Re: [PATCH ipsec-next v2 3/6] libbpf: Add BPF_CORE_WRITE_BITFIELD() macro

[Andrii Nakryiko]

On Fri, Dec 1, 2023 at 11:11 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Thu, Nov 30, 2023 at 5:33 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
> >
> > On Tue, Nov 28, 2023 at 07:59:01PM +0200, Eduard Zingerman wrote:
> > > On Tue, 2023-11-28 at 10:54 -0700, Daniel Xu wrote:
> > > > Similar to reading from CO-RE bitfields, we need a CO-RE aware bitfield
> > > > writing wrapper to make the verifier happy.
> > > >
> > > > Two alternatives to this approach are:
> > > >
> > > > 1. Use the upcoming `preserve_static_offset` [0] attribute to disable
> > > >    CO-RE on specific structs.
> > > > 2. Use broader byte-sized writes to write to bitfields.
> > > >
> > > > (1) is a bit a bit hard to use. It requires specific and
> > > > not-very-obvious annotations to bpftool generated vmlinux.h. It's also
> > > > not generally available in released LLVM versions yet.
> > > >
> > > > (2) makes the code quite hard to read and write. And especially if
> > > > BPF_CORE_READ_BITFIELD() is already being used, it makes more sense to
> > > > to have an inverse helper for writing.
> > > >
> > > > [0]: https://reviews.llvm.org/D133361
> > > > From: Eduard Zingerman <eddyz87@xxxxxxxxx>
> > > >
> > > > Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> > > > ---
> > >
> > > Could you please also add a selftest (or several) using __retval()
> > > annotation for this macro?
> >
> > Good call about adding tests -- I found a few bugs with the code from
> > the other thread. But boy did they take a lot of brain cells to figure
> > out.
> >
> > There was some 6th grade algebra involved too -- I'll do my best to
> > explain it in the commit msg for v3.
> >
> >
> > Here are the fixes in case you are curious:
> >
> > diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> > index 7a764f65d299..8f02c558c0ff 100644
> > --- a/tools/lib/bpf/bpf_core_read.h
> > +++ b/tools/lib/bpf/bpf_core_read.h
> > @@ -120,7 +120,9 @@ enum bpf_enum_value_kind {
> >         unsigned int byte_size = __CORE_RELO(s, field, BYTE_SIZE);      \
> >         unsigned int lshift = __CORE_RELO(s, field, LSHIFT_U64);        \
> >         unsigned int rshift = __CORE_RELO(s, field, RSHIFT_U64);        \
> > -       unsigned int bit_size = (rshift - lshift);                      \
> > +       unsigned int bit_size = (64 - rshift);                          \
> > +       unsigned int hi_size = lshift;                                  \
> > +       unsigned int lo_size = (rshift - lshift);                       \
>
> nit: let's drop unnecessary ()
>
> >         unsigned long long nval, val, hi, lo;                           \
> >                                                                         \
> >         asm volatile("" : "+r"(p));                                     \
> > @@ -131,13 +133,13 @@ enum bpf_enum_value_kind {
> >         case 4: val = *(unsigned int *)p; break;                        \
> >         case 8: val = *(unsigned long long *)p; break;                  \
> >         }                                                               \
> > -       hi = val >> (bit_size + rshift);                                \
> > -       hi <<= bit_size + rshift;                                       \
> > -       lo = val << (bit_size + lshift);                                \
> > -       lo >>= bit_size + lshift;                                       \
> > +       hi = val >> (64 - hi_size);                                     \
> > +       hi <<= 64 - hi_size;                                            \
> > +       lo = val << (64 - lo_size);                                     \
> > +       lo >>= 64 - lo_size;                                            \
> >         nval = new_val;                                                 \
> > -       nval <<= lshift;                                                \
> > -       nval >>= rshift;                                                \
> > +       nval <<= (64 - bit_size);                                       \
> > +       nval >>= (64 - bit_size - lo_size);                             \
> >         val = hi | nval | lo;                                           \
>
> this looks.. unusual. I'd imagine we calculate a mask, mask out bits
> we are replacing, and then OR with new values, roughly (assuming all
> the right left/right shift values and stuff)
>
> /* clear bits */
> val &= ~(bitfield_mask << shift);

we can also calculate shifted mask with just

bitfield_mask = (-1ULL) << some_left_shift >> some_right_shift;
val &= ~bitfield_mask;

> /* set bits */
> val |= (nval & bitfield_mask) << shift;
>
> ?
>
> >         switch (byte_size) {                                            \
> >         case 1: *(unsigned char *)p      = val; break;                  \
> >
> >
> > Thanks,
> > Daniel