linux-kernel - Re: [PATCH] fs: avoid NULL pointer dereference for nobh on write error

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Message-ID: <CA+55aFz19F6Lzsigv9CQzsF4gBDWQf5f8oBKHt4jT9zxCeTi-w@mail.gmail.com>
Date:   Fri, 10 Nov 2017 12:38:29 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH] fs: avoid NULL pointer dereference for nobh on write error

On Thu, Nov 9, 2017 at 1:49 PM,  <jglisse@...hat.com> wrote:
>
> (Result of code inspection only, i do not have a bug, nor know a bug
> that would be explain by this issue. Is there a kernel trace database
> one can query for that ?)

This is intentional.

See the comment above the code you added:

         * Be careful: the buffer linked list is a NULL terminated one, rather
         * than the circular one we're used to.

and then nobh_write_end() does:

        struct buffer_head *head = fsdata;
...
        while (head) {
                bh = head;
                head = head->b_this_page;
                free_buffer_head(bh);
        }

so it *depends* on the bh list being NULL-terminated.

So your patch is definitely wrong and breaks that nobh_write_end() case.

Which is not to say that there couldn't be a NULL pointer dereference
in some error path exactly because this code intentionally breaks the
normal rules.

But no, I'm definitely not applying this patch as-is, and not just before 4.14.

                       Linus