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

Andrii Nakryiko
2023-12-02T00:41:13Z

On Fri, Dec 1, 2023 at 4:13 PM Daniel Xu <dxu@dxuuu.xyz> 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@dxuuu.xyz> 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@gmail.com>
> > > Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
> > > Co-authored-by: Jonathan Lemon <jlemon@aviatrix.com>
> > > Signed-off-by: Jonathan Lemon <jlemon@aviatrix.com>
> > > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> > > ---
> > > 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