image seems to have rec->dir.DirID == 2.

That is, the only approach that can avoid hitting BUG() without removing BUG()
would be to verify that rec.type is HFS_CDR_DIR and rec.dir.DirID is HFS_ROOT_CNID.

--- a/fs/hfs/super.c
+++ b/fs/hfs/super.c
@@ -354,7 +354,7 @@ static int hfs_fill_super(struct super_block *sb, struct fs_context *fc)
                        goto bail_hfs_find;
                }
                hfs_bnode_read(fd.bnode, &rec, fd.entryoffset, fd.entrylength);
-               if (rec.type != HFS_CDR_DIR)
+               if (rec.type != HFS_CDR_DIR || rec.dir.DirID != cpu_to_be32(HFS_ROOT_CNID))
                        res = -EIO;
        }
        if (res)

Is this condition correct?

Discussion on what values should be filtered by hfs_read_inode() is
out of scope for this syzbot report.