bpf: Use named fields for certain bpf uapi structs

bpf: Use named fields for certain bpf uapi structs

Martin and Vadim reported a verifier failure with bpf_dynptr usage.
The issue is mentioned but Vadim workarounded the issue with source
change ([1]). The below describes what is the issue and why there
is a verification failure.

  int BPF_PROG(skb_crypto_setup) {
    struct bpf_dynptr algo, key;
    ...

    bpf_dynptr_from_mem(..., ..., 0, &algo);
    ...
  }

The bpf program is using vmlinux.h, so we have the following definition in
vmlinux.h:
  struct bpf_dynptr {
        long: 64;
        long: 64;
  };
Note that in uapi header bpf.h, we have
  struct bpf_dynptr {
        long: 64;
        long: 64;
} __attribute__((aligned(8)));

So we lost alignment information for struct bpf_dynptr by using vmlinux.h.
Let us take a look at a simple program below:
  $ cat align.c
  typedef unsigned long long __u64;
  struct bpf_dynptr_no_align {
        __u64 :64;
        __u64 :64;
  };
  struct bpf_dynptr_yes_align {
        __u64 :64;
        __u64 :64;
  } __attribute__((aligned(8)));

  void bar(void *, void *);
  int foo() {
    struct bpf_dynptr_no_align a;
    struct bpf_dynptr_yes_align b;
    bar(&a, &b);
    return 0;
  }
  $ clang --target=bpf -O2 -S -emit-llvm align.c

Look at the generated IR file align.ll:
  ...
  %a = alloca %struct.bpf_dynptr_no_align, align 1
  %b = alloca %struct.bpf_dynptr_yes_align, align 8
  ...

The compiler dictates the alignment for struct bpf_dynptr_no_align is 1 and
the alignment for struct bpf_dynptr_yes_align is 8. So theoretically compiler
could allocate variable %a with alignment 1 although in reallity the compiler
may choose a different alignment by considering other local variables.

In [1], the verification failure happens because variable 'algo' is allocated
on the stack with alignment 4 (fp-28). But the verifer wants its alignment
to be 8.

To fix the issue, the RFC patch ([1]) tried to add '__attribute__((aligned(8)))'
to struct bpf_dynptr plus other similar structs. Andrii suggested that
we could directly modify uapi struct with named fields like struct 'bpf_iter_num':
  struct bpf_iter_num {
        /* opaque iterator state; having __u64 here allows to preserve correct
         * alignment requirements in vmlinux.h, generated from BTF
         */
        __u64 __opaque[1];
  } __attribute__((aligned(8)));

Indeed, adding named fields for those affected structs in this patch can preserve
alignment when bpf program references them in vmlinux.h. With this patch,
the verification failure in [1] can also be resolved.

  [1] https://lore.kernel.org/bpf/1b100f73-7625-4c1f-3ae5-50ecf84d3ff0@linux.dev/
  [2] https://lore.kernel.org/bpf/20231103055218.2395034-1-yonghong.song@linux.dev/

Cc: Vadim Fedorenko <vadfed@meta.com>
Cc: Martin KaFai Lau <martin.lau@linux.dev>
Suggested-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231104024900.1539182-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>