lkml.org 
[lkml]   [2022]   [Mar]   [17]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [RESEND PATCH v7 7/8] kernfs: Replace per-fs rwsem with hashed rwsems.
On Thu, Mar 17, 2022 at 06:26:11PM +1100, Imran Khan wrote:

> diff --git a/fs/kernfs/symlink.c b/fs/kernfs/symlink.c
> index 9d4103602554..cbdd1be5f0a8 100644
> --- a/fs/kernfs/symlink.c
> +++ b/fs/kernfs/symlink.c
> @@ -113,12 +113,19 @@ static int kernfs_getlink(struct inode *inode, char *path)
> struct kernfs_node *kn = inode->i_private;
> struct kernfs_node *parent = kn->parent;
> struct kernfs_node *target = kn->symlink.target_kn;
> - struct rw_semaphore *rwsem;
> + struct kernfs_rwsem_token token;
> int error;
>
> - rwsem = kernfs_down_read(parent);
> + /**
> + * Lock both parent and target, to avoid their movement
> + * or removal in the middle of path construction.
> + * If a competing remove or rename for parent or target
> + * wins, it will be reflected in result returned from
> + * kernfs_get_target_path.
> + */
> + kernfs_down_read_double_nodes(target, parent, &token);
> error = kernfs_get_target_path(parent, target, path);
> - kernfs_up_read(rwsem);
> + kernfs_up_read_double_nodes(target, parent, &token);
>
> return error;
> }

No. Read through the kernfs_get_target_path(). Why would locking these
two specific nodes be sufficient for anything useful? That code relies
upon ->parent of *many* nodes being stable. Which is not going to be
guaranteed by anything of that sort.

And it's not just "we might get garbage if we race" - it's "we might
walk into kfree'd object and proceed to walk the pointer chain".

Or have this loop
kn = target;
while (kn->parent && kn != base) {
len += strlen(kn->name) + 1;
kn = kn->parent;
}
see the names that are not identical to what we see in
kn = target;
while (kn->parent && kn != base) {
int slen = strlen(kn->name);

len -= slen;
memcpy(s + len, kn->name, slen);
if (len)
s[--len] = '/';

kn = kn->parent;
}
done later in the same function. With obvious unpleasant effects.
Or a different set of nodes, for that matter.

This code really depends upon the tree being stable. No renames of
any sort allowed during that thing.

\
 
 \ /
  Last update: 2022-03-18 01:08    [W:0.140 / U:0.872 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site