linux-kernel - Re: [RFC PATCH 1/4] sysctl: API extension for handling sysctl

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Message-ID: <Ype+08iXbbWghf3i@example.org>
Date:   Wed, 1 Jun 2022 21:32:35 +0200
From:   Alexey Gladkov <legion@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christian Brauner <brauner@...nel.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Linux Containers <containers@...ts.linux.dev>,
        linux-fsdevel@...r.kernel.org,
        Luis Chamberlain <mcgrof@...nel.org>,
        Vasily Averin <vvs@...tuozzo.com>
Subject: Re: [RFC PATCH 1/4] sysctl: API extension for handling sysctl

On Wed, Jun 01, 2022 at 08:19:14PM +0100, Matthew Wilcox wrote:
> On Wed, Jun 01, 2022 at 03:20:29PM +0200, Alexey Gladkov wrote:
> > +struct ctl_fops {
> > +	int (*open) (struct ctl_context *, struct inode *, struct file *);
> > +	int (*release) (struct ctl_context *, struct inode *, struct file *);
> > +	ssize_t (*read) (struct ctl_context *, struct file *, char *, size_t *, loff_t *);
> > +	ssize_t (*write) (struct ctl_context *, struct file *, char *, size_t *, loff_t *);
> > +};
> 
> Why not pass the iocb in ->read and ->write?  We're still regretting not
> doing that with file_operations.

Because buf and len can be modified in BPF_CGROUP_RUN_PROG_SYSCTL.
We need to pass the result of this hook to read/write callback.

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/fs/proc/proc_sysctl.c#n605
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/bpf/cgroup.c#n1441

-- 
Rgrds, legion