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); \ 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; \ switch (byte_size) { \ case 1: *(unsigned char *)p = val; break; \ Thanks, Daniel
