On Mon, Feb 26, 2024 at 5:04 PM Kui-Feng Lee <thinker.li@xxxxxxxxx> wrote:
>
> Convert st_ops->data to the shadow type of the struct_ops map. The shadow
> type of a struct_ops type is a variant of the original struct type
> providing a way to access/change the values in the maps of the struct_ops
> type.
>
> bpf_map__initial_value() will return st_ops->data for struct_ops types. The
> skeleton is going to use it as the pointer to the shadow type of the
> original struct type.
>
> One of the main differences between the original struct type and the shadow
> type is that all function pointers of the shadow type are converted to
> pointers of struct bpf_program. Users can replace these bpf_program
> pointers with other BPF programs. The st_ops->progs[] will be updated
> before updating the value of a map to reflect the changes made by users.
>
> Signed-off-by: Kui-Feng Lee <thinker.li@xxxxxxxxx>
> ---
> tools/lib/bpf/libbpf.c | 21 ++++++++++++++++++++-
> 1 file changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
> index 465b50235a01..2d22344fb127 100644
> --- a/tools/lib/bpf/libbpf.c
> +++ b/tools/lib/bpf/libbpf.c
> @@ -1102,6 +1102,9 @@ static int bpf_map__init_kern_struct_ops(struct bpf_map *map)
> if (btf_is_ptr(mtype)) {
> struct bpf_program *prog;
>
> + /* Update the value from the shadow type */
> + st_ops->progs[i] = *(struct bpf_program **)mdata;
> +
it's unsettling to just cast a pointer like this without any
validation. It's too easy for users to set either some garbage there
or struct bpf_program * pointer from some other skeleton.
Luckily, validation is pretty simple, we can just iterate over all
bpf_object's programs and check if any of them matches the value of
the mdata pointer. If not, error out with meaningful error.
Also, even if the bpf_program pointer is correct, it could be a
program of the wrong type, so I think we should add a bit more
validation here, given these pointers are set by users directly after
bpf_object is opened.
> prog = st_ops->progs[i];
> if (!prog)
> continue;
> @@ -9308,7 +9311,9 @@ static struct bpf_map *find_struct_ops_map_by_offset(struct bpf_object *obj,
> return NULL;
> }
>
> -/* Collect the reloc from ELF and populate the st_ops->progs[] */
> +/* Collect the reloc from ELF, populate the st_ops->progs[], and update
> + * st_ops->data for shadow type.
> + */
> static int bpf_object__collect_st_ops_relos(struct bpf_object *obj,
> Elf64_Shdr *shdr, Elf_Data *data)
> {
> @@ -9422,6 +9427,14 @@ static int bpf_object__collect_st_ops_relos(struct bpf_object *obj,
> }
>
> st_ops->progs[member_idx] = prog;
> +
> + /* st_ops->data will be expose to users, being returned by
typo: exposed
> + * bpf_map__initial_value() as a pointer to the shadow
> + * type. All function pointers in the original struct type
> + * should be converted to a pointer to struct bpf_program
> + * in the shadow type.
> + */
> + *((struct bpf_program **)(st_ops->data + moff)) = prog;
> }
>
> return 0;
> @@ -9880,6 +9893,12 @@ int bpf_map__set_initial_value(struct bpf_map *map,
>
> void *bpf_map__initial_value(struct bpf_map *map, size_t *psize)
> {
> + if (bpf_map__is_struct_ops(map)) {
> + if (psize)
> + *psize = map->def.value_size;
> + return map->st_ops->data;
> + }
> +
> if (!map->mmaped)
> return NULL;
> *psize = map->def.value_size;
> --
> 2.34.1
>
