linux-kernel - [PATCH] bpf: Specify access type of bpf_sysctl_get

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Message-ID: <20250527165412.533335-1-jmarchan@redhat.com>
Date: Tue, 27 May 2025 18:54:12 +0200
From: "Jerome Marchand" <jmarchan@...hat.com>
To: bpf@...r.kernel.org
Cc: Martin KaFai Lau <martin.lau@...ux.dev>,
	Alexei Starovoitov <ast@...nel.org>,
	Daniel Borkmann <daniel@...earbox.net>,
	Andrii Nakryiko <andrii@...nel.org>,
	linux-kernel@...r.kernel.org,
	Jerome Marchand <jmarchan@...hat.com>
Subject: [PATCH] bpf: Specify access type of bpf_sysctl_get_name args

The second argument of bpf_sysctl_get_name() helper is a pointer to a
buffer that is being written to. However that isn't specify in the
prototype.

Until commit 37cce22dbd51a ("bpf: verifier: Refactor helper access
type tracking"), all helper accesses were considered as a possible
write access by the verifier, so no big harm was done. However, since
then, the verifier might make wrong asssumption about the content of
that address which might lead it to make faulty optimizations (such as
removing code that was wrongly labeled dead). This is what happens in
test_sysctl selftest to the tests related to sysctl_get_name.

Correctly mark the second argument of bpf_sysctl_get_name() as
ARG_PTR_TO_UNINIT_MEM.

Signed-off-by: Jerome Marchand <jmarchan@...hat.com>
---
 kernel/bpf/cgroup.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
index 84f58f3d028a3..09c02a592d24a 100644
--- a/kernel/bpf/cgroup.c
+++ b/kernel/bpf/cgroup.c
@@ -2104,7 +2104,7 @@ static const struct bpf_func_proto bpf_sysctl_get_name_proto = {
 	.gpl_only	= false,
 	.ret_type	= RET_INTEGER,
 	.arg1_type	= ARG_PTR_TO_CTX,
-	.arg2_type	= ARG_PTR_TO_MEM,
+	.arg2_type	= ARG_PTR_TO_UNINIT_MEM,
 	.arg3_type	= ARG_CONST_SIZE,
 	.arg4_type	= ARG_ANYTHING,
 };
-- 
2.49.0