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SubjectRe: [PATCH] fs-writeback: fix NULL pointer dereference in __mark_inode_dirty
On Wed, 02 Mar 2011 09:35:55 +0100
"Andreas Bie__mann" <andreas.devel@googlemail.com> wrote:

> Dear Jason A. Donenfeld,
>
> Am 01.03.2011 10:00, schrieb Jason A. Donenfeld:
> > Can you make an isolated test case to trigger this bug?
>
> in my case it is easily reproduceable. I have an SD-card in our embedded
> device (AVR32 AP7000). Some random data is continuously written to an
> FAT filesystem on that device. When you pull the card out of the slot
> you trigger that NULL pointer dereference.
>
> I will try to reproduce that error on my workstation but this will need
> some time. Maybe I can not hit that race on my quad core workstation but
> I will give it a try.
>

afaik this regression didn't get fixed. Jens put out a patch for
George to test but there hasn't been any feedback on that yet. Could
you guys please give it a spin?

From: Jens Axboe <axboe@kernel.dk>

When we move the potential dirty list entries to the
default_backing_dev_info, reassign the sb->s_bdi as well.
default_backing_dev_info will always be around. I hope this can fix it up
for 2.6.38 and we can add the proper ref counting for .39.

Cc: Anton Altaparmakov <aia21@cam.ac.uk>
Cc: George Spelvin <linux@horizon.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andreas Biemann <biessmann@corscience.de>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Tested-by: Torsten Hilbrich <torsten.hilbrich@secunet.com>
Cc: <stable@kernel.org> [2.6.38.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
fs/super.c | 2 ++
fs/sync.c | 4 ++--
mm/backing-dev.c | 2 +-
3 files changed, 5 insertions(+), 3 deletions(-)
diff -puN fs/super.c~vfs-fix-null-pointer-oops-in-sync_inodes_sb fs/super.c
--- a/fs/super.c~vfs-fix-null-pointer-oops-in-sync_inodes_sb
+++ a/fs/super.c
@@ -72,6 +72,7 @@ static struct super_block *alloc_super(s
#else
INIT_LIST_HEAD(&s->s_files);
#endif
+ s->s_bdi = &default_backing_dev_info;
INIT_LIST_HEAD(&s->s_instances);
INIT_HLIST_BL_HEAD(&s->s_anon);
INIT_LIST_HEAD(&s->s_inodes);
@@ -1006,6 +1007,7 @@ vfs_kern_mount(struct file_system_type *
}
BUG_ON(!mnt->mnt_sb);
WARN_ON(!mnt->mnt_sb->s_bdi);
+ WARN_ON(mnt->mnt_sb->s_bdi == &default_backing_dev_info);
mnt->mnt_sb->s_flags |= MS_BORN;

error = security_sb_kern_mount(mnt->mnt_sb, flags, secdata);
diff -puN fs/sync.c~vfs-fix-null-pointer-oops-in-sync_inodes_sb fs/sync.c
--- a/fs/sync.c~vfs-fix-null-pointer-oops-in-sync_inodes_sb
+++ a/fs/sync.c
@@ -33,7 +33,7 @@ static int __sync_filesystem(struct supe
* This should be safe, as we require bdi backing to actually
* write out data in the first place
*/
- if (!sb->s_bdi || sb->s_bdi == &noop_backing_dev_info)
+ if (sb->s_bdi == &noop_backing_dev_info)
return 0;

if (sb->s_qcop && sb->s_qcop->quota_sync)
@@ -79,7 +79,7 @@ EXPORT_SYMBOL_GPL(sync_filesystem);

static void sync_one_sb(struct super_block *sb, void *arg)
{
- if (!(sb->s_flags & MS_RDONLY) && sb->s_bdi)
+ if (!(sb->s_flags & MS_RDONLY))
__sync_filesystem(sb, *(int *)arg);
}
/*
diff -puN mm/backing-dev.c~vfs-fix-null-pointer-oops-in-sync_inodes_sb mm/backing-dev.c
--- a/mm/backing-dev.c~vfs-fix-null-pointer-oops-in-sync_inodes_sb
+++ a/mm/backing-dev.c
@@ -598,7 +598,7 @@ static void bdi_prune_sb(struct backing_
spin_lock(&sb_lock);
list_for_each_entry(sb, &super_blocks, s_list) {
if (sb->s_bdi == bdi)
- sb->s_bdi = NULL;
+ sb->s_bdi = &default_backing_dev_info;
}
spin_unlock(&sb_lock);
}
_

btw, Christoph: would this not have been be a less hacky hack?

--- a/fs/fs-writeback.c~a
+++ a/fs/fs-writeback.c
@@ -73,7 +73,7 @@ static inline struct backing_dev_info *i
{
struct super_block *sb = inode->i_sb;

- if (strcmp(sb->s_type->name, "bdev") == 0)
+ if (sb == blockdev_superblock)
return inode->i_mapping->backing_dev_info;

return sb->s_bdi;
_


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