[v4,0/3] fanotify accounting for fs/splice.c

  Always generate modify out, access in for splice;
this gets automatically merged with no ugly special cases.

No changes to 2/3 or 3/3.

Ahelenia Ziemiańska (3):
  splice: always fsnotify_access(in), fsnotify_modify(out) on success
  splice: fsnotify_access(fd)/fsnotify_modify(fd) in vmsplice
  splice: fsnotify_access(in), fsnotify_modify(out) on success in tee

 fs/splice.c | 38 ++++++++++++++++++++------------------
 1 file changed, 20 insertions(+), 18 deletions(-)

Interdiff against v3:
  

On Tue, Jun 27, 2023 at 11:50 PM Ahelenia Ziemiańska
<nabijaczleweli@nabijaczleweli.xyz> wrote:
>
> Always generate modify out, access in for splice;
> this gets automatically merged with no ugly special cases.

Ahelenia,

Obviously, you are new to sending patches to the kernel
and you appear to be a very enthusiastic and fast learner,
so I assume you won't mind getting some tips that you won't
find in any document.

a) CC LTP only on tests, not on the kernel patches

b) Please don't post these "diff" patches.
Developers (and bots) should be able to understand which
upstream commit a patch is based on (git format-patch provides that info).

I mean you can send those diff patches as part of a conversation to
explain yourself, that's fine, just don't post them as if they are patches
for review.

c) When there are prospect reviewers that have not reviewed v1
(especially inotify maintainer), it is better to wait at least one day posting
v2 and v3 and v4 ;), because:
1. It is better to accumulate review comments from several reviewers
2. Different reviewers may disagree, so if you are just following my
    advice you may need to go back and forth until everyone is happy
3. It's racy - reviewers may be in the middle of review of v1 without
    realizing that v2,v3,v4 is already in their inbox, so that's creating
    extra work for them - not a good outcome

Going to review v4....

Thanks,
Amir.
  

Hello!

On Tue 27-06-23 22:50:46, Ahelenia Ziemiańska wrote:
> Always generate modify out, access in for splice;
> this gets automatically merged with no ugly special cases.
> 
> No changes to 2/3 or 3/3.

Thanks for the patches Ahelena! The code looks fine to me but to be honest
I still have one unresolved question so let me think about it loud here for
documentation purposes :). Do we want fsnotify (any filesystem
notification framework like inotify or fanotify) to actually generate
events on FIFOs? FIFOs are virtual objects and are not part of the
filesystem as such (well, the inode itself and the name is), hence
*filesystem* notification framework does not seem like a great fit to watch
for changes or accesses there. And if we say "yes" for FIFOs, then why not
AF_UNIX sockets? Where do we draw the line? And is it all worth the
trouble?

I understand the convenience of inotify working on FIFOs for the "tail -f"
usecase but then wouldn't this better be fixed in tail(1) itself by using
epoll(7) for FIFOs which, as I've noted in my other reply, does not have
the problem that poll(2) has when there are no writers?

Another issue with FIFOs is that they do not have a concept of file
position. For hierarchical storage usecase we are introducing events that
will report file ranges being modified / accessed and officially supporting
FIFOs is one more special case to deal with.

What is supporting your changes is that fsnotify mostly works for FIFOs
already now (normal reads & writes generate notification) so splice not
working could be viewed as an inconsistency. Sockets (although they are
visible in the filesystem) cannot be open so for them the illusion of being
a file is even weaker.

So overall I guess I'm slightly in favor of making fsnotify generate events
on FIFOs even with splice, provided Amir does not see a big trouble in
supporting this with his upcoming HSM changes.

								Honza

> Ahelenia Ziemiańska (3):
>   splice: always fsnotify_access(in), fsnotify_modify(out) on success
>   splice: fsnotify_access(fd)/fsnotify_modify(fd) in vmsplice
>   splice: fsnotify_access(in), fsnotify_modify(out) on success in tee
> 
>  fs/splice.c | 38 ++++++++++++++++++++------------------
>  1 file changed, 20 insertions(+), 18 deletions(-)
> 
> Interdiff against v3:
> diff --git a/fs/splice.c b/fs/splice.c
> index 2ecfccbda956..bdbabc2ebfff 100644
> --- a/fs/splice.c
> +++ b/fs/splice.c
> @@ -1184,10 +1184,6 @@ long do_splice(struct file *in, loff_t *off_in, struct file *out,
>  			out->f_pos = offset;
>  		else
>  			*off_out = offset;
> -
> -		// splice_write-> already marked out
> -		// as modified via vfs_iter_write()
> -		goto noaccessout;
>  	} else if (opipe) {
>  		if (off_out)
>  			return -ESPIPE;
> @@ -1211,11 +1207,10 @@ long do_splice(struct file *in, loff_t *off_in, struct file *out,
>  	} else
>  		return -EINVAL;
>  
> -	if (ret > 0)
> +	if (ret > 0) {
>  		fsnotify_modify(out);
> -noaccessout:
> -	if (ret > 0)
>  		fsnotify_access(in);
> +	}
>  
>  	return ret;
>  }
> -- 
> 2.39.2
  

On Wed, Jun 28, 2023 at 2:38 PM Jan Kara <jack@suse.cz> wrote:
>
> Hello!
>
> On Tue 27-06-23 22:50:46, Ahelenia Ziemiańska wrote:
> > Always generate modify out, access in for splice;
> > this gets automatically merged with no ugly special cases.
> >
> > No changes to 2/3 or 3/3.
>
> Thanks for the patches Ahelena! The code looks fine to me but to be honest
> I still have one unresolved question so let me think about it loud here for
> documentation purposes :). Do we want fsnotify (any filesystem
> notification framework like inotify or fanotify) to actually generate
> events on FIFOs? FIFOs are virtual objects and are not part of the
> filesystem as such (well, the inode itself and the name is), hence
> *filesystem* notification framework does not seem like a great fit to watch
> for changes or accesses there. And if we say "yes" for FIFOs, then why not
> AF_UNIX sockets? Where do we draw the line? And is it all worth the
> trouble?
>
> I understand the convenience of inotify working on FIFOs for the "tail -f"
> usecase but then wouldn't this better be fixed in tail(1) itself by using
> epoll(7) for FIFOs which, as I've noted in my other reply, does not have
> the problem that poll(2) has when there are no writers?
>
> Another issue with FIFOs is that they do not have a concept of file
> position. For hierarchical storage usecase we are introducing events that
> will report file ranges being modified / accessed and officially supporting
> FIFOs is one more special case to deal with.
>
> What is supporting your changes is that fsnotify mostly works for FIFOs
> already now (normal reads & writes generate notification) so splice not
> working could be viewed as an inconsistency. Sockets (although they are
> visible in the filesystem) cannot be open so for them the illusion of being
> a file is even weaker.
>
> So overall I guess I'm slightly in favor of making fsnotify generate events
> on FIFOs even with splice, provided Amir does not see a big trouble in
> supporting this with his upcoming HSM changes.
>

I've also thought about this.

The thing about the HSM events is that they are permission events
and just like FAN_ACCESS_PERM, they originate from the common
access control helpers {rw,remap}_verify_area(), which also happen
to have the file range info (with ppos NULL for pipes).

Ahelenia's patches do not add any new rw_verify_area() to pipes
so no new FAN_ACCESS_PERM events were added.

If we could go back to the design of fanotify we would have probably
made it explicit that permission events are only allowed on regular
files and dirs. For the new HSM events we can (and will) do that.

In any case, the new events are supposed to be delivered with
file access range records, so delivering HSM events on pipes
wouldn't make any sense.

So I do not see any problem with these patches wrt upcomping
HSM events.

However, note that these patches create more inconsistencies
between IN_ACCESS and FAN_ACCESS_PERM on pipes.

We can leave it at that if we want, but fixing the inconsistencies
by adding more FAN_ACCESS_PERM events on pipes - this
is not something that I wouldn't be comfortable with.

If anything, we can remove FAN_ACCESS_PERM events from
special files and see if anybody complains.

I don't know of any users of FAN_ACCESS_PERM and even for
FAN_OPEN_PERM, I don't think that AV-vendors have anything
useful to do with open permission events on special files.

Thanks,
Amir.
  

Hi!

On Wed, Jun 28, 2023 at 01:38:53PM +0200, Jan Kara wrote:
> On Tue 27-06-23 22:50:46, Ahelenia Ziemiańska wrote:
> > Always generate modify out, access in for splice;
> > this gets automatically merged with no ugly special cases.
> > 
> > No changes to 2/3 or 3/3.
> Thanks for the patches Ahelena! The code looks fine to me but to be honest
> I still have one unresolved question so let me think about it loud here for
> documentation purposes :). Do we want fsnotify (any filesystem
> notification framework like inotify or fanotify) to actually generate
> events on FIFOs? FIFOs are virtual objects and are not part of the
> filesystem as such (well, the inode itself and the name is), hence
> *filesystem* notification framework does not seem like a great fit to watch
> for changes or accesses there. And if we say "yes" for FIFOs, then why not
> AF_UNIX sockets? Where do we draw the line? And is it all worth the
> trouble?
As a relative outsider (I haven't used inotify before this, and have not
 been subjected to it or its peripheries before),
I interpreted inotify as being the Correct solution for:
  1. stuff you can find in a normal
     (non-/dev, you don't want to touch devices)
     filesystem traversal
  2. stuff you can open
where, going down the list in inode(7):
  S_IFSOCK   can't open
  S_IFLNK    can't open
  S_IFREG    yes!
  S_IFBLK    it's a device
  S_IFDIR    yes!
  S_IFCHR    it's a device
  S_IFIFO    yes!

It appears that I'm not the only one who's interpreted it that way,
especially since neither regular files nor pipes are pollable.
(Though, under that same categorisation, I wouldn't be surprised
 if anonymous pipes had been refused, for example, since those are
 conventionally unnameable.)

To this end, I'd say we're leaving the line precisely where it was drawn
before, even if by accident.

> I understand the convenience of inotify working on FIFOs for the "tail -f"
> usecase but then wouldn't this better be fixed in tail(1) itself by using
> epoll(7) for FIFOs which, as I've noted in my other reply, does not have
> the problem that poll(2) has when there are no writers?
Yes, epoll in ET mode returns POLLHUP only once, but you /also/ need the
inotify anyway for regular files, which epoll refuses
(and, with -F, you may want both epoll for a pipe and inotify for the
 directory it's contained in).
Is it possible to do? yes. Is it more annoying than just having pipes
report when they were written to? very much so.

inotify actually working(*) is presumably why coreutils tail doesn't use
epoll ‒ inotify already provides all required events(*), you can use the
same code for regular files and fifos, and with one fewer level of
indirection: there's just no need(*).

(*: except with a magic syscall only I use apparently)
  

Hi!

On Wed 28-06-23 20:54:28, Ahelenia Ziemiańska wrote:
> On Wed, Jun 28, 2023 at 01:38:53PM +0200, Jan Kara wrote:
> > On Tue 27-06-23 22:50:46, Ahelenia Ziemiańska wrote:
> > > Always generate modify out, access in for splice;
> > > this gets automatically merged with no ugly special cases.
> > > 
> > > No changes to 2/3 or 3/3.
> > Thanks for the patches Ahelena! The code looks fine to me but to be honest
> > I still have one unresolved question so let me think about it loud here for
> > documentation purposes :). Do we want fsnotify (any filesystem
> > notification framework like inotify or fanotify) to actually generate
> > events on FIFOs? FIFOs are virtual objects and are not part of the
> > filesystem as such (well, the inode itself and the name is), hence
> > *filesystem* notification framework does not seem like a great fit to watch
> > for changes or accesses there. And if we say "yes" for FIFOs, then why not
> > AF_UNIX sockets? Where do we draw the line? And is it all worth the
> > trouble?
> As a relative outsider (I haven't used inotify before this, and have not
>  been subjected to it or its peripheries before),
> I interpreted inotify as being the Correct solution for:
>   1. stuff you can find in a normal
>      (non-/dev, you don't want to touch devices)
>      filesystem traversal
>   2. stuff you can open
> where, going down the list in inode(7):
>   S_IFSOCK   can't open
>   S_IFLNK    can't open
>   S_IFREG    yes!
>   S_IFBLK    it's a device
>   S_IFDIR    yes!
>   S_IFCHR    it's a device
>   S_IFIFO    yes!
> 
> It appears that I'm not the only one who's interpreted it that way,
> especially since neither regular files nor pipes are pollable.
> (Though, under that same categorisation, I wouldn't be surprised
>  if anonymous pipes had been refused, for example, since those are
>  conventionally unnameable.)
> 
> To this end, I'd say we're leaving the line precisely where it was drawn
> before, even if by accident.

I agree, although I'd note that there are S_IFREG inodes under /sys or
/proc where it would be too difficult to provide fsnotify events (exactly
because the file contents is not "data stored somewhere" but rather
something "generated on the fly") so the illusion is not perfect already.

> > I understand the convenience of inotify working on FIFOs for the "tail -f"
> > usecase but then wouldn't this better be fixed in tail(1) itself by using
> > epoll(7) for FIFOs which, as I've noted in my other reply, does not have
> > the problem that poll(2) has when there are no writers?
> Yes, epoll in ET mode returns POLLHUP only once, but you /also/ need the
> inotify anyway for regular files, which epoll refuses
> (and, with -F, you may want both epoll for a pipe and inotify for the
>  directory it's contained in).
> Is it possible to do? yes. Is it more annoying than just having pipes
> report when they were written to? very much so.
> 
> inotify actually working(*) is presumably why coreutils tail doesn't use
> epoll ‒ inotify already provides all required events(*), you can use the
> same code for regular files and fifos, and with one fewer level of
> indirection: there's just no need(*).
> 
> (*: except with a magic syscall only I use apparently)

Yeah, I've slept to this and I still think adding fsnotify events to splice
is a nicer option so feel free to add:

Acked-by: Jan Kara <jack@suse.cz>

to all kernel patches in your series. Since the changes are in splice code,
Christian or Al Viro (who you already have on CC list) should be merging
this so please make sure to also include them in the v5 submission.

								Honza