linux-kernel - Re: [PATCH] fuse:rely on fuse_perm for exec when no mode bits set

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Date:   Mon, 11 May 2020 14:25:53 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Chakra Divi <chakragithub@...il.com>
Cc:     linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] fuse:rely on fuse_perm for exec when no mode bits set

On Fri, May 8, 2020 at 12:14 PM Chakra Divi <chakragithub@...il.com> wrote:
>
> On Tue, Apr 28, 2020 at 1:51 PM Miklos Szeredi <miklos@...redi.hu> wrote:
> >
> > On Mon, Apr 27, 2020 at 3:46 PM Chakra Divi <chakragithub@...il.com> wrote:
> > >
> > > On Tue, Apr 21, 2020 at 4:21 PM Chakra Divi <chakragithub@...il.com> wrote:
> > > >
> > > > On Mon, Apr 20, 2020 at 4:55 PM Miklos Szeredi <miklos@...redi.hu> wrote:
> > > > >
> > > > > On Wed, Apr 1, 2020 at 11:31 AM Chakra Divi <chakragithub@...il.com> wrote:
> > > > > >
> > > > > > In current code, for exec we are checking mode bits
> > > > > > for x bit set even though the fuse_perm_getattr returns
> > > > > > success. Changes in this patch avoids mode bit explicit
> > > > > > check, leaves the exec checking to fuse file system
> > > > > > in uspace.
> > > > >
> > > > > Why is this needed?
> > > >
> > > > Thanks for responding Miklos. We have an use case with our remote file
> > > > system mounted on fuse , where permissions checks will happen remotely
> > > > without the need of mode bits. In case of read, write it worked
> > > > without issues. But for executable files, we found that fuse kernel is
> > > > explicitly checking 'x' mode bit set on the file. We want this
> > > > checking also to be pushed to remote instead of kernel doing it - so
> > > > modified the kernel code to send getattr op to usespace in exec case
> > > > too.
> > >
> > > Any help on this Miklos....
> >
> > I still don't understand what you are requesting.  What your patch
> > does is unconditionally allow execution, even without any 'x' bits in
> > the mode.  What does that achieve?
>
> Thanks for the help Miklos. We have a network based filesystem that
> supports acls.
> As our filesystem give granular access, we wipe out the mode bits and
> completely rely on ACLs.

Are you using POSIX ACLs?   Why can't you translate the ACL's back
into mode bits (that's what all filesystems do)?

>
> Fuse works well for all other ops (with default_permissions disabled )
>  as all the checks are done at the filesystems.
> But only executables have problems because fuse kernel rejects the
> execution by doing access checks on mode bit.
> To push this check to filesystem, in the above patch - i'm relying on
> return value from fuse_perm_getattr() ignoring the mode bits.
>
> When the fuse module is asked to rely on filesystem for access checks,
> why do we need this explicit check for executables?

Because there's no other check.  Have you noticed that with your patch
*all* files become executable?  I guess that's not what you wanted...

Thanks,
Miklos