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

[Daniel Xu]

On Fri, Dec 01, 2023 at 01:23:14PM -0700, Daniel Xu via Devel 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 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).
> 
> 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>

Just pointing out I inserted Eduard's tags here. Eduard - I hope that's
OK. Not sure what the usual procedure for this is.

> 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(+)
> 

[..]