[RFC PATCH] getting misc stats/attributes via xattr API

[Christian Brauner]

On Mon, May 09, 2022 at 05:20:50PM +0300, Amir Goldstein wrote:
> On Mon, May 9, 2022 at 3:48 PM Christian Brauner <brauner at kernel.org> wrote:
> >
> > On Tue, May 03, 2022 at 02:23:23PM +0200, Miklos Szeredi wrote:
> > > This is a simplification of the getvalues(2) prototype and moving it to the
> > > getxattr(2) interface, as suggested by Dave.
> > >
> > > The patch itself just adds the possibility to retrieve a single line of
> > > /proc/$$/mountinfo (which was the basic requirement from which the fsinfo
> > > patchset grew out of).
> > >
> > > But this should be able to serve Amir's per-sb iostats, as well as a host of
> > > other cases where some statistic needs to be retrieved from some object.  Note:
> > > a filesystem object often represents other kinds of objects (such as processes
> > > in /proc) so this is not limited to fs attributes.
> > >
> > > This also opens up the interface to setting attributes via setxattr(2).
> > >
> > > After some pondering I made the namespace so:
> > >
> > > : - root
> > > bar - an attribute
> > > foo: - a folder (can contain attributes and/or folders)
> > >
> > > The contents of a folder is represented by a null separated list of names.
> > >
> > > Examples:
> > >
> > > $ getfattr -etext -n ":" .
> > > # file: .
> > > :="mnt:\000mntns:"
> > >
> > > $ getfattr -etext -n ":mnt:" .
> > > # file: .
> > > :mnt:="info"
> > >
> > > $ getfattr -etext -n ":mnt:info" .
> > > # file: .
> > > :mnt:info="21 1 254:0 / / rw,relatime - ext4 /dev/root rw\012"
> >
> > Hey Miklos,
> >
> > One comment about this. We really need to have this interface support
> > giving us mount options like "relatime" back in numeric form (I assume
> > this will be possible.). It is royally annoying having to maintain a
> > mapping table in userspace just to do:
> >
> > relatime -> MS_RELATIME/MOUNT_ATTR_RELATIME
> > ro       -> MS_RDONLY/MOUNT_ATTR_RDONLY
> >
> > A library shouldn't be required to use this interface. Conservative
> > low-level software that keeps its shared library dependencies minimal
> > will need to be able to use that interface without having to go to an
> > external library that transforms text-based output to binary form (Which
> > I'm very sure will need to happen if we go with a text-based
> > interface.).
> >
> 
> No need for a library.
> We can export:
> 
> :mnt:attr:flags (in hex format)

So a binary attribute or a hex value as a string which we have to parse
in userspace into proper hex?

> 
> > >
> > > $ getfattr -etext -n ":mntns:" .
> > > # file: .
> > > :mntns:="21:\00022:\00024:\00025:\00023:\00026:\00027:\00028:\00029:\00030:\00031:"
> > >
> > > $ getfattr -etext -n ":mntns:28:" .
> > > # file: .
> > > :mntns:28:="info"
> > >
> > > Comments?
> >
> > I'm not a fan of text-based APIs and I'm particularly not a fan of the
> > xattr APIs. But at this point I'm ready to compromise on a lot as long
> > as it gets us values out of the kernel in some way. :)
> >
> > I had to use xattrs extensively in various low-level userspace projects
> > and they continue to be a source of races and memory bugs.
> >
> > A few initial questions:
> >
> > * The xattr APIs often require the caller to do sm like (copying some go
> >   code quickly as I have that lying around):
> >
> >         for _, x := range split {
> >                 xattr := string(x)
> >                 // Call Getxattr() twice: First, to determine the size of the
> >                 // buffer we need to allocate to store the extended attributes,
> >                 // second, to actually store the extended attributes in the
> >                 // buffer. Also, check if the size of the extended attribute
> >                 // hasn't increased between the two calls.
> >                 pre, err = unix.Getxattr(path, xattr, nil)
> >                 if err != nil || pre < 0 {
> >                         return nil, err
> >                 }
> >
> >                 dest = make([]byte, pre)
> >                 post := 0
> >                 if pre > 0 {
> >                         post, err = unix.Getxattr(path, xattr, dest)
> >                         if err != nil || post < 0 {
> >                                 return nil, err
> >                         }
> >                 }
> >
> >                 if post > pre {
> >                         return nil, fmt.Errorf("Extended attribute '%s' size increased from %d to %d during retrieval", xattr, pre, post)
> >                 }
> >
> >                 xattrs[xattr] = string(dest)
> >         }
> >
> >   This pattern of requesting the size first by passing empty arguments,
> >   then allocating the buffer and then passing down that buffer to
> >   retrieve that value is really annoying to use and error prone (I do
> >   of course understand why it exists.).
> >
> >   For real xattrs it's not that bad because we can assume that these
> >   values don't change often and so the race window between
> >   getxattr(GET_SIZE) and getxattr(GET_VALUES) often doesn't matter. But
> >   fwiw, the post > pre check doesn't exist for no reason; we do indeed
> >   hit that race.
> 
> It is not really a race, you can do {} while (errno != ERANGE) and there
> will be no race.

I don't know what your definition of your race is but if a value can
change between two calls and I have to call it in a loop until I get a
consistent value then that's a race. At least I know of no better way of
calling it.

> 
> >
> >   In addition, it is costly having to call getxattr() twice. Again, for
> >   retrieving xattrs it often doesn't matter because it's not a super
> >   common operation but for mount and other info it might matter.
> >
> 
> samba and many other projects that care about efficiency solved this
> a long time ago with an opportunistic buffer - never start with NULL buffer
> most values will fit in a 1K buffer.

I'm glad that it's been solved a long time ago. It's still not a good
property for an interface.

> 
> >   Will we have to use the same pattern for mnt and other info as well?
> >   If so, I worry that the race is way more likely than it is for real
> >   xattrs.
> >
> > * Would it be possible to support binary output with this interface?
> >   I really think users would love to have an interfact where they can
> >   get a struct with binary info back. I'm not advocating to make the
> >   whole interface binary but I wouldn't mind having the option to
> >   support it.
> >   Especially for some information at least. I'd really love to have a
> >   way go get a struct mount_info or whatever back that gives me all the
> >   details about a mount encompassed in a single struct.
> >
> 
> I suggested that up thread and Greg has explicitly and loudly
> NACKed it - so you will have to take it up with him

This is a vfs API and ultimately I would think that if we agree as a
subsystem that it would be desirable to have a way of providing binary
output in the form of well-defined structs in some form then we are free
to do so.

> 
> >   Callers like systemd will have to parse text and will end up
> >   converting everything from text into binary anyway; especially for
> >   mount information. So giving them an option for this out of the box
> >   would be quite good.
> >
> >   Interfaces like statx aim to be as fast as possible because we exptect
> >   them to be called quite often. Retrieving mount info is quite costly
> >   and is done quite often as well. Maybe not for all software but for a
> >   lot of low-level software. Especially when starting services in
> >   systemd a lot of mount parsing happens similar when starting
> >   containers in runtimes.
> >
> 
> This API is not for *everything*. Obviously it does not replace statx and some
> info (like the cifs OFFLINE flag) should be added to statx.

I'm not sure why you bring up this API as a replacement for statx().
That was never part of the discussion. And I didn't think I gave the
impression I was saying this.

This is about mount information and you can't get a lot of meaningful
mount information from statx(). Though tbh, I really think a mountx() or
similar system call wouldn't hurt...

> Whether or not mount info needs to get a special treatment like statx
> is not proven.

Hm, if we take that argument I could also claim that whether or not
mount info needs to be given in textual form is not proven. Iow, I'm not
sure what this is an argument for or against.

In fact, most well-used vfs information providing APIs apart from a few
such as the xattr APIs are based on well-defined structs. And I
personally at least consider that to be a good thing.

But I'm willing to compromise and support the textual thing. But I'd
still like to have the possibility to have some information provided in
binary form. I don't think that needs to be off the table completely.

> Miklos claims this is a notification issue-
> With David Howells' mount notification API, systemd can be pointed at the new
> mount that was added/removed/changed and then systemd will rarely need to
> parse thousands of mounts info.

You seem to be replying to things I didn't say. :)

The notification issue is orthogonal to that and yes, we need that.
I'm just saying that I want the ability in principal for some properties
to be given in binary form in addition to textual form. Performance may
be an aspect of this but that's orthogonal to performance issues when
being notified about mount changes and reacting to it in e.g. a service.

> 
> > * If we decide to go forward with this interface - and I think I
> >   mentioned this in the lsfmm session - could we please at least add a
> >   new system call? It really feels wrong to retrieve mount and other
> >   information through the xattr interfaces. They aren't really xattrs.
> >
> >   Imho, xattrs are a bit like a wonky version of streams already (One of
> >   the reasons I find them quite unpleasant.). Making mount and other
> >   information retrievable directly through the getxattr() interface will
> >   turn them into a full-on streams implementation imho. I'd prefer not
> >   to do that (Which is another reason I'd prefer at least a separate
> >   system call.).
> 
> If you are thinking about a read() like interface for xattr or any alternative
> data stream then Linus has NACKed it times before.

Please don't just make up an argument for me and then counter it. :D

> 
> However, we could add getxattr_multi() as Dave Chinner suggested for
> enumerating multiple keys+ (optional) values.
> In contrast to listxattr(), getxattr_multi() could allow a "short read"
> at least w.r.t the size of the vector.

Having "xattr" in the system call name is just confusing. These are
fundamentally not "real" xattrs and we shouldn't mix semantics. There
should be a clear distinction between traditional xattrs and this vfs
and potentially fs information providing interface.

Just thinking about what the manpage would look like. We would need to
add a paragraph to xattr(7) explaining that in addition to the system.*,
security.*, user.* and other namespaces we now also have a set of
namespaces that function as ways to get information about mounts or
other things instead of information attached to specific inodes.

That's super random imho. If I were to be presented with this manpage
I'd wonder if someone was too lazy to add a proper new system call with
it's own semantics for this and just stuffed it into an existing API
because it provided matching system call arguments. We can add a new
system call. It's not that we're running out of them.

Fwiw, and I'm genuinly _not_ trolling you could call it:
fsinfo(int dfd, const char *path, const char *key, char *value);