Re: [PATCH 1/3] fs, xfs: introduce S_IOMAP_IMMUTABLE

Darrick J. Wong
2017-07-31T18:21:55Z

On Mon, Jul 31, 2017 at 01:42:13PM -0400, Colin Walters wrote:
>
>
> On Mon, Jul 31, 2017, at 12:32 PM, Colin Walters wrote:
> > On Mon, Jul 31, 2017, at 12:29 PM, Dan Williams wrote:
> > >
> > > How is S_CONTENTS_IMMUTABLE different than S_IMMUTABLE?
> >
> > We still want the ability to make hardlinks.
>
> Also of course, symmetrically, to unlink. If we used S_IMMUTABLE for /etc/sudoers,
> it'd still be racy since one would have to transiently remove the flag in order
> to replace it with a new version.
>
> Related to this topic is the fact that S_IMMUTABLE is itself mutable; I
> think once S_IMMUTABLE_CONTENTS is set, it would not be able to made
> mutable again.
>
> Also I just remembered that since then memfd_create() and more notably
> fcntl(F_ADD_SEALS) landed - in fact it already has flags for what we want
> here AFAICS. Your S_IOMAP_IMMUTABLE is fcntl(F_ADD_SEALS, F_SEAL_SHRINK | F_SEAL_GROW)

I don't think F_SEAL_{SHRINK,GROW} prevents reflinking or CoW of file data,
which are two things that cannot happen under S_IOMAP_IMMUTABLE that
aren't size changes. From the implementation it looks like shrink and
grow are only supposed to disallow changes to i_size, not i_blocks (or
the file block map).

Then again, I suppose F_SEAL_* only work on shmem, so maybe it simply
isn't defined for any other filesystem...? e.g. it doesn't prohibit
reflink, but the only fs implementing seals doesn't support reflink.

<shrug>

Seals cannot be removed, which is too strict for the S_IOMAP_IMMUTABLE
user cases being presented.

> and mine just adds in F_SEAL_WRITE. I think there was some discussion
> of the seals for persistent files when memfd_create() landed, but I can't
> find it offhand.

--D

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