Hi,
On Sun, Nov 26, 2023 at 10:14:21PM +0200, Eduard Zingerman wrote:
> On Sat, 2023-11-25 at 20:22 -0800, Yonghong Song wrote:
> [...]
> > --- a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> > +++ b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> > @@ -6,7 +6,10 @@
> > * modify it under the terms of version 2 of the GNU General Public
> > * License as published by the Free Software Foundation.
> > */
> > -#define BPF_NO_PRESERVE_ACCESS_INDEX
> > +#if __has_attribute(preserve_static_offset)
> > +struct __attribute__((preserve_static_offset)) erspan_md2;
> > +struct __attribute__((preserve_static_offset)) erspan_metadata;
> > +#endif
> > #include "vmlinux.h"
> [...]
> > int bpf_skb_get_fou_encap(struct __sk_buff *skb_ctx,
> > @@ -174,9 +177,13 @@ int erspan_set_tunnel(struct __sk_buff *skb)
> > __u8 hwid = 7;
> >
> > md.version = 2;
> > +#if __has_attribute(preserve_static_offset)
> > md.u.md2.dir = direction;
> > md.u.md2.hwid = hwid & 0xf;
> > md.u.md2.hwid_upper = (hwid >> 4) & 0x3;
> > +#else
> > + /* Change bit-field store to byte(s)-level stores. */
> > +#endif
> > #endif
> >
> > ret = bpf_skb_set_tunnel_opt(skb, &md, sizeof(md));
> >
> > ====
> >
> > Eduard, could you double check whether this is a valid use case
> > to solve this kind of issue with preserve_static_offset attribute?
>
> 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()?
Thanks,
Daniel
