On Tue, Mar 12, 2024 at 11:32 AM Kui-Feng Lee <thinker.li@xxxxxxxxx> wrote:
>
> Accept additional fields of a struct_ops type with all zero values even if
> these fields are not in the corresponding type in the kernel. This provides
> a way to be backward compatible. User space programs can use the same map
> on a machine running an old kernel by clearing fields that do not exist in
> the kernel.
>
> Signed-off-by: Kui-Feng Lee <thinker.li@xxxxxxxxx>
> ---
> tools/lib/bpf/libbpf.c | 30 +++++++++++++++++++++++-------
> 1 file changed, 23 insertions(+), 7 deletions(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index efab29b8935b..715879796046 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1131,11 +1131,33 @@ static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
> __u32 kern_member_idx;
> const char *mname;
>
> + mtype = skip_mods_and_typedefs(btf, member->type, &mtype_id);
you don't need to move this up here, btf__resolve_size() can skip
modifiers and typedefs just fine, so let's keep this one below and
just pass member->type to btf__resolve_size()
> mname = btf__name_by_offset(btf, member->name_off);
> + moff = member->offset / 8;
> + mdata = data + moff;
> + msize = btf__resolve_size(btf, mtype_id);
> + if (msize < 0) {
> + pr_warn("struct_ops init_kern %s: fails to resolve the size of member %s\n",
s/fails/failed/
> + map->name, mname);
> + return msize;
> + }
> +
> kern_member = find_member_by_name(kern_btf, kern_type, mname);
> if (!kern_member) {
> pr_warn("struct_ops init_kern %s: Cannot find member %s in kernel BTF\n",
> map->name, mname);
> +
> + /* Skip all zeros or null fields if they are not
> + * presented in the kernel BTF.
> + */
> + if (btf_is_ptr(mtype)) {
> + if (!st_ops->progs[i])
> + continue;
so, this is both unnecessary to check for NULL (libbpf_is_mem_zeroed
will do it just fine for pointers as well), but it's also wrong
because user could have set this program pointer through skeleton's
shadow type, while here you won't yet see. So let's drop this
btf_is_ptr() special case, and just do generic libbpf_is_mem_zeroed()
check
> + } else {
> + if (libbpf_is_mem_zeroed(mdata, msize))
> + continue;
I think it's worth emitting informational message here, something like
pr_info("struct_ops %s: member %s not found in kernel, skipping it as
it's set to zero\n", ...")
?
and move that pr_warn("Cannot find member %s in kernel BTF") after
this check, so we don't have scary-looking error-level message
> + }
> +
> return -ENOTSUP;
> }
>
> @@ -1147,13 +1169,8 @@ static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
> return -ENOTSUP;
> }
>
> - moff = member->offset / 8;
> kern_moff = kern_member->offset / 8;
> -
> - mdata = data + moff;
> kern_mdata = kern_data + kern_moff;
> -
> - mtype = skip_mods_and_typedefs(btf, member->type, &mtype_id);
> kern_mtype = skip_mods_and_typedefs(kern_btf, kern_member->type,
> &kern_mtype_id);
> if (BTF_INFO_KIND(mtype->info) !=
> @@ -1230,9 +1247,8 @@ static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
> continue;
> }
>
> - msize = btf__resolve_size(btf, mtype_id);
> kern_msize = btf__resolve_size(kern_btf, kern_mtype_id);
> - if (msize < 0 || kern_msize < 0 || msize != kern_msize) {
> + if (kern_msize < 0 || msize != kern_msize) {
> pr_warn("struct_ops init_kern %s: Error in size of member %s: %zd != %zd(kernel)\n",
> map->name, mname, (ssize_t)msize,
> (ssize_t)kern_msize);
> --
> 2.34.1
>
