Re: [PATCH] fuse: callback for invalidating cached children dentries for a directory

[Miklos Szeredi <miklos@szeredi.hu>]

On Tue, May 31, 2016 at 11:04 AM, Ashish Sangwan
<ashishsangwan2@gmail.com> wrote:
> This patch solves a long standing bug.
> "([bug #15](https://github.com/libfuse/libfuse/issues/15)): if the
> `default_permissions` mount option is not used, the results of the
> first permission check performed by the file system for a directory
> entry will be re-used for subsequent accesses as long as the inode of
> the accessed entry is present in the kernel cache - even if the
> permissions have since changed, and even if the subsequent access is
> made by a different user.
> This bug needs to be fixed in the Linux kernel and has been known
> since 2006 but unfortunately no fix has been applied yet. If you
> depend on correct permission handling for FUSE file systems, the only
> workaround is to use `default_permissions` (which does not currently
> support ACLs), or to completely disable caching of directory entry
> attributes. Alternatively, the severity of the bug can be somewhat
> reduced by not using the `allow_other` mount option."
>
> This patch introduce a callback which the user space implementation can use
> to invalidate the cached entries of a parent directory, for example when
> the execute permissions are revoked and force real lookup.

This doesn't solve the real problem.

As I just mentioned, the bigger problem is that when doing cached
lookups we ignore the credentials of the current process.  Without
that (and without default_permission) we just can't make a good
decision whether to allow the cached lookup or not.

Thanks,
Miklos