[PATCH bpf-next 1/2] bpf: Fix context type resolving for extension programs

[Jiri Olsa <jolsa@kernel.org>]

Eelco reported we can't properly access arguments if the tracing
program is attached to extension program.

Having following program:

  SEC("classifier/test_pkt_md_access")
  int test_pkt_md_access(struct __sk_buff *skb)

with its extension:

  SEC("freplace/test_pkt_md_access")
  int test_pkt_md_access_new(struct __sk_buff *skb)

and tracing that extension with:

  SEC("fentry/test_pkt_md_access_new")
  int BPF_PROG(fentry, struct sk_buff *skb)

It's not possible to access skb argument in the fentry program,
with following error from verifier:

  ; int BPF_PROG(fentry, struct sk_buff *skb)
  0: (79) r1 = *(u64 *)(r1 +0)
  invalid bpf_context access off=0 size=8

The problem is that btf_ctx_access gets the context type for the
traced program, which is in this case the extension.

But when we trace extension program, we want to get the context
type of the program that the extension is attached to, so we can
access the argument properly in the trace program.

Reported-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
---
 kernel/bpf/btf.c | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index f9ac6935ab3c..37ad01c32e5a 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3859,6 +3859,14 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	}
 
 	info->reg_type = PTR_TO_BTF_ID;
+
+	/* When we trace extension program, we want to get the context
+	 * type of the program that the extension is attached to, so
+	 * we can access the argument properly in the trace program.
+	 */
+	if (tgt_prog && tgt_prog->type == BPF_PROG_TYPE_EXT)
+		tgt_prog = tgt_prog->aux->linked_prog;
+
 	if (tgt_prog) {
 		ret = btf_translate_to_vmlinux(log, btf, t, tgt_prog->type, arg);
 		if (ret > 0) {
-- 
2.26.2