Re: [PATCH ipsec-next v3 3/9] libbpf: Add BPF_CORE_WRITE_BITFIELD() macro

[Andrii Nakryiko]

On Fri, Dec 1, 2023 at 4:13 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
>
> On Fri, Dec 01, 2023 at 03:49:30PM -0800, Andrii Nakryiko wrote:
> > On Fri, Dec 1, 2023 at 12:24 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
> > >
> > > === Motivation ===
> > >
> > > 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 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.
> > >
> > > === Implementation details ===
> > >
> > > Since the logic is a bit non-obvious, I thought it would be helpful
> > > to explain exactly what's going on.
> > >
> > > To start, it helps by explaining what LSHIFT_U64 (lshift) and RSHIFT_U64
> > > (rshift) is designed to mean. Consider the core of the
> > > BPF_CORE_READ_BITFIELD() algorithm:
> > >
> > >         val <<= __CORE_RELO(s, field, LSHIFT_U64);
> > >                 val = val >> __CORE_RELO(s, field, RSHIFT_U64);
> >
> > nit: indentation is off?
>
> Oops, it's cuz I only deleted the SIGNED check. Will fix.
> >
> > >
> > > Basically what happens is we lshift to clear the non-relevant (blank)
> > > higher order bits. Then we rshift to bring the relevant bits (bitfield)
> > > down to LSB position (while also clearing blank lower order bits). To
> > > illustrate:
> > >
> > >         Start:    ........XXX......
> > >         Lshift:   XXX......00000000
> > >         Rshift:   00000000000000XXX
> > >
> > > where `.` means blank bit, `0` means 0 bit, and `X` means bitfield bit.
> > >
> > > After the two operations, the bitfield is ready to be interpreted as a
> > > regular integer.
> > >
> > > Next, we want to build an alternative (but more helpful) mental model
> > > on lshift and rshift. That is, to consider:
> > >
> > > * rshift as the total number of blank bits in the u64
> > > * lshift as number of blank bits left of the bitfield in the u64
> > >
> > > Take a moment to consider why that is true by consulting the above
> > > diagram.
> > >
> > > With this insight, we can how define the following relationship:
> > >
> > >               bitfield
> > >                  _
> > >                 | |
> > >         0.....00XXX0...00
> > >         |      |   |    |
> > >         |______|   |    |
> > >          lshift    |    |
> > >                    |____|
> > >               (rshift - lshift)
> > >
> > > That is, we know the number of higher order blank bits is just lshift.
> > > And the number of lower order blank bits is (rshift - lshift).
> > >
> >
> > Nice diagrams and description, thanks!
>
> Thanks!
>
> >
> > > Finally, we can examine the core of the write side algorithm:
> > >
> > >         mask = (~0ULL << rshift) >> lshift;   // 1
> > >         nval = new_val;                       // 2
> > >         nval = (nval << rpad) & mask;         // 3
> > >         val = (val & ~mask) | nval;           // 4
> > >
> > > (1): Compute a mask where the set bits are the bitfield bits. The first
> > >      left shift zeros out exactly the number of blank bits, leaving a
> > >      bitfield sized set of 1s. The subsequent right shift inserts the
> > >      correct amount of higher order blank bits.
> > > (2): Place the new value into a word sized container, nval.
> > > (3): Place nval at the correct bit position and mask out blank bits.
> > > (4): Mix the bitfield in with original surrounding blank bits.
> > >
> > > [0]: https://reviews.llvm.org/D133361
> > > Co-authored-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> > > Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> > > Co-authored-by: Jonathan Lemon <jlemon@xxxxxxxxxxxx>
> > > Signed-off-by: Jonathan Lemon <jlemon@xxxxxxxxxxxx>
> > > Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> > > ---
> > >  tools/lib/bpf/bpf_core_read.h | 34 ++++++++++++++++++++++++++++++++++
> > >  1 file changed, 34 insertions(+)
> > >
> > > diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> > > index 1ac57bb7ac55..a7ffb80e3539 100644
> > > --- a/tools/lib/bpf/bpf_core_read.h
> > > +++ b/tools/lib/bpf/bpf_core_read.h
> > > @@ -111,6 +111,40 @@ enum bpf_enum_value_kind {
> > >         val;                                                                  \
> > >  })
> > >
> > > +/*
> > > + * Write to a bitfield, identified by s->field.
> > > + * This is the inverse of BPF_CORE_WRITE_BITFIELD().
> > > + */
> > > +#define BPF_CORE_WRITE_BITFIELD(s, field, new_val) ({                  \
> > > +       void *p = (void *)s + __CORE_RELO(s, field, BYTE_OFFSET);       \
> > > +       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 rpad = rshift - lshift;                            \
> > > +       unsigned long long nval, mask, val;                             \
> > > +                                                                       \
> > > +       asm volatile("" : "+r"(p));                                     \
> > > +                                                                       \
> > > +       switch (byte_size) {                                            \
> > > +       case 1: val = *(unsigned char *)p; break;                       \
> > > +       case 2: val = *(unsigned short *)p; break;                      \
> > > +       case 4: val = *(unsigned int *)p; break;                        \
> > > +       case 8: val = *(unsigned long long *)p; break;                  \
> > > +       }                                                               \
> > > +                                                                       \
> > > +       mask = (~0ULL << rshift) >> lshift;                             \
> > > +       nval = new_val;                                                 \
> > > +       nval = (nval << rpad) & mask;                                   \
> > > +       val = (val & ~mask) | nval;                                     \
> >
> > I'd simplify it to not need nval at all
> >
> > val = (val & ~mask) | ((new_val << rpad) & mask);
> >
> > I actually find it easier to follow and make sure we are not doing
> > anything unexpected. First part before |, we take old value and clear
> > bits we are about to set, second part after |, we take bitfield value,
> > shift it in position, and just in case mask it out if it's too big to
> > fit. Combine, done.
> >
> > Other than that, it looks good.
>
> I mostly left it there for the cast. Cuz injecting the `unsigned long
> long` cast made the line really long. How about this instead?
>
> diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> index a7ffb80e3539..7325a12692a3 100644
> --- a/tools/lib/bpf/bpf_core_read.h
> +++ b/tools/lib/bpf/bpf_core_read.h
> @@ -120,8 +120,8 @@ 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 long long mask, val, nval = new_val;                   \
>         unsigned int rpad = rshift - lshift;                            \
> -       unsigned long long nval, mask, val;                             \
>                                                                         \
>         asm volatile("" : "+r"(p));                                     \
>                                                                         \
> @@ -133,9 +133,7 @@ enum bpf_enum_value_kind {
>         }                                                               \
>                                                                         \
>         mask = (~0ULL << rshift) >> lshift;                             \
> -       nval = new_val;                                                 \
> -       nval = (nval << rpad) & mask;                                   \
> -       val = (val & ~mask) | nval;                                     \
> +       val = (val & ~mask) | ((nval << rpad) & mask);                  \

sgtm

>                                                                         \
>         switch (byte_size) {                                            \
>         case 1: *(unsigned char *)p      = val; break;                  \
>
>
> Thanks,
> Daniel