Re: [PATCH 1/1] fsopen: fsconfig syscall restart fix

[Al Viro <viro@zeniv.linux.org.uk>]

On Wed, Sep 23, 2020 at 07:46:36PM +0300, Alexander Mikhalitsyn wrote:
> During execution of vfs_fsconfig_locked function we can get ERESTARTNOINTR
> error (or other interrupt error). But we changing fs context fc->phase
> field to transient states and our entry fc->phase checks in switch cases
> (see FS_CONTEXT_CREATE_PARAMS, FS_CONTEXT_RECONF_PARAMS) will always fail
> after syscall restart which will lead to returning -EBUSY to the userspace.
> 
> The idea of the fix is to save entry-time fs_context phase field value and
> recover fc->phase value to the original one before exiting with
> "interrupt error" (ERESTARTNOINTR or similar).

If you have e.g. vfs_create_tree() fail in the middle of ->get_tree(),
the only thing you can do to that thing is to discard it.  The state is
*NOT* required to be recoverable after a failure exit - quite a bit of
config might've been consumed and freed by that point.

CREATE and RECONFIGURE are simply not restartable.