Re: [PATCH ipsec-next v1 6/7] bpf: selftests: test_tunnel: Disable CO-RE relocations

[Daniel Xu]

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.

Just wondering, though: will bpftool be able to generate the appropriate
annotations for uapi structs? IIUC uapi structs look the same in BTF as
any other struct.

> 
> [...]
> 

Thanks,
Daniel