vfswrap_getxattrat_do_async and unshare(CLONE_FS)

[Jeremy Allison]

On Mon, Oct 24, 2022 at 11:46:22PM +0200, Florian Weimer via samba-technical wrote:
>As far as I can tell, vfswrap_getxattrat_do_async relies on
>unshare(CLONE_FS) to do fgetxattrat emulation with a thread-local fchdir
>and getxattr.  There do not seem to be any other uses in the sources.
>
>I think the more usual way to emulate the missing fgetxattrat system
>call is to open the file with openat and O_PATH, and then use getxattr
>on the synthetic path under /proc/self/fd.  While these paths present as
>symbolic links, they actually are not, so there is no race possible.
>(fgetxattr cannot operate on the open file descriptor directly.)
>
>Why wasn't the /proc-based approach chosen for Samba?  It looks a bit
>simpler to implement, and does not do strange things to the process
>state behind glibc's back.

As I recall it was done that way as there are various OEMs with fuse-based
filesystems that are very slow on fetching EA's, which we have to do
on every directory entry returned. Doing this inside a pthread using
unshare(CLONE_FS) to keep a per-thread directory was found to allow
these filesystems to have a decent performance.

This was before the VFS-rewrite to use fd's everywhere as I recall,
so it's possible it may no longer be required. But I'd wait for
Metze to chime in here as he was deeply involved in this I think.