Re: [PATCH bpf-next v4 04/10] libbpf: Add BPF_CORE_WRITE_BITFIELD() macro

[Andrii Nakryiko]

On Mon, Dec 4, 2023 at 12:57 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);
>
> 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 now 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).
>
> Finally, we can examine the core of the write side algorithm:
>
>         mask = (~0ULL << rshift) >> lshift;              // 1
>         val = (val & ~mask) | ((nval << rpad) & mask);   // 2
>
> 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. On the left of the `|`, mask out the bitfield bits. This creates
>    0s where the new bitfield bits will go. On the right of the `|`,
>    bring nval into the correct bit position and mask out any bits
>    that fall outside of the bitfield. Finally, by bor'ing the two
>    halves, we get the final set of bits to write back.
>
> [0]: https://reviews.llvm.org/D133361
> Co-developed-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> Signed-off-by: Eduard Zingerman <eddyz87@xxxxxxxxx>
> Co-developed-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 | 32 ++++++++++++++++++++++++++++++++
>  1 file changed, 32 insertions(+)
>

LGTM

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

> diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h
> index 1ac57bb7ac55..7325a12692a3 100644
> --- a/tools/lib/bpf/bpf_core_read.h
> +++ b/tools/lib/bpf/bpf_core_read.h
> @@ -111,6 +111,38 @@ 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 long long mask, val, nval = new_val;                   \
> +       unsigned int rpad = rshift - lshift;                            \
> +                                                                       \
> +       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;                             \
> +       val = (val & ~mask) | ((nval << rpad) & mask);                  \
> +                                                                       \
> +       switch (byte_size) {                                            \
> +       case 1: *(unsigned char *)p      = val; break;                  \
> +       case 2: *(unsigned short *)p     = val; break;                  \
> +       case 4: *(unsigned int *)p       = val; break;                  \
> +       case 8: *(unsigned long long *)p = val; break;                  \
> +       }                                                               \
> +})
> +
>  #define ___bpf_field_ref1(field)       (field)
>  #define ___bpf_field_ref2(type, field) (((typeof(type) *)0)->field)
>  #define ___bpf_field_ref(args...)                                          \
> --
> 2.42.1
>