Re: [PATCH v7 5/7] virtiofsd: Add capability to change/restore umask

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Vivek Goyal (vgoyal@redhat.com) wrote:
> On Mon, Jun 28, 2021 at 05:12:13PM +0100, Dr. David Alan Gilbert wrote:
> > * Vivek Goyal (vgoyal@redhat.com) wrote:
> > > When parent directory has default acl and a file is created in that
> > > directory, then umask is ignored and final file permissions are
> > > determined using default acl instead. (man 2 umask).
> > > 
> > > Currently, fuse applies the umask and sends modified mode in create
> > > request accordingly. fuse server can set FUSE_DONT_MASK and tell
> > > fuse client to not apply umask and fuse server will take care of
> > > it as needed.
> > > 
> > > With posix acls enabled, requirement will be that we want umask
> > > to determine final file mode if parent directory does not have
> > > default acl.
> > > 
> > > So if posix acls are enabled, opt in for FUSE_DONT_MASK. virtiofsd
> > > will set umask of the thread doing file creation. And host kernel
> > > should use that umask if parent directory does not have default
> > > acls, otherwise umask does not take affect.
> > > 
> > > Miklos mentioned that we already call unshare(CLONE_FS) for
> > > every thread. That means umask has now become property of per
> > > thread and it should be ok to manipulate it in file creation path.
> > > 
> > > This patch only adds capability to change umask and restore it. It
> > > does not enable it yet. Next few patches will add capability to enable it
> > > based on if user enabled posix_acl or not.
> > > 
> > > This should fix fstest generic/099.
> > > 
> > > Reported-by: Luis Henriques <lhenriques@suse.de>
> > > Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
> > > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > ---
> > >  tools/virtiofsd/passthrough_ll.c | 22 ++++++++++++++++------
> > >  1 file changed, 16 insertions(+), 6 deletions(-)
> > > 
> > > diff --git a/tools/virtiofsd/passthrough_ll.c b/tools/virtiofsd/passthrough_ll.c
> > > index 9f5cd98fb5..0c9084ea15 100644
> > > --- a/tools/virtiofsd/passthrough_ll.c
> > > +++ b/tools/virtiofsd/passthrough_ll.c
> > > @@ -122,6 +122,7 @@ struct lo_inode {
> > >  struct lo_cred {
> > >      uid_t euid;
> > >      gid_t egid;
> > > +    mode_t umask;
> > >  };
> > >  
> > >  enum {
> > > @@ -172,6 +173,8 @@ struct lo_data {
> > >      /* An O_PATH file descriptor to /proc/self/fd/ */
> > >      int proc_self_fd;
> > >      int user_killpriv_v2, killpriv_v2;
> > > +    /* If set, virtiofsd is responsible for setting umask during creation */
> > > +    bool change_umask;
> > >  };
> > >  
> > >  static const struct fuse_opt lo_opts[] = {
> > > @@ -1134,7 +1137,8 @@ static void lo_lookup(fuse_req_t req, fuse_ino_t parent, const char *name)
> > >   * ownership of caller.
> > >   * TODO: What about selinux context?
> > >   */
> > > -static int lo_change_cred(fuse_req_t req, struct lo_cred *old)
> > > +static int lo_change_cred(fuse_req_t req, struct lo_cred *old,
> > > +                          bool change_umask)
> > >  {
> > >      int res;
> > >  
> > > @@ -1154,11 +1158,14 @@ static int lo_change_cred(fuse_req_t req, struct lo_cred *old)
> > >          return errno_save;
> > >      }
> > >  
> > > +    if (change_umask) {
> > > +        old->umask = umask(req->ctx.umask);
> > > +    }
> > >      return 0;
> > >  }
> > >  
> > >  /* Regain Privileges */
> > > -static void lo_restore_cred(struct lo_cred *old)
> > > +static void lo_restore_cred(struct lo_cred *old, bool restore_umask)
> > >  {
> > >      int res;
> > >  
> > > @@ -1173,6 +1180,9 @@ static void lo_restore_cred(struct lo_cred *old)
> > >          fuse_log(FUSE_LOG_ERR, "setegid(%u): %m\n", old->egid);
> > >          exit(1);
> > >      }
> > > +
> > > +    if (restore_umask)
> > > +        umask(old->umask);
> > >  }
> > >  
> > >  static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t parent,
> > > @@ -1202,7 +1212,7 @@ static void lo_mknod_symlink(fuse_req_t req, fuse_ino_t parent,
> > >          return;
> > >      }
> > >  
> > > -    saverr = lo_change_cred(req, &old);
> > > +    saverr = lo_change_cred(req, &old, lo->change_umask && !S_ISLNK(mode));
> > 
> > Can you explain what these ISLNK checks are for (insid mknod_symlink, so
> > is that always true or irrelevant?)
> 
> I think I put this check in because if we are creating symlink then we
> don't have to change umask as symlink will always get a some fix
> mode (usually 777) and umask will not have an affect. So this is
> just an optimization to avoid switching umask in some cases. I 
> can't think of any other reason.

But this is in 'lo_mknod_symlink' - so when do we call that except for
making symlinks?

Dave

> thanks
> Vivek
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK