Re: [PATCH 1/5] fuse: check attributes staleness on fuse_iget()

[Miklos Szeredi]

On Tue, 11 Jul 2023 at 06:36, Jiachen Zhang
<zhangjiachen.jaycee@xxxxxxxxxxxxx> wrote:
>
> Function fuse_direntplus_link() might call fuse_iget() to initialize a new
> fuse_inode and change its attributes. If fi->attr_version is always
> initialized with 0, even if the attributes returned by the FUSE_READDIR
> request is staled, as the new fi->attr_version is 0, fuse_change_attributes
> will still set the staled attributes to inode. This wrong behaviour may
> cause file size inconsistency even when there is no changes from
> server-side.
>
> To reproduce the issue, consider the following 2 programs (A and B) are
> running concurrently,
>
>         A                                               B
> ----------------------------------      --------------------------------
> { /fusemnt/dir/f is a file path in a fuse mount, the size of f is 0. }
>
> readdir(/fusemnt/dir) start
> //Daemon set size 0 to f direntry
>                                         fallocate(f, 1024)
>                                         stat(f) // B see size 1024
>                                         echo 2 > /proc/sys/vm/drop_caches
> readdir(/fusemnt/dir) reply to kernel
> Kernel set 0 to the I_NEW inode
>
>                                         stat(f) // B see size 0
>
> In the above case, only program B is modifying the file size, however, B
> observes file size changing between the 2 'readonly' stat() calls. To fix
> this issue, we should make sure readdirplus still follows the rule of
> attr_version staleness checking even if the fi->attr_version is lost due to
> inode eviction. So this patch increases fc->attr_version on inode eviction,
> and compares request attr_version and the fc->attr_version when a
> FUSE_READDIRPLUS request is finished.

Thanks for the report.  It's really interesting that you are the first
to notice this misbehavior, even though it's been there since the
beginning.

The fix looks correct but suboptimal: fc->attr_version will get
incremented due to non-evict events as well, which could lead to false
positives.   I'd add another counter (e.g. fc->evict_ctr) and manage
that separately for lookup type operations (LOOKUP/READDIRPLUS).

Thanks,
Miklos