linux-kernel - Re: [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls for prog_bpf

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Message-ID: <CA+khW7g4mLw9W+CY651FaE-2SF0XBeaGKa5Le7ZnTBTK7eD30Q@mail.gmail.com>
Date:   Mon, 28 Feb 2022 14:10:39 -0800
From:   Hao Luo <haoluo@...gle.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Joe Burton <jevburton.kernel@...il.com>,
        Tejun Heo <tj@...nel.org>, joshdon@...gle.com, sdf@...gle.com,
        bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v1 1/9] bpf: Add mkdir, rmdir, unlink syscalls
 for prog_bpf_syscall

Hi Kumar,

On Sat, Feb 26, 2022 at 9:18 PM Kumar Kartikeya Dwivedi
<memxor@...il.com> wrote:
>
> On Sat, Feb 26, 2022 at 05:13:31AM IST, Hao Luo wrote:
> > This patch allows bpf_syscall prog to perform some basic filesystem
> > operations: create, remove directories and unlink files. Three bpf
> > helpers are added for this purpose. When combined with the following
> > patches that allow pinning and getting bpf objects from bpf prog,
> > this feature can be used to create directory hierarchy in bpffs that
> > help manage bpf objects purely using bpf progs.
> >
> > The added helpers subject to the same permission checks as their syscall
> > version. For example, one can not write to a read-only file system;
> > The identity of the current process is checked to see whether it has
> > sufficient permission to perform the operations.
> >
> > Only directories and files in bpffs can be created or removed by these
> > helpers. But it won't be too hard to allow these helpers to operate
> > on files in other filesystems, if we want.
> >
> > Signed-off-by: Hao Luo <haoluo@...gle.com>
> > ---
> > + *
> > + * long bpf_mkdir(const char *pathname, int pathname_sz, u32 mode)
> > + *   Description
> > + *           Attempts to create a directory name *pathname*. The argument
> > + *           *pathname_sz* specifies the length of the string *pathname*.
> > + *           The argument *mode* specifies the mode for the new directory. It
> > + *           is modified by the process's umask. It has the same semantic as
> > + *           the syscall mkdir(2).
> > + *   Return
> > + *           0 on success, or a negative error in case of failure.
> > + *
> > + * long bpf_rmdir(const char *pathname, int pathname_sz)
> > + *   Description
> > + *           Deletes a directory, which must be empty.
> > + *   Return
> > + *           0 on sucess, or a negative error in case of failure.
> > + *
> > + * long bpf_unlink(const char *pathname, int pathname_sz)
> > + *   Description
> > + *           Deletes a name and possibly the file it refers to. It has the
> > + *           same semantic as the syscall unlink(2).
> > + *   Return
> > + *           0 on success, or a negative error in case of failure.
> >   */
> >
>
> How about only introducing bpf_sys_mkdirat and bpf_sys_unlinkat? That would be
> more useful for other cases in future, and when AT_FDCWD is passed, has the same
> functionality as these, but when openat/fget is supported, it would work
> relative to other dirfds as well. It can also allow using dirfd of the process
> calling read for a iterator (e.g. if it sets the fd number using skel->bss).
> unlinkat's AT_REMOVEDIR flag also removes the need for a bpf_rmdir.
>
> WDYT?
>

The idea sounds good to me, more flexible. But I don't have a real use
case for using the added 'dirfd' at this moment. For all the use cases
I can think of, absolute paths will suffice, I think. Unless other
reviewers have opposition, I will try switching to mkdirat and
unlinkat in v2.