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[34.141.217.155]) by smtp.gmail.com with ESMTPSA id 4fb4d7f45d1cf-64751035b75sm14363010a12.20.2025.12.01.10.48.36 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 01 Dec 2025 10:48:37 -0800 (PST) Date: Mon, 1 Dec 2025 18:48:33 +0000 From: Matt Bobrowski To: Shuran Liu Cc: song@kernel.org, bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, eddyz87@gmail.com, yonghong.song@linux.dev, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@fomichev.me, haoluo@google.com, jolsa@kernel.org, rostedt@goodmis.org, mhiramat@kernel.org, mathieu.desnoyers@efficios.com, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org, Zesen Liu , Peili Gao , Haoran Ni Subject: Re: [PATCH bpf 1/2] bpf: mark bpf_d_path() buffer as writeable Message-ID: References: <20251201143813.5212-1-electronlsr@gmail.com> <20251201143813.5212-2-electronlsr@gmail.com> Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20251201143813.5212-2-electronlsr@gmail.com> On Mon, Dec 01, 2025 at 10:38:12PM +0800, Shuran Liu wrote: > Commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type > tracking") started distinguishing read vs write accesses performed by > helpers. > > The second argument of bpf_d_path() is a pointer to a buffer that the > helper fills with the resulting path. However, its prototype currently > uses ARG_PTR_TO_MEM without MEM_WRITE. > > Before 37cce22dbd51, helper accesses were conservatively treated as > potential writes, so this mismatch did not cause issues. Since that > commit, the verifier may incorrectly assume that the buffer contents > are unchanged across the helper call and base its optimizations on this > wrong assumption. This can lead to misbehaviour in BPF programs that > read back the buffer, such as prefix comparisons on the returned path. > > Fix this by marking the second argument of bpf_d_path() as > ARG_PTR_TO_MEM | MEM_WRITE so that the verifier correctly models the > write to the caller-provided buffer. > > Fixes: 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking") > Co-developed-by: Zesen Liu > Signed-off-by: Zesen Liu > Co-developed-by: Peili Gao > Signed-off-by: Peili Gao > Co-developed-by: Haoran Ni > Signed-off-by: Haoran Ni > Signed-off-by: Shuran Liu Reviewed-by: Matt Bobrowski > --- > kernel/trace/bpf_trace.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c > index 4f87c16d915a..49e0bdaa7a1b 100644 > --- a/kernel/trace/bpf_trace.c > +++ b/kernel/trace/bpf_trace.c > @@ -965,7 +965,7 @@ static const struct bpf_func_proto bpf_d_path_proto = { > .ret_type = RET_INTEGER, > .arg1_type = ARG_PTR_TO_BTF_ID, > .arg1_btf_id = &bpf_d_path_btf_ids[0], > - .arg2_type = ARG_PTR_TO_MEM, > + .arg2_type = ARG_PTR_TO_MEM | MEM_WRITE, > .arg3_type = ARG_CONST_SIZE_OR_ZERO, > .allowed = bpf_d_path_allowed, > }; > -- > 2.52.0 >