Re: [PATCH v2 bpf-next 5/7] bpf: support BPF ksym variables in kernel modules
From: Hao Luo <hidden>
Date: 2021-01-11 19:00:50
Also in:
bpf
Acked-by: Hao Luo <redacted>, with a suggestion on adding a comment. On Fri, Jan 8, 2021 at 2:09 PM Andrii Nakryiko [off-list ref] 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@kernel.org> --- 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(-)
[...]
quoted hunk ↗ jump to hunk
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