On Sat 27 Jan 2024 09:58:02 AM +08, Zhang Yi wrote;
<...>
> +static ext4_lblk_t ext4_ext_determine_insert_hole(struct inode *inode,
> + struct ext4_ext_path *path,
> + ext4_lblk_t lblk)
> +{
> + ext4_lblk_t hole_start, len;
> + struct extent_status es;
> +
> + hole_start = lblk;
> + len = ext4_ext_find_hole(inode, path, &hole_start);
> +again:
> + ext4_es_find_extent_range(inode, &ext4_es_is_delayed, hole_start,
> + hole_start + len - 1, &es);
> + if (!es.es_len)
> + goto insert_hole;
> +
> + /*
> + * There's a delalloc extent in the hole, handle it if the delalloc
> + * extent is in front of, behind and straddle the queried range.
> + */
> + if (lblk >= es.es_lblk + es.es_len) {
> + /*
> + * The delalloc extent is in front of the queried range,
> + * find again from the queried start block.
> + */
> + len -= lblk - hole_start;
> + hole_start = lblk;
> + goto again;
It's looks like it's easy to trigger an infinite loop here using fstest
generic/039. If I understand it correctly (which doesn't happen as often
as I'd like), this is due to an integer overflow in the 'if' condition,
and should be fixed with the patch below.
From 3117af2f8dacad37a2722850421f31075ae9e88d Mon Sep 17 00:00:00 2001
From: "Luis Henriques (SUSE)" <[email protected]>
Date: Thu, 9 May 2024 15:53:01 +0100
Subject: [PATCH] ext4: fix infinite loop caused by integer overflow
An integer overflow will happen if the extent_status len is set to
EXT_MAX_BLOCKS (0xffffffff). This may cause an infinite loop in function
ext4_ext_determine_insert_hole(), easily reproducible using fstest
generic/039.
Fixes: 6430dea07e85 ("ext4: correct the hole length returned by ext4_map_blocks()")
Signed-off-by: Luis Henriques (SUSE) <[email protected]>
---
fs/ext4/extents.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/fs/ext4/extents.c b/fs/ext4/extents.c
index e57054bdc5fd..193121b394f9 100644
--- a/fs/ext4/extents.c
+++ b/fs/ext4/extents.c
@@ -4064,7 +4064,7 @@ static ext4_lblk_t ext4_ext_determine_insert_hole(struct inode *inode,
* There's a delalloc extent in the hole, handle it if the delalloc
* extent is in front of, behind and straddle the queried range.
*/
- if (lblk >= es.es_lblk + es.es_len) {
+ if (lblk >= ((__u64) es.es_lblk) + es.es_len) {
/*
* The delalloc extent is in front of the queried range,
* find again from the queried start block.
On Thu, May 09, 2024 at 04:16:34PM +0100, Luis Henriques wrote:
>
> It's looks like it's easy to trigger an infinite loop here using fstest
> generic/039. If I understand it correctly (which doesn't happen as often
> as I'd like), this is due to an integer overflow in the 'if' condition,
> and should be fixed with the patch below.
Thanks for the report. However, I can't reproduce the failure, and
looking at generic/039, I don't see how it could be relevant to the
code path in question. Generic/039 creates a test symlink with two
hard links in the same directory, syncs the file system, and then
removes one of the hard links, and then drops access to the block
device using dmflakey. So I don't see how the extent code would be
involved at all. Are you sure that you have the correct test listed?
Looking at the code in question in fs/ext4/extents.c:
again:
ext4_es_find_extent_range(inode, &ext4_es_is_delayed, hole_start,
hole_start + len - 1, &es);
if (!es.es_len)
goto insert_hole;
* There's a delalloc extent in the hole, handle it if the delalloc
* extent is in front of, behind and straddle the queried range.
*/
- if (lblk >= es.es_lblk + es.es_len) {
+ if (lblk >= ((__u64) es.es_lblk) + es.es_len) {
/*
* The delalloc extent is in front of the queried range,
* find again from the queried start block.
len -= lblk - hole_start;
hole_start = lblk;
goto again;
lblk and es.es_lblk are both __u32. So the infinite loop is
presumably because es.es_lblk + es.es_len has overflowed. This should
never happen(tm), and in fact we have a test for this case which
*should* have gotten tripped when ext4_es_find_extent_range() calls
__es_tree_search() in fs/ext4/extents_status.c:
static inline ext4_lblk_t ext4_es_end(struct extent_status *es)
{
BUG_ON(es->es_lblk + es->es_len < es->es_lblk);
return es->es_lblk + es->es_len - 1;
}
So the patch is harmless, and I can see how it might fix what you were
seeing --- but I'm a bit nervous that I can't reproduce it and the
commit description claims that it reproduces easily; and we should
have never allowed the entry to have gotten introduced into the
extents status tree in the first place, and if it had been introduced,
it should have been caught before it was returned by
ext4_es_find_extent_range().
Can you give more details about the reproducer; can you double check
the test id, and how easily you can trigger the failure, and what is
the hardware you used to run the test?
Many thanks,
- Ted
On Thu 09 May 2024 12:39:53 PM -04, Theodore Ts'o wrote;
> On Thu, May 09, 2024 at 04:16:34PM +0100, Luis Henriques wrote:
>>
>> It's looks like it's easy to trigger an infinite loop here using fstest
>> generic/039. If I understand it correctly (which doesn't happen as often
>> as I'd like), this is due to an integer overflow in the 'if' condition,
>> and should be fixed with the patch below.
>
> Thanks for the report. However, I can't reproduce the failure, and
> looking at generic/039, I don't see how it could be relevant to the
> code path in question. Generic/039 creates a test symlink with two
> hard links in the same directory, syncs the file system, and then
> removes one of the hard links, and then drops access to the block
> device using dmflakey. So I don't see how the extent code would be
> involved at all. Are you sure that you have the correct test listed?
Yep, I just retested and it's definitely generic/039. I'm using a simple
test environment, with virtme-ng.
> Looking at the code in question in fs/ext4/extents.c:
>
> again:
> ext4_es_find_extent_range(inode, &ext4_es_is_delayed, hole_start,
> hole_start + len - 1, &es);
> if (!es.es_len)
> goto insert_hole;
>
> * There's a delalloc extent in the hole, handle it if the delalloc
> * extent is in front of, behind and straddle the queried range.
> */
> - if (lblk >= es.es_lblk + es.es_len) {
> + if (lblk >= ((__u64) es.es_lblk) + es.es_len) {
> /*
> * The delalloc extent is in front of the queried range,
> * find again from the queried start block.
> len -= lblk - hole_start;
> hole_start = lblk;
> goto again;
>
> lblk and es.es_lblk are both __u32. So the infinite loop is
> presumably because es.es_lblk + es.es_len has overflowed. This should
> never happen(tm), and in fact we have a test for this case which
If I instrument the code, I can see that es.es_len is definitely set to
EXT_MAX_BLOCKS, which will overflow.
> *should* have gotten tripped when ext4_es_find_extent_range() calls
> __es_tree_search() in fs/ext4/extents_status.c:
>
> static inline ext4_lblk_t ext4_es_end(struct extent_status *es)
> {
> BUG_ON(es->es_lblk + es->es_len < es->es_lblk);
> return es->es_lblk + es->es_len - 1;
> }
>
> So the patch is harmless, and I can see how it might fix what you were
> seeing --- but I'm a bit nervous that I can't reproduce it and the
> commit description claims that it reproduces easily; and we should
> have never allowed the entry to have gotten introduced into the
> extents status tree in the first place, and if it had been introduced,
> it should have been caught before it was returned by
> ext4_es_find_extent_range().
>
> Can you give more details about the reproducer; can you double check
> the test id, and how easily you can trigger the failure, and what is
> the hardware you used to run the test?
So, here's few more details that may clarify, and that I should have added
to the commit description:
When the test hangs, the test is blocked mounting the flakey device:
mount -t ext4 -o acl,user_xattr /dev/mapper/flakey-test /mnt/scratch
which will eventually call into ext4_ext_map_blocks(), triggering the bug.
Also, some more code instrumentation shows that after the call to
ext4_ext_find_hole(), the 'hole_start' will be set to '1' and 'len' to
'0xfffffffe'. This '0xfffffffe' value is a bit odd, but it comes from the
fact that, in ext4_ext_find_hole(), the call to
ext4_ext_next_allocated_block() will return EXT_MAX_BLOCKS and 'len' will
thus be set to 'EXT_MAX_BLOCKS - 1'.
Does this make sense?
Cheers,
--
Luis