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

[Yonghong Song <yonghong.song@linux.dev>]

On 11/22/23 1:20 PM, Daniel Xu wrote:
> Switching to vmlinux.h definitions seems to make the verifier very
> unhappy with bitfield accesses. The error is:
>
>      ; md.u.md2.dir = direction;
>      33: (69) r1 = *(u16 *)(r2 +11)
>      misaligned stack access off (0x0; 0x0)+-64+11 size 2
>
> It looks like disabling CO-RE relocations seem to make the error go
> away.

Thanks for reporting. I did some preliminary investigation and the
failure is due to that we do not support CORE-based bitfield store
yet. Besides disabling CORE-relocation as in this patch, there
are a few ways to do this:
   - Change the code to avoid bitfield store and use 1/2/4/8 byte(s)
     store. A little bit ugly but it should work.
   - Use to-be-supported 'preserve_static_offset'
     (https://reviews.llvm.org/D133361)
     to preserve the offset. This might work (I didn't
     try it yet).
   - Eduard did some early study trying to remove CORE attribute
     (preserve_access_index) from UAPI structures. In this particular
     case, erspan_metadata is in /usr/include/linux/erspan.h.

We will also investigate whether we could store bitfield store
directly with CORE.

>
> Co-developed-by: Antony Antony <antony.antony@secunet.com>
> Signed-off-by: Antony Antony <antony.antony@secunet.com>
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> ---
>   tools/testing/selftests/bpf/progs/test_tunnel_kern.c | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> index 3065a716544d..ec7e04e012ae 100644
> --- a/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> +++ b/tools/testing/selftests/bpf/progs/test_tunnel_kern.c
> @@ -6,6 +6,7 @@
>    * 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

This is a temporary workaround and hopefully we can lift it in the
near future. Please add a comment here with prefix 'Workaround' to
explain why this is needed and later on we can earliy search the
keyword and remember to tackle this.

>   #include "vmlinux.h"
>   #include <bpf/bpf_helpers.h>
>   #include <bpf/bpf_endian.h>