bug in handling bad inodes in 2.4 series

bug in handling bad inodes in 2.4 series

[Daniel Mack]

hi all,

i figured out a bug in at least the 2.4.16, .17 and .18-pre9 kernel releases
regarding the handling of bad inodes which causes the names_cache to get 
confused. i informed al viro about this but found it better to also post
my perception to the lkml for getting more response and feedback.
you can easily reproduce the effect by making an inode bad using debugfs
(or using a bad one if you have, of course) and open() it with the flag 
O_CREAT set. watching /proc/slabinfo will show you the weirdness. 
consider /tmp/bad is a bad inode:

# cd /tmp
# ls | grep bad
bad
# cat bad
cat: bad: Input/output error
# cat /proc/slabinfo | grep names
names_cache            0      2   4096    0    2    1 :      2     333     2    0    0
# echo foo >bad
sh: bad: Input/output error
# cat /proc/slabinfo | grep names
names_cache       4294967295      2   4096    1    2    1

boom! 4294967295 == (unsigned long) -1, the cache length is growing backwards.
this "echo foo >bad" opens the file with O_CREAT set which forces the effect.

what confuses me is that the system remains stable after that happened, the
only effect i got was that i wasn't able to rename() a file on any filesystem 
anymore. what the rename() syscall does on kernel side is getting memory from 
the names_cache twice by calling __getname() which gives out the same pointer
twice when the kernel is poisoned this way. 
anyway, it's an ugly bug that needs to be fixed.

the bug is most likely in fs/namei.c, open_namei() - at least i fixed my
machine here with this:

--- linux-2.4.17-orig/fs/namei.c        Wed Oct 17 23:46:29 2001
+++ linux-2.4.17-uml/fs/namei.c Fri Feb  8 02:53:36 2002
@@ -1052,6 +1052,11 @@
        error = -ENOENT;
        if (!dentry->d_inode)
                goto exit_dput;
+
+       error = -EIO;
+       if (is_bad_inode(dentry->d_inode))
+               goto exit_dput;
+
        if (dentry->d_inode->i_op && dentry->d_inode->i_op->follow_link)
                goto do_link;

an open() does not make any sense on a bad inode so i see no reason for
not breaking the branch at this point.

any comments? please let me know if you're able to trigger this effect
on older 2.4 kernels. btw, version 2.2.19 is not tainted.


greets,
daniel
--