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

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Message-ID: <d4abeee2-e291-4da4-9e0e-7880a9c213e3@I-love.SAKURA.ne.jp>
Date: Fri, 25 Jul 2025 08:20:28 +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 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.

> 
>>
>>>
>>> instead of
>>>
>>> +               if (inode->i_ino < HFS_FIRSTUSER_CNID && ((1U << inode->i_ino) & bad_cnid_list))
>>> +                       make_bad_inode(inode);
>>>
>>> , the reproducer still hits BUG() for 0, 1, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 and 15
>>> because hfs_write_inode() handles only 2, 3 and 4.
>>>
>>
>> How can we go into hfs_write_inode() if we created the bad inode for invalid
>> inode ID? How is it possible?

Calling make_bad_inode() for some of values between 0...15 at hfs_read_inode() will prevent
that inode from going into hfs_write_inode(). But regarding the values between 0...15 which
were not calling make_bad_inode() at hfs_read_inode() will not prevent that inode from going
into hfs_write_inode().

Since hfs_write_inode() calls BUG() for values 0...15 except 2...4, any values between 0...15
except 2...4 which were not calling make_bad_inode() at hfs_read_inode() will hit BUG().

If we don't remove BUG(), the values which hfs_read_inode() does not need to call
make_bad_inode() will be limited to only 2...4.

And since you say that hfs_read_inode() should call make_bad_inode() for 3...4, the only value
hfs_read_inode() can accept (from the point of view of avoid hitting BUG() in hfs_write_inode())
will be 2.

> 
> are all of 0, 1, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 and 15 invalid value for hfs_read_inode() ?
> 
> If all of 0, 1, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14 and 15 are invalid value for hfs_read_inode(),
> and 3 and 4 are also invalid value for hfs_read_inode(), hfs_read_inode() would accept only 2.
> Something is crazily wrong.
> 
> Can we really filter some of values between 0 and 15 at hfs_read_inode() ?
> 

Can the attempt to filter some of values between 0 and 15 at hfs_read_inode() make sense,
without the attempt to remove BUG() from hfs_write_inode() ?

I think that we need to remove BUG() from hfs_write_inode(), even if you try to filter
at hfs_read_inode().