Re: [PATCH bpf-next 3/3] libbpf: Add pin option to automount BPF filesystem before pinning

[Andrii Nakryiko <andrii.nakryiko@gmail.com>]

On Tue, Oct 22, 2019 at 12:04 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
>
> > On Tue, Oct 22, 2019 at 9:08 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
> >>
> >> From: Toke Høiland-Jørgensen <toke@redhat.com>
> >>
> >> While the current map pinning functions will check whether the pin path is
> >> contained on a BPF filesystem, it does not offer any options to mount the
> >> file system if it doesn't exist. Since we now have pinning options, add a
> >> new one to automount a BPF filesystem at the pinning path if that is not
> >
> > Next thing we'll be adding extra options to mount BPF FS... Can we
> > leave the task of auto-mounting BPF FS to tools/applications?
>
> Well, there was a reason I put this into a separate patch: I wasn't sure
> it really fit here. My reasoning is the following: If we end up with a
> default auto-pinning that works really well, people are going to just
> use that. And end up really confused when bpffs is not mounted. And it
> seems kinda silly to make every application re-implement the same mount
> check and logic.
>
> Or to put it another way: If we agree that the reasonable default thing
> is to just pin things in /sys/fs/bpf, let's make it as easy as possible
> for applications to do that right.
>

This reminds me the setrlimit() issue, though. And we decided that
library shouldn't be manipulating global resources on behalf of users.
I think this is a similar one.

> >> already pointing at a bpffs.
> >>
> >> The mounting logic itself is copied from the iproute2 BPF helper functions.
> >>
> >> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> >> ---
> >>  tools/lib/bpf/libbpf.c |   47 +++++++++++++++++++++++++++++++++++++++++++++++
> >>  tools/lib/bpf/libbpf.h |    5 ++++-
> >>  2 files changed, 51 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> >> index aea3916de341..f527224bb211 100644
> >> --- a/tools/lib/bpf/libbpf.c
> >> +++ b/tools/lib/bpf/libbpf.c
> >> @@ -37,6 +37,7 @@
> >>  #include <sys/epoll.h>
> >>  #include <sys/ioctl.h>
> >>  #include <sys/mman.h>
> >> +#include <sys/mount.h>
> >>  #include <sys/stat.h>
> >>  #include <sys/types.h>
> >>  #include <sys/vfs.h>
> >> @@ -4072,6 +4073,35 @@ int bpf_map__unpin(struct bpf_map *map, const char *path)
> >>         return 0;
> >>  }
> >>
> >> +static int mount_bpf_fs(const char *target)
> >> +{
> >> +       bool bind_done = false;
> >> +
> >> +       while (mount("", target, "none", MS_PRIVATE | MS_REC, NULL)) {
> >
> > what does this loop do? we need some comments explaining what's going
> > on here
>
> Well, as it says in the commit message I stole this from iproute2. I
> think the "--make-private, --bind" dance is there to make sure we don't
> mess up some other mount points at this path. Which seems like a good
> idea, and one of those things that most people probably won't think
> about when just writing an application that wants to mount the fs; which
> is another reason to put this into libbpf :)

I think this is exactly a reason to not do this and rely on
applications to know and set up their environment properly. All these
races, accidentally stomping on someone else's FS, etc, that sounds
like a really good excuse to not do this in libbpf. Definitely not
until we get a real experience, driven by production use cases, on how
to go about that correctly. It might be added as a helper, but I think
application has to call it explicitly.

>
> >> +               if (errno != EINVAL || bind_done) {
> >> +                       pr_warning("mount --make-private %s failed: %s\n",
> >> +                                  target, strerror(errno));
> >> +                       return -1;
> >> +               }
> >> +
> >> +               if (mount(target, target, "none", MS_BIND, NULL)) {
> >> +                       pr_warning("mount --bind %s %s failed: %s\n",
> >> +                                  target, target, strerror(errno));
> >> +                       return -1;
> >> +               }
> >> +
> >> +               bind_done = true;
> >> +       }
> >> +
> >> +       if (mount("bpf", target, "bpf", 0, "mode=0700")) {
> >> +               fprintf(stderr, "mount -t bpf bpf %s failed: %s\n",
> >> +                       target, strerror(errno));
> >> +               return -1;
> >> +       }
> >> +
> >> +       return 0;
> >> +}
> >> +
> >>  static int get_pin_path(char *buf, size_t buf_len,
> >>                         struct bpf_map *map, struct bpf_object_pin_opts *opts,
> >>                         bool mkdir)
> >> @@ -4102,6 +4132,23 @@ static int get_pin_path(char *buf, size_t buf_len,
> >
> > Nothing in `get_pin_path` indicates that it's going to do an entire FS
> > mount, please split this out of get_pin_path.
>
> Regardless of the arguments above, that is certainly a fair point ;)
>
> -Toke