linux-kernel - Re: [RFC PATCH 2/3] add statmnt(2) syscall

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Date:   Tue, 26 Sep 2023 16:06:05 +0200
From:   Miklos Szeredi <miklos@...redi.hu>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     Miklos Szeredi <mszeredi@...hat.com>,
        Christian Brauner <brauner@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-api@...r.kernel.org, linux-man@...r.kernel.org,
        linux-security-module@...r.kernel.org, Karel Zak <kzak@...hat.com>,
        Ian Kent <raven@...maw.net>,
        David Howells <dhowells@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Christian Brauner <christian@...uner.io>,
        Amir Goldstein <amir73il@...il.com>
Subject: Re: [RFC PATCH 2/3] add statmnt(2) syscall

On Tue, 26 Sept 2023 at 15:49, Florian Weimer <fweimer@...hat.com> wrote:
>
> * Miklos Szeredi:
>
> > On Mon, Sep 18, 2023 at 3:51 PM Christian Brauner <brauner@...nel.org> wrote:
> >
> >> I really would prefer a properly typed struct and that's what everyone
> >> was happy with in the session as well. So I would not like to change the
> >> main parameters.
> >
> > I completely  agree.  Just would like to understand this point:
> >
> >   struct statmnt *statmnt(u64 mntid, u64 mask, unsigned int flags);
> >
> > What's not properly typed about this interface?
> >
> > I guess the answer is that it's not a syscall interface, which will
> > have an added [void *buf, size_t bufsize], while the buffer sizing is
> > done by a simple libc wrapper.
> >
> > Do you think that's a problem?  If so, why?
>
> Try-and-resize interfaces can be quite bad for data obtained from the
> network.

In this particular case it's all local information.

>  If the first call provides the minimum buffer size (like
> getgroups, but unlike readlink or the glibc *_r interfaces for NSS),
> this could at least allow us to avoid allocating too much.  In
> userspace, we cannot reduce the size of the heap allocation without
> knowing where the pointers are and what they mean.

Does it matter if the heap allocation is say 32k instead of 589bytes?
 The returned strings are not limited in size, but are quite unlikely
to be over PATH_MAX.

E.g. getdents apparently uses 32k buffers, which is really a tiny
amount of heap these days, but more than enough for the purpose.  Not
sure if this is hard coded into libc or if it's the result of some
heuristic based on available memory, but I don't see why similar
treatment couldn't be applied to the statmount(2) syscall.

> I also don't quite understand the dislike of variable-sized records.
> Don't getdents, inotify, Netlink all use them?  And I think at least for
> Netlink, more stuff is added all the time?

What do you mean by variable sized records?

Thanks,
Miklos