Re: [PATCH bpf-next 4/6] libbpf: add kernel module BTF support for CO-RE relocations

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Thu, Nov 19, 2020 at 03:22:42PM -0800, Andrii Nakryiko wrote:
> Teach libbpf to search for candidate types for CO-RE relocations across kernel
> modules BTFs, in addition to vmlinux BTF. If at least one candidate type is
> found in vmlinux BTF, kernel module BTFs are not iterated. If vmlinux BTF has
> no matching candidates, then find all kernel module BTFs and search for all
> matching candidates across all of them.
> 
> Kernel's support for module BTFs are inferred from the support for BTF name
> pointer in BPF UAPI.
> 
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  tools/lib/bpf/libbpf.c | 185 ++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 172 insertions(+), 13 deletions(-)
> 

[...]

> +static int probe_module_btf(void)
> +{
> +	static const char strs[] = "\0int";
> +	__u32 types[] = {
> +		/* int */
> +		BTF_TYPE_INT_ENC(1, BTF_INT_SIGNED, 0, 32, 4),
> +	};
> +	struct bpf_btf_info info;
> +	__u32 len = sizeof(info);
> +	char name[16];
> +	int fd, err;
> +
> +	fd = libbpf__load_raw_btf((char *)types, sizeof(types), strs, sizeof(strs));
> +	if (fd < 0)
> +		return 0; /* BTF not supported at all */
> +
> +	len = sizeof(info);

nit: reinit of len

> +	memset(&info, 0, sizeof(info));

use len in memset

> +	info.name = ptr_to_u64(name);
> +	info.name_len = sizeof(name);
> +
> +	/* check that BPF_OBJ_GET_INFO_BY_FD supports specifying name pointer;
> +	 * kernel's module BTF support coincides with support for
> +	 * name/name_len fields in struct bpf_btf_info.
> +	 */
> +	err = bpf_obj_get_info_by_fd(fd, &info, &len);
> +	close(fd);
> +	return !err;
> +}

[...]