On Fri, Dec 1, 2023 at 11:11 AM Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx> wrote: > > On Thu, Nov 30, 2023 at 5:33 PM Daniel Xu <dxu@xxxxxxxxx> wrote: > > > > On Tue, Nov 28, 2023 at 07:59:01PM +0200, Eduard Zingerman wrote: > > > On Tue, 2023-11-28 at 10:54 -0700, Daniel Xu wrote: > > > > 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 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. > > > > > > > > [0]: https://reviews.llvm.org/D133361 > > > > From: Eduard Zingerman <eddyz87@xxxxxxxxx> > > > > > > > > Signed-off-by: Daniel Xu <dxu@xxxxxxxxx> > > > > --- > > > > > > Could you please also add a selftest (or several) using __retval() > > > annotation for this macro? > > > > Good call about adding tests -- I found a few bugs with the code from > > the other thread. But boy did they take a lot of brain cells to figure > > out. > > > > There was some 6th grade algebra involved too -- I'll do my best to > > explain it in the commit msg for v3. > > > > > > Here are the fixes in case you are curious: > > > > diff --git a/tools/lib/bpf/bpf_core_read.h b/tools/lib/bpf/bpf_core_read.h > > index 7a764f65d299..8f02c558c0ff 100644 > > --- a/tools/lib/bpf/bpf_core_read.h > > +++ b/tools/lib/bpf/bpf_core_read.h > > @@ -120,7 +120,9 @@ 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 int bit_size = (rshift - lshift); \ > > + unsigned int bit_size = (64 - rshift); \ > > + unsigned int hi_size = lshift; \ > > + unsigned int lo_size = (rshift - lshift); \ > > nit: let's drop unnecessary () > > > unsigned long long nval, val, hi, lo; \ > > \ > > asm volatile("" : "+r"(p)); \ > > @@ -131,13 +133,13 @@ enum bpf_enum_value_kind { > > case 4: val = *(unsigned int *)p; break; \ > > case 8: val = *(unsigned long long *)p; break; \ > > } \ > > - hi = val >> (bit_size + rshift); \ > > - hi <<= bit_size + rshift; \ > > - lo = val << (bit_size + lshift); \ > > - lo >>= bit_size + lshift; \ > > + hi = val >> (64 - hi_size); \ > > + hi <<= 64 - hi_size; \ > > + lo = val << (64 - lo_size); \ > > + lo >>= 64 - lo_size; \ > > nval = new_val; \ > > - nval <<= lshift; \ > > - nval >>= rshift; \ > > + nval <<= (64 - bit_size); \ > > + nval >>= (64 - bit_size - lo_size); \ > > val = hi | nval | lo; \ > > this looks.. unusual. I'd imagine we calculate a mask, mask out bits > we are replacing, and then OR with new values, roughly (assuming all > the right left/right shift values and stuff) > > /* clear bits */ > val &= ~(bitfield_mask << shift); we can also calculate shifted mask with just bitfield_mask = (-1ULL) << some_left_shift >> some_right_shift; val &= ~bitfield_mask; > /* set bits */ > val |= (nval & bitfield_mask) << shift; > > ? > > > switch (byte_size) { \ > > case 1: *(unsigned char *)p = val; break; \ > > > > > > Thanks, > > Daniel
