Re: [PATCH v2 bpf-next 1/2] bpf: Add bits iterator

[John Fastabend <john.fastabend@gmail.com>]

Yafang Shao wrote:
> On Fri, Mar 1, 2024 at 6:19 AM John Fastabend <john.fastabend@gmail.com> wrote:
> >
> > Andrii Nakryiko wrote:
> > > On Wed, Feb 28, 2024 at 6:16 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> > > >
> > > > On Wed, Feb 28, 2024 at 2:04 PM Andrii Nakryiko
> > > > <andrii.nakryiko@gmail.com> wrote:
> > > > >
> > > > > On Tue, Feb 27, 2024 at 6:25 PM Yafang Shao <laoar.shao@gmail.com> wrote:
> > > > > >
> > > > > > On Wed, Feb 28, 2024 at 9:24 AM Andrii Nakryiko
> > > > > > <andrii.nakryiko@gmail.com> wrote:
> > > > > > >
> > > > > > > On Sun, Feb 25, 2024 at 2:07 AM Yafang Shao <laoar.shao@gmail.com> wrote:
> > > > > > > >
> > > > > > > > Add three new kfuncs for the bits iterator:
> > > > > > > > - bpf_iter_bits_new
> > > > > > > >   Initialize a new bits iterator for a given memory area. Due to the
> > > > > > > >   limitation of bpf memalloc, the max number of bits that can be iterated
> > > > > > > >   over is limited to (4096 * 8).
> > > > > > > > - bpf_iter_bits_next
> > > > > > > >   Get the next bit in a bpf_iter_bits
> > > > > > > > - bpf_iter_bits_destroy
> > > > > > > >   Destroy a bpf_iter_bits
> > > > > > > >
> > > > > > > > The bits iterator facilitates the iteration of the bits of a memory area,
> > > > > > > > such as cpumask. It can be used in any context and on any address.
> > > > > > > >
> > > > > > > > Signed-off-by: Yafang Shao <laoar.shao@gmail.com>
> > > > > > > > ---

[...]

> > > > > > > > +/**
> > > > > > > > + * bpf_iter_bits_new() - Initialize a new bits iterator for a given memory area
> > > > > > > > + * @it: The new bpf_iter_bits to be created
> > > > > > > > + * @unsafe_ptr__ign: A ponter pointing to a memory area to be iterated over
> > > > > > > > + * @nr_bits: The number of bits to be iterated over. Due to the limitation of
> > > > > > > > + * memalloc, it can't greater than (4096 * 8).
> > > > > > > > + *
> > > > > > > > + * This function initializes a new bpf_iter_bits structure for iterating over
> > > > > > > > + * a memory area which is specified by the @unsafe_ptr__ign and @nr_bits. It
> > > > > > > > + * copy the data of the memory area to the newly created bpf_iter_bits @it for
> > > > > > > > + * subsequent iteration operations.
> > > > > > > > + *
> > > > > > > > + * On success, 0 is returned. On failure, ERR is returned.
> > > > > > > > + */
> > > > > > > > +__bpf_kfunc int
> > > > > > > > +bpf_iter_bits_new(struct bpf_iter_bits *it, const void *unsafe_ptr__ign, u32 nr_bits)
> > > > > > > > +{
> > > > > > > > +       struct bpf_iter_bits_kern *kit = (void *)it;
> > > > > > > > +       u32 size = BITS_TO_BYTES(nr_bits);
> > > > > > > > +       int err;
> > > > > > > > +
> > > > > > > > +       BUILD_BUG_ON(sizeof(struct bpf_iter_bits_kern) != sizeof(struct bpf_iter_bits));
> > > > > > > > +       BUILD_BUG_ON(__alignof__(struct bpf_iter_bits_kern) !=
> > > > > > > > +                    __alignof__(struct bpf_iter_bits));
> > > > > > > > +
> > > > > > > > +       if (!unsafe_ptr__ign || !nr_bits) {
> > > > > > > > +               kit->bits = NULL;
> > > > > > > > +               return -EINVAL;
> > > > > > > > +       }
> > > > > > > > +
> > > > > > > > +       kit->bits = bpf_mem_alloc(&bpf_global_ma, size);
> > > > > > > > +       if (!kit->bits)
> > > > > > > > +               return -ENOMEM;
> > > > > > >
> > > > > > > it's probably going to be a pretty common case to do bits iteration
> > > > > > > for nr_bits<=64, right?
> > > > > >
> > > > > > It's highly unlikely.
> > > > > > Consider the CPU count as an example; There are 256 CPUs on our AMD
> > > > > > EPYC servers.
> > > > >
> > > > > Also consider u64-based bit masks (like struct backtrack_state in
> > > > > verifier code, which has u32 reg_mask and u64 stack_mask). This
> > > > > iterator is a generic bits iterator, there are tons of cases of
> > > > > u64/u32 masks in practice.
> > > >
> > > > Should we optimize it as follows?
> > > >
> > > >     if (nr_bits <= 64) {
> > > >         // do the optimization
> > > >     } else {
> > > >         // fallback to memalloc
> > > >     }
> > > >
> > >
> > > Yep, that's what I'm proposing
> >
> > When I suggested why not just open code this in BPF earlier I was
> > mostly thinking of these u64 and u32 masks we have lots of them
> > in our code base as well.
> >
> > I have something like this which might be even better than 3
> > calls depending on your use case,
> >
> >  int find_next_bit(uint64_t bits, int last_bit)
> >  {
> >     int i = last_bit;
> >     for (i = 0; i < sizeof(uint64_t) * 8; i++) {
> >         if (bits & (1 << i))
> >            return i;
> >     }
> >     return -1;
> >   }
> >
> > Verifier seems plenty happy with above.
> 
> I'm not quite following.
> Regarding the find_next_bit() function you mentioned, it seems it only
> retrieves one bit at a time, necessitating a for loop for execution,
> correct? Consequently, the verifier will likely fail the for loop.

In practice for small sizes uint64_t and uint32_t we don't see any
issue. Just a comment that this can be open coded without much
trouble in many cases. Not against the helper at all.

> 
> -- 
> Regards
> Yafang