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

Dan Williams
2017-07-31T16:27:42Z

On Mon, Jul 31, 2017 at 9:02 AM, Colin Walters <walters@verbum.org> wrote:
> On Sat, Jul 29, 2017, at 03:43 PM, Dan Williams wrote:
>> An inode with this flag set indicates that the file's block map cannot
>> be changed, no size change, deletion, hole-punch, range collapse, or
>> reflink.
>>
>> The implementation of toggling the flag and sealing the state of the
>> extent map is saved for a later patch. The functionality provided by
>> S_IOMAP_IMMUTABLE, once toggle support is added, will be a superset of
>> that provided by S_SWAPFILE, and it is targeted to replace it.
>>
>> For now, only xfs and the core vfs are updated to consider the new flag.
>
> Quite a while ago I started a request for O_OBJECT:
> http://www.spinics.net/lists/linux-fsdevel/msg75085.html
> A few months ago I was thinking about that more and realized
> it'd likely be more palatable to land as an inode flag, like
> you're doing here.
>
> Now, S_IOMAP_IMMUTABLE would be quite close to the semantics
> I want for ostree, except we also want to disallow
> changes to the inode uid, gid or mode. (Extended attributes are
> a whole other story; but I'd like to at least disallow changes to the
> security. namespace).
>
> The goal here is mostly about resilience to *accidental* changes;
> think an admin doing `cp /path/to/binary /usr/bin/bash` which
> does open(O_TRUNC), which would hence corrupt all hardlinks.
>
> S_IOMAP_IMMUTABLE would give a lot of great protection against
> those types of accidental changes - most of them are either going
> to be open(O_TRUNC) or O_APPEND. Since you're touching various
> write paths here, perhaps we can also add
> S_CONTENTS_IMMUTABLE or something at the same time?
>
> If this lands as is - I'm quite likely to change ostree to use it;
> any objections to that? As mentioned in the thread, there are several
> other cases of "content immutable" files in userspace, such as
> QEMU "qcow2", git objects. And really the most classic example is
> /etc/sudoers and the need for a special "visudo" program to really
> ensure that editors don't do in-place overwrites.
>
> But it'd be great if we can use this push to also land "content immutabilty"
> or however we decide to call it.

How is S_CONTENTS_IMMUTABLE different than S_IMMUTABLE?