On Tue, Nov 28, 2023 at 10:13:50AM -0600, Daniel Xu wrote:
> On Mon, Nov 27, 2023 at 08:06:01PM -0800, Yonghong Song wrote:
> >
> > On 11/27/23 7:01 PM, Daniel Xu wrote:
> > > On Mon, Nov 27, 2023 at 02:45:11PM -0600, Daniel Xu wrote:
> > > > On Sun, Nov 26, 2023 at 09:53:04PM -0800, Yonghong Song wrote:
> > > > > On 11/27/23 12:44 AM, Yonghong Song wrote:
> > > > > > On 11/26/23 8:52 PM, Eduard Zingerman wrote:
> > > > > > > On Sun, 2023-11-26 at 18:04 -0600, Daniel Xu wrote:
> > > > > > > [...]
> > > > > > > > > Tbh I'm not sure. This test passes with preserve_static_offset
> > > > > > > > > because it suppresses preserve_access_index. In general clang
> > > > > > > > > translates bitfield access to a set of IR statements like:
> > > > > > > > >
> > > > > > > > > C:
> > > > > > > > > struct foo {
> > > > > > > > > unsigned _;
> > > > > > > > > unsigned a:1;
> > > > > > > > > ...
> > > > > > > > > };
> > > > > > > > > ... foo->a ...
> > > > > > > > >
> > > > > > > > > IR:
> > > > > > > > > %a = getelementptr inbounds %struct.foo, ptr %0, i32 0, i32 1
> > > > > > > > > %bf.load = load i8, ptr %a, align 4
> > > > > > > > > %bf.clear = and i8 %bf.load, 1
> > > > > > > > > %bf.cast = zext i8 %bf.clear to i32
> > > > > > > > >
> > > > > > > > > With preserve_static_offset the getelementptr+load are replaced by a
> > > > > > > > > single statement which is preserved as-is till code generation,
> > > > > > > > > thus load with align 4 is preserved.
> > > > > > > > >
> > > > > > > > > On the other hand, I'm not sure that clang guarantees that load or
> > > > > > > > > stores used for bitfield access would be always aligned according to
> > > > > > > > > verifier expectations.
> > > > > > > > >
> > > > > > > > > I think we should check if there are some clang knobs that prevent
> > > > > > > > > generation of unaligned memory access. I'll take a look.
> > > > > > > > Is there a reason to prefer fixing in compiler? I'm not opposed to it,
> > > > > > > > but the downside to compiler fix is it takes years to propagate and
> > > > > > > > sprinkles ifdefs into the code.
> > > > > > > >
> > > > > > > > Would it be possible to have an analogue of BPF_CORE_READ_BITFIELD()?
> > > > > > > Well, the contraption below passes verification, tunnel selftest
> > > > > > > appears to work. I might have messed up some shifts in the macro,
> > > > > > > though.
> > > > > > I didn't test it. But from high level it should work.
> > > > > >
> > > > > > > Still, if clang would peek unlucky BYTE_{OFFSET,SIZE} for a particular
> > > > > > > field access might be unaligned.
> > > > > > clang should pick a sensible BYTE_SIZE/BYTE_OFFSET to meet
> > > > > > alignment requirement. This is also required for BPF_CORE_READ_BITFIELD.
> > > > > >
> > > > > > > ---
> > > > > > >
> > > > > > > diff --git a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> > > > > > > b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> > > > > > > index 3065a716544d..41cd913ac7ff 100644
> > > > > > > --- a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> > > > > > > +++ b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> > > > > > > @@ -9,6 +9,7 @@
> > > > > > > #include "vmlinux.h"
> > > > > > > #include <bpf/bpf_helpers.h>
> > > > > > > #include <bpf/bpf_endian.h>
> > > > > > > +#include <bpf/bpf_core_read.h>
> > > > > > > #include "bpf_kfuncs.h"
> > > > > > > #include "bpf_tracing_net.h"
> > > > > > > @@ -144,6 +145,38 @@ int ip6gretap_get_tunnel(struct __sk_buff *skb)
> > > > > > > return TC_ACT_OK;
> > > > > > > }
> > > > > > > +#define BPF_CORE_WRITE_BITFIELD(s, field, new_val) ({ \
> > > > > > > + void *p = (void *)s + __CORE_RELO(s, field, BYTE_OFFSET); \
> > > > > > > + unsigned byte_size = __CORE_RELO(s, field, BYTE_SIZE); \
> > > > > > > + unsigned lshift = __CORE_RELO(s, field, LSHIFT_U64); \
> > > > > > > + unsigned rshift = __CORE_RELO(s, field, RSHIFT_U64); \
> > > > > > > + unsigned bit_size = (rshift - lshift); \
> > > > > > > + unsigned long long nval, val, hi, lo; \
> > > > > > > + \
> > > > > > > + asm volatile("" : "=r"(p) : "0"(p)); \
> > > > > > Use 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; \
> > > > > > > + } \
> > > > > > > + hi = val >> (bit_size + rshift); \
> > > > > > > + hi <<= bit_size + rshift; \
> > > > > > > + lo = val << (bit_size + lshift); \
> > > > > > > + lo >>= bit_size + lshift; \
> > > > > > > + nval = new_val; \
> > > > > > > + nval <<= lshift; \
> > > > > > > + nval >>= rshift; \
> > > > > > > + val = hi | nval | lo; \
> > > > > > > + 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; \
> > > > > > > + } \
> > > > > > > +})
> > > > > > I think this should be put in libbpf public header files but not sure
> > > > > > where to put it. bpf_core_read.h although it is core write?
> > > > > >
> > > > > > But on the other hand, this is a uapi struct bitfield write,
> > > > > > strictly speaking, CORE write is really unnecessary here. It
> > > > > > would be great if we can relieve users from dealing with
> > > > > > such unnecessary CORE writes. In that sense, for this particular
> > > > > > case, I would prefer rewriting the code by using byte-level
> > > > > > stores...
> > > > > or preserve_static_offset to clearly mean to undo bitfield CORE ...
> > > > Ok, I will do byte-level rewrite for next revision.
> > > [...]
> > >
> > > This patch seems to work: https://pastes.dxuuu.xyz/0glrf9 .
> > >
> > > But I don't think it's very pretty. Also I'm seeing on the internet that
> > > people are saying the exact layout of bitfields is compiler dependent.
> >
> > Any reference for this (exact layout of bitfields is compiler dependent)?
> >
> > > So I am wondering if these byte sized writes are correct. For that
> > > matter, I am wondering how the GCC generated bitfield accesses line up
> > > with clang generated BPF bytecode. Or why uapi contains a bitfield.
> >
> > One thing for sure is memory layout of bitfields should be the same
> > for both clang and gcc as it is determined by C standard. Register
> > representation and how to manipulate could be different for different
> > compilers.
>
> I was reading this thread:
> https://github.com/Lora-net/LoRaMac-node/issues/697. It's obviously not
> authoritative, but they sure sound confident!
>
> I think I've also heard it before a long time ago when I was working on
> adding bitfield support to bpftrace.
Wikipedia [0] also claims this:
The layout of bit fields in a C struct is
implementation-defined. For behavior that remains predictable
across compilers, it may be preferable to emulate bit fields
with a primitive and bit operators:
[0]: https://en.wikipedia.org/wiki/Bit_field#C_programming_language
