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SubjectRe: __getblk infinite loop
On Thu, 4 Sep 2008 23:24:11 -0400 Bob Copeland <me@bobcopeland.com> wrote:

> Hi all,
>
> Eric Sesterhenn and I were puzzling over a lockup found by his fsfuzzer.
>
> sb_bread() calls __getblk, which says:
>
> /*
> * __getblk will locate (and, if necessary, create) the buffer_head
> * which corresponds to the passed block_device, block and size. The
> * returned buffer has its reference count incremented.
> *
> * __getblk() cannot fail - it just keeps trying. If you pass it an
> * illegal block number, __getblk() will happily return a buffer_head
> * which represents the non-existent block. Very weird.
> *
> * __getblk() will lock up the machine if grow_dev_page's try_to_free_buffers()
> * attempt is failing. FIXME, perhaps?
> */
>
> In fact the following will cause an infinite loop when mounting omfs
> loopback (on 32 bit x86 at least):
>
> diff --git a/fs/omfs/inode.c b/fs/omfs/inode.c
> index a95fe59..80eacc8 100644
> --- a/fs/omfs/inode.c
> +++ b/fs/omfs/inode.c
> @@ -413,6 +413,15 @@ static int omfs_fill_super(struct super_block *sb, void *data, int silent)
> sector_t start;
> int ret = -EINVAL;
>
> + if (1) {
> + sector_t foo = 0x1d4000004ULL;
> +
> + sb_set_blocksize(sb, 2048);
> + bh = sb_bread(sb, foo);
> + brelse(bh);
> + goto end;
> + }
> +
> save_mount_options(sb, (char *) data);
>
> sbi = kzalloc(sizeof(struct omfs_sb_info), GFP_KERNEL);
>
> What's supposed to happen here? I would have thought that sb_bread
> would realize foo was outside the block dev and bail out, but instead
> it just gets stuck. Do I need to bounds-check anything passed to
> sb_bread?

That loop does lock up on people occasionally - last time was in isofs,
because it had done an insane set_blocksize() earlier on.

Yes, it's always a case of garbage in, garbage out (or nothing out, as
the case may be).

No, it's not particularly programmer-friendly behaviour.



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