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

[Yonghong Song]

On 3/3/22 10:56 AM, Hao Luo wrote:
> On Wed, Mar 2, 2022 at 12:55 PM Yonghong Song <yhs@xxxxxx> 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@xxxxxxxxxx>
> > > ---
> > >    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.

There is no need. The verifier should ensure the memory held by pathname 
will have at least size of pathname_sz.

> > > + *   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
> > [...]