Re: [PATCH v4 bpf-next 3/7] bpf: add fd_array_cnt attribute for prog_load

[Anton Protopopov <aspsk@isovalent.com>]

On 24/12/04 10:08AM, Andrii Nakryiko wrote:
> On Wed, Dec 4, 2024 at 4:19 AM Anton Protopopov <aspsk@isovalent.com> wrote:
> >
> > On 24/12/03 01:25PM, Andrii Nakryiko wrote:
> > > On Tue, Dec 3, 2024 at 5:48 AM Anton Protopopov <aspsk@isovalent.com> wrote:
> > > >
> > > > The fd_array attribute of the BPF_PROG_LOAD syscall may contain a set
> > > > of file descriptors: maps or btfs. This field was introduced as a
> > > > sparse array. Introduce a new attribute, fd_array_cnt, which, if
> > > > present, indicates that the fd_array is a continuous array of the
> > > > corresponding length.
> > > >
> > > > If fd_array_cnt is non-zero, then every map in the fd_array will be
> > > > bound to the program, as if it was used by the program. This
> > > > functionality is similar to the BPF_PROG_BIND_MAP syscall, but such
> > > > maps can be used by the verifier during the program load.
> > > >
> > > > Signed-off-by: Anton Protopopov <aspsk@isovalent.com>
> > > > ---
> > > >  include/uapi/linux/bpf.h       | 10 ++++
> > > >  kernel/bpf/syscall.c           |  2 +-
> > > >  kernel/bpf/verifier.c          | 98 ++++++++++++++++++++++++++++------
> > > >  tools/include/uapi/linux/bpf.h | 10 ++++
> > > >  4 files changed, 104 insertions(+), 16 deletions(-)
> > > >
> > >
> > > [...]
> > >
> > > > +/*
> > > > + * The add_fd_from_fd_array() is executed only if fd_array_cnt is non-zero. In
> > > > + * this case expect that every file descriptor in the array is either a map or
> > > > + * a BTF. Everything else is considered to be trash.
> > > > + */
> > > > +static int add_fd_from_fd_array(struct bpf_verifier_env *env, int fd)
> > > > +{
> > > > +       struct bpf_map *map;
> > > > +       CLASS(fd, f)(fd);
> > > > +       int ret;
> > > > +
> > > > +       map = __bpf_map_get(f);
> > > > +       if (!IS_ERR(map)) {
> > > > +               ret = __add_used_map(env, map);
> > > > +               if (ret < 0)
> > > > +                       return ret;
> > > > +               return 0;
> > > > +       }
> > > > +
> > > > +       /*
> > > > +        * Unlike "unused" maps which do not appear in the BPF program,
> > > > +        * BTFs are visible, so no reason to refcnt them now
> > >
> > > What does "BTFs are visible" mean? I find this behavior surprising,
> > > tbh. Map is added to used_maps, but BTF is *not* added to used_btfs?
> > > Why?
> >
> > This functionality is added to catch maps, and work with them during
> > verification, which aren't otherwise referenced by program code. The
> > actual application is those "instructions set" maps for static keys.
> > All other objects are "visible" during verification.
> 
> That's your specific intended use case, but API is semantically more
> generic and shouldn't tailor to your specific interpretation on how it
> will/should be used. I think this is a landmine to add reference to
> just BPF maps and not to BTF objects, we won't be able to retrofit the
> proper and uniform treatment later without extra flags or backwards
> compatibility breakage.
> 
> Even though we don't need extra "detached" BTF objects associated with
> BPF program, right now, I can anticipate some interesting use case
> where we might want to attach additional BTF objects to BPF programs
> (for whatever reasons, BTFs are a convenient bag of strings and
> graph-based types, so could be useful for extra
> debugging/metadata/whatever information).
> 
> So I can see only two ways forward. Either we disable BTFs in fd_array
> if fd_array_cnt>0, which will prevent its usage from light skeleton,
> so not great. Or we bump refcount both BPF maps and BTFs in fd_array.
> 
> 
> The latter seems saner and I don't think is a problem at all, we
> already have used_btfs that function similarly to used_maps.

This makes total sense to treat all BPF objects in fd_array the same
way. With BTFs the problem is that, currently, a btf fd can end up
either in used_btfs or kfunc_btf_tab. I will take a look at how easy
it is to merge those two.

> >
> > > > +        */
> > > > +       if (!IS_ERR(__btf_get_by_fd(f)))
> > > > +               return 0;
> > > > +
> > > > +       verbose(env, "fd %d is not pointing to valid bpf_map or btf\n", fd);
> > > > +       return PTR_ERR(map);
> > > > +}
> > > > +
> > > > +static int process_fd_array(struct bpf_verifier_env *env, union bpf_attr *attr, bpfptr_t uattr)
> > > > +{
> > > > +       size_t size = sizeof(int);
> > > > +       int ret;
> > > > +       int fd;
> > > > +       u32 i;
> > > > +
> > > > +       env->fd_array = make_bpfptr(attr->fd_array, uattr.is_kernel);
> > > > +
> > > > +       /*
> > > > +        * The only difference between old (no fd_array_cnt is given) and new
> > > > +        * APIs is that in the latter case the fd_array is expected to be
> > > > +        * continuous and is scanned for map fds right away
> > > > +        */
> > > > +       if (!attr->fd_array_cnt)
> > > > +               return 0;
> > > > +
> > > > +       for (i = 0; i < attr->fd_array_cnt; i++) {
> > > > +               if (copy_from_bpfptr_offset(&fd, env->fd_array, i * size, size))
> > >
> > > potential overflow in `i * size`? Do we limit fd_array_cnt anywhere to
> > > less than INT_MAX/4?
> >
> > Right. So, probably cap to (UINT_MAX/size)?
> 
> either that or use check_mul_overflow()

Ok, will fix it, thanks.

> >
> > > > +                       return -EFAULT;
> > > > +
> > > > +               ret = add_fd_from_fd_array(env, fd);
> > > > +               if (ret)
> > > > +                       return ret;
> > > > +       }
> > > > +
> > > > +       return 0;
> > > > +}
> > > > +
> > >
> > > [...]