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 >
