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

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Date:   Thu, 3 Mar 2022 10:56:53 -0800
From:   Hao Luo <haoluo@...gle.com>
To:     Yonghong Song <yhs@...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>,
        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

On Wed, Mar 2, 2022 at 12:55 PM Yonghong Song <yhs@...com> wrote:
>
>
>
> On 2/25/22 3:43 PM, 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>
> > ---
> >   include/linux/bpf.h            |   1 +
> >   include/uapi/linux/bpf.h       |  26 +++++
> >   kernel/bpf/inode.c             |   9 +-
> >   kernel/bpf/syscall.c           | 177 +++++++++++++++++++++++++++++++++
> >   tools/include/uapi/linux/bpf.h |  26 +++++
> >   5 files changed, 236 insertions(+), 3 deletions(-)
> >
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index f19abc59b6cd..fce5e26179f5 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -1584,6 +1584,7 @@ int bpf_link_new_fd(struct bpf_link *link);
> >   struct file *bpf_link_new_file(struct bpf_link *link, int *reserved_fd);
> >   struct bpf_link *bpf_link_get_from_fd(u32 ufd);
> >
> > +bool bpf_path_is_bpf_dir(const struct path *path);
> >   int bpf_obj_pin_user(u32 ufd, const char __user *pathname);
> >   int bpf_obj_get_user(const char __user *pathname, int flags);
> >
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index afe3d0d7f5f2..a5dbc794403d 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -5086,6 +5086,29 @@ union bpf_attr {
> >    *  Return
> >    *          0 on success, or a negative error in case of failure. On error
> >    *          *dst* buffer is zeroed out.
> > + *
> > + * long bpf_mkdir(const char *pathname, int pathname_sz, u32 mode)
>
> Can we make pathname_sz to be u32 instead of int? pathname_sz should
> never be negative any way.
>
> Also, I think it is a good idea to add 'u64 flags' parameter for all
> three helpers, so we have room in the future to tune for new use cases.
>

SG. Will make this change.

Actually, I think I need to cap patthname_sz from above, to ensure
pathname_sz isn't too big. Is that necessary? I see there are other
helpers that don't have this type of check.

> > + *   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.
> >    */
> >   #define __BPF_FUNC_MAPPER(FN)               \
> >       FN(unspec),                     \
> > @@ -5280,6 +5303,9 @@ union bpf_attr {
> >       FN(xdp_load_bytes),             \
> >       FN(xdp_store_bytes),            \
> >       FN(copy_from_user_task),        \
> > +     FN(mkdir),                      \
> > +     FN(rmdir),                      \
> > +     FN(unlink),                     \
> >       /* */
> >
> >   /* integer value in 'imm' field of BPF_CALL instruction selects which helper
> [...]