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

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2023-12-01 at 13:51 -0700, Daniel Xu wrote:
> 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@gmail.com>
> > Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> 
> Just pointing out I inserted Eduard's tags here. Eduard - I hope that's
> OK. Not sure what the usual procedure for this is.

Not that I did a big contribution, you and Andrii figured out a much
better (and correct) expression :) I'm fine with and without this tag,
thank you for working on this.