Re: [PATCH ipsec-next v2 3/6] libbpf: Add BPF_CORE_WRITE_BITFIELD() macro

[Daniel Xu]

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