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Date:   Fri, 11 Sep 2020 11:59:20 +0200
From:   Toke Høiland-Jørgensen <>
To:     Alexei Starovoitov <>
Cc:     Daniel Borkmann <>,
        Martin KaFai Lau <>,
        Song Liu <>, Yonghong Song <>,
        Andrii Nakryiko <>,
        John Fastabend <>,
        Jiri Olsa <>,
        Eelco Chaudron <>,
        KP Singh <>,,
Subject: [PATCH RESEND bpf-next v3 5/9] bpf: Fix context type resolving for
 extension programs

From: Toke Høiland-Jørgensen <>

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

Having following program:

  int test_pkt_md_access(struct __sk_buff *skb)

with its extension:

  int test_pkt_md_access_new(struct __sk_buff *skb)

and tracing that extension with:

  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.

This version of the patch is tweaked slightly from Jiri's original one,
since the refactoring in the previous patches means we have to get the
target prog type from the new variable in prog->aux instead of directly
from the target prog.

Reported-by: Eelco Chaudron <>
Suggested-by: Jiri Olsa <>
Signed-off-by: Toke Høiland-Jørgensen <>
 kernel/bpf/btf.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index e10f13f8251c..1a48253ba168 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -3863,7 +3863,14 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 	info->reg_type = PTR_TO_BTF_ID;
 	if (tgt_prog) {
-		ret = btf_translate_to_vmlinux(log, btf, t, tgt_prog->type, arg);
+		enum bpf_prog_type tgt_type;
+		if (tgt_prog->type == BPF_PROG_TYPE_EXT)
+			tgt_type = tgt_prog->aux->tgt_prog_type;
+		else
+			tgt_type = tgt_prog->type;
+		ret = btf_translate_to_vmlinux(log, btf, t, tgt_type, arg);
 		if (ret > 0) {
 			info->btf_id = ret;
 			return true;

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