Acked-by: Hao Luo <haoluo@xxxxxxxxxx>, with a suggestion on adding a comment.
On Fri, Jan 8, 2021 at 2:09 PM Andrii Nakryiko <andrii@xxxxxxxxxx> wrote:
>
> Add support for directly accessing kernel module variables from BPF programs
> using special ldimm64 instructions. This functionality builds upon vmlinux
> ksym support, but extends ldimm64 with src_reg=BPF_PSEUDO_BTF_ID to allow
> specifying kernel module BTF's FD in insn[1].imm field.
>
> During BPF program load time, verifier will resolve FD to BTF object and will
> take reference on BTF object itself and, for module BTFs, corresponding module
> as well, to make sure it won't be unloaded from under running BPF program. The
> mechanism used is similar to how bpf_prog keeps track of used bpf_maps.
>
> One interesting change is also in how per-CPU variable is determined. The
> logic is to find .data..percpu data section in provided BTF, but both vmlinux
> and module each have their own .data..percpu entries in BTF. So for module's
> case, the search for DATASEC record needs to look at only module's added BTF
> types. This is implemented with custom search function.
>
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> ---
> include/linux/bpf.h | 10 +++
> include/linux/bpf_verifier.h | 3 +
> include/linux/btf.h | 3 +
> kernel/bpf/btf.c | 31 +++++++-
> kernel/bpf/core.c | 23 ++++++
> kernel/bpf/verifier.c | 149 ++++++++++++++++++++++++++++-------
> 6 files changed, 189 insertions(+), 30 deletions(-)
[...]
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 17270b8404f1..af94c6871ab8 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9703,6 +9703,31 @@ static int do_check(struct bpf_verifier_env *env)
> return 0;
> }
>
> +static int find_btf_percpu_datasec(struct btf *btf)
> +{
> + const struct btf_type *t;
> + const char *tname;
> + int i, n;
> +
It would be good to add a short comment here explaining the reason why
the search for DATASEC in the module case needs to skip entries.
> + n = btf_nr_types(btf);
> + if (btf_is_module(btf))
> + i = btf_nr_types(btf_vmlinux);
> + else
> + i = 1;
> +
> + for(; i < n; i++) {
> + t = btf_type_by_id(btf, i);
> + if (BTF_INFO_KIND(t->info) != BTF_KIND_DATASEC)
> + continue;
> +
> + tname = btf_name_by_offset(btf, t->name_off);
> + if (!strcmp(tname, ".data..percpu"))
> + return i;
> + }
> +
> + return -ENOENT;
> +}
[...]
> 2.24.1
>
