Add check to ensure that ARG_PTR_TO_MEM is used with either MEM_WRITE or
MEM_RDONLY.

Using ARG_PTR_TO_MEM alone without tags does not make sense because:

- If the helper does not change the argument, missing MEM_RDONLY causes the
verifier to incorrectly reject a read-only buffer.
- If the helper does change the argument, missing MEM_WRITE causes the
verifier to incorrectly assume the memory is unchanged, leading to errors
in code optimization.

Co-developed-by: Shuran Liu <electronlsr@gmail.com>
Signed-off-by: Shuran Liu <electronlsr@gmail.com>
Co-developed-by: Peili Gao <gplhust955@gmail.com>
Signed-off-by: Peili Gao <gplhust955@gmail.com>
Co-developed-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Haoran Ni <haoran.ni.cs@gmail.com>
Signed-off-by: Zesen Liu <ftyghome@gmail.com>
---
 kernel/bpf/verifier.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index f0ca69f888fa..c7ebddb66385 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10349,10 +10349,27 @@ static bool check_btf_id_ok(const struct bpf_func_proto *fn)
 	return true;
 }
 
+static bool check_mem_arg_rw_flag_ok(const struct bpf_func_proto *fn)
+{
+	int i;
+
+	for (i = 0; i < ARRAY_SIZE(fn->arg_type); i++) {
+		enum bpf_arg_type arg_type = fn->arg_type[i];
+
+		if (base_type(arg_type) != ARG_PTR_TO_MEM)
+			continue;
+		if (!(arg_type & (MEM_WRITE | MEM_RDONLY)))
+			return false;
+	}
+
+	return true;
+}
+
 static int check_func_proto(const struct bpf_func_proto *fn, int func_id)
 {
 	return check_raw_mode_ok(fn) &&
 	       check_arg_pair_ok(fn) &&
+		   check_mem_arg_rw_flag_ok(fn) &&
 	       check_btf_id_ok(fn) ? 0 : -EINVAL;
 }
 

-- 
2.43.0

> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index f0ca69f888fa..c7ebddb66385 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -10349,10 +10349,27 @@ static bool check_btf_id_ok(const struct bpf_func_proto *fn)
>  	return true;
>  }
>
> +static bool check_mem_arg_rw_flag_ok(const struct bpf_func_proto *fn)
> +{
> +	int i;
> +
> +	for (i = 0; i < ARRAY_SIZE(fn->arg_type); i++) {
> +		enum bpf_arg_type arg_type = fn->arg_type[i];
> +
> +		if (base_type(arg_type) != ARG_PTR_TO_MEM)
> +			continue;
> +		if (!(arg_type & (MEM_WRITE | MEM_RDONLY)))
> +			return false;

Does this check also apply to ARG_PTR_TO_FIXED_SIZE_MEM? Looking at the
definition in include/linux/bpf.h:

    ARG_PTR_TO_FIXED_SIZE_MEM = MEM_FIXED_SIZE | ARG_PTR_TO_MEM,

Since base_type() masks off the flags, base_type(ARG_PTR_TO_FIXED_SIZE_MEM)
returns ARG_PTR_TO_MEM. But ARG_PTR_TO_FIXED_SIZE_MEM does not include
MEM_WRITE or MEM_RDONLY in its definition.

Several helpers in net/core/filter.c use ARG_PTR_TO_FIXED_SIZE_MEM without
additional flags:

    bpf_tcp_raw_gen_syncookie_ipv4_proto:
        .arg1_type = ARG_PTR_TO_FIXED_SIZE_MEM,

    bpf_tcp_raw_gen_syncookie_ipv6_proto:
        .arg1_type = ARG_PTR_TO_FIXED_SIZE_MEM,

    bpf_tcp_raw_check_syncookie_ipv4_proto:
        .arg1_type = ARG_PTR_TO_FIXED_SIZE_MEM,
        .arg2_type = ARG_PTR_TO_FIXED_SIZE_MEM,

    bpf_tcp_raw_check_syncookie_ipv6_proto:
        .arg1_type = ARG_PTR_TO_FIXED_SIZE_MEM,
        .arg2_type = ARG_PTR_TO_FIXED_SIZE_MEM,

Would these helpers fail check_func_proto() after this change, causing
BPF programs using them to fail verification? Should these prototypes
be updated to include MEM_RDONLY (since they read from the memory), or
should the check skip ARG_PTR_TO_FIXED_SIZE_MEM?

> +	}
> +
> +	return true;
> +}

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/20781625347

You're right. I'll add the missing flags to these prototypes and address this in v2.


> On Jan 7, 2026, at 20:44, bot+bpf-ci@kernel.org wrote:
> 
> Would these helpers fail check_func_proto() after this change, causing
> BPF programs using them to fail verification? Should these prototypes
> be updated to include MEM_RDONLY (since they read from the memory), or
> should the check skip ARG_PTR_TO_FIXED_SIZE_MEM?