linux-kernel - [PATCH bpf-next 1/2] bpf: Allow access to const void pointer arguments in tracing programs

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Message-ID: <20250412170626.3638516-2-kafai.wan@hotmail.com>
Date: Sun, 13 Apr 2025 01:06:25 +0800
From: KaFai Wan <mannkafai@...il.com>
To: martin.lau@...ux.dev,
	ast@...nel.org,
	daniel@...earbox.net,
	andrii@...nel.org,
	eddyz87@...il.com,
	song@...nel.org,
	yonghong.song@...ux.dev,
	john.fastabend@...il.com,
	kpsingh@...nel.org,
	sdf@...ichev.me,
	haoluo@...gle.com,
	jolsa@...nel.org,
	mykolal@...com,
	shuah@...nel.org,
	memxor@...il.com
Cc: bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-kselftest@...r.kernel.org,
	kafai.wan@...mail.com,
	leon.hwang@...ux.dev
Subject: [PATCH bpf-next 1/2] bpf: Allow access to const void pointer arguments in tracing programs

Adding support to access arguments with const void pointer arguments
in tracing programs.

Currently we allow tracing programs to access void pointers. If we try to
access argument which is pointer to const void like 2nd argument in kfree,
verifier will fail to load the program with;

0: R1=ctx() R10=fp0
; asm volatile ("r2 = *(u64 *)(r1 + 8); ");
0: (79) r2 = *(u64 *)(r1 +8)
func 'kfree' arg1 type UNKNOWN is not a struct

Adding is_void_ptr to generic void  pointer check.

Cc: Leon Hwang <leon.hwang@...ux.dev>
Signed-off-by: KaFai Wan <kafai.wan@...mail.com>
---
 kernel/bpf/btf.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/kernel/bpf/btf.c b/kernel/bpf/btf.c
index 16ba36f34dfa..e11d3afd0562 100644
--- a/kernel/bpf/btf.c
+++ b/kernel/bpf/btf.c
@@ -6383,6 +6383,14 @@ struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
 		return prog->aux->attach_btf;
 }
 
+static bool is_void_ptr(struct btf *btf, const struct btf_type *t)
+{
+	/* skip modifiers */
+	t = btf_type_skip_modifiers(btf, t->type, NULL);
+
+	return t->type == 0;
+}
+
 static bool is_int_ptr(struct btf *btf, const struct btf_type *t)
 {
 	/* skip modifiers */
@@ -6776,7 +6784,7 @@ bool btf_ctx_access(int off, int size, enum bpf_access_type type,
 		}
 	}
 
-	if (t->type == 0)
+	if (is_void_ptr(btf, t))
 		/* This is a pointer to void.
 		 * It is the same as scalar from the verifier safety pov.
 		 * No further pointer walking is allowed.
-- 
2.43.0