Re: [PATCH bpf-next v1 05/11] bpf: add a path ancestor iterator

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] bpf: deadlock in bpf_iter_path_ancestors_next due to GFP_KERNEL under VFS locks
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commit 0d24b2f0561971ff2a9d89456692d9ec1ea13279
Author: Justin Suess <utilityemal77@gmail.com>

bpf: add a path ancestor iterator

This commit introduces a sleepable BPF iterator for evaluating a path's
ancestry using the stepwise vfs_walk_ancestors() engine. It adds kfuncs
to manage the iterator and hands each position to the program as an
acquired reference by allocating a copy of the path structure.

> diff --git a/fs/bpf_fs_kfuncs.c b/fs/bpf_fs_kfuncs.c
> index 08f0847c49708..02398f5a46d5a 100644
> --- a/fs/bpf_fs_kfuncs.c
> +++ b/fs/bpf_fs_kfuncs.c
[ ... ]
> +__bpf_kfunc struct path *
> +bpf_iter_path_ancestors_next(struct bpf_iter_path_ancestors *it)
> +{
> +	struct bpf_path_ancestors_kern *kit = (void *)it;
> +	struct path *pos = bpf_path_ancestors_step(kit);
> +	struct path *held;
> +
> +	if (!pos)
> +		return NULL;
> +	/*
> +	 * The position must outlive the walk's own view of it, so it gets a
> +	 * reference and a struct path of its own to live in: struct path is
> +	 * a value type, with nothing a BPF reference could be taken on
> +	 * otherwise.  Sleepable, so no atomic allocation.
> +	 */
> +	held = kmalloc_obj(*held);

[Severity: High]
Can this allocation cause a deadlock if the iterator is used from a sleepable
LSM hook?

If a sleepable BPF LSM program is attached to a hook like
security_inode_setattr or security_inode_unlink, it is called by the VFS
with the inode's i_rwsem held. 

The kmalloc_obj() macro appears to default to GFP_KERNEL allocations, which
includes the __GFP_FS flag. Under memory pressure, this could trigger direct
memory reclaim.

If reclaim attempts to write back dirty pages or shrink the dcache/icache,
it might try to acquire the same i_rwsem, resulting in a deadlock.

> +	if (!held) {
> +		kit->step = -ENOMEM;
> +		return NULL;
> +	}
> +	*held = *pos;
> +	path_get(held);
> +	return held;
> +}

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Sashiko AI review · https://sashiko.dev/#/patchset/20261013164800.2906577-1-utilityemal77@gmail.com?part=5