Re: [RFC PATCH bpf 1/2] bpf: Fix memory access tags in helper prototypes

[Amery Hung]

On Sat, Dec 20, 2025 at 3:35 AM Zesen Liu <ftyghome@xxxxxxxxx> wrote:
>
> After commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking"),
> the verifier started relying on the access type tags in helper
> function prototypes to perform memory access optimizations.
>
> Currently, several helper functions utilizing ARG_PTR_TO_MEM lack the
> corresponding MEM_RDONLY or MEM_WRITE tags. This omission causes the
> verifier to incorrectly assume that the buffer contents are unchanged
> across the helper call. Consequently, the verifier may optimize away
> subsequent reads based on this wrong assumption, leading to correctness
> issues.
>
> Similar issues were recently addressed for specific helpers in commit
> ac44dcc788b9 ("bpf: Fix verifier assumptions of bpf_d_path's output buffer")
> and commit 2eb7648558a7 ("bpf: Specify access type of bpf_sysctl_get_name args").
>
> Fix these prototypes by adding the correct memory access tags.
>
> Fixes: 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking")
> Co-developed-by: Shuran Liu <electronlsr@xxxxxxxxx>
> Signed-off-by: Shuran Liu <electronlsr@xxxxxxxxx>
> Co-developed-by: Peili Gao <gplhust955@xxxxxxxxx>
> Signed-off-by: Peili Gao <gplhust955@xxxxxxxxx>
> Co-developed-by: Haoran Ni <haoran.ni.cs@xxxxxxxxx>
> Signed-off-by: Haoran Ni <haoran.ni.cs@xxxxxxxxx>
> Signed-off-by: Zesen Liu <ftyghome@xxxxxxxxx>
> ---
>  kernel/bpf/helpers.c     | 2 +-
>  kernel/trace/bpf_trace.c | 6 +++---
>  net/core/filter.c        | 8 ++++----
>  3 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index db72b96f9c8c..f66284f8ec2c 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -1077,7 +1077,7 @@ const struct bpf_func_proto bpf_snprintf_proto = {
>         .func           = bpf_snprintf,
>         .gpl_only       = true,
>         .ret_type       = RET_INTEGER,
> -       .arg1_type      = ARG_PTR_TO_MEM_OR_NULL,
> +       .arg1_type      = ARG_PTR_TO_MEM_OR_NULL | MEM_WRITE,
>         .arg2_type      = ARG_CONST_SIZE_OR_ZERO,
>         .arg3_type      = ARG_PTR_TO_CONST_STR,
>         .arg4_type      = ARG_PTR_TO_MEM | PTR_MAYBE_NULL | MEM_RDONLY,
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index fe28d86f7c35..59c2394981c7 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -1022,7 +1022,7 @@ const struct bpf_func_proto bpf_snprintf_btf_proto = {
>         .func           = bpf_snprintf_btf,
>         .gpl_only       = false,
>         .ret_type       = RET_INTEGER,
> -       .arg1_type      = ARG_PTR_TO_MEM,
> +       .arg1_type      = ARG_PTR_TO_MEM | MEM_WRITE,
>         .arg2_type      = ARG_CONST_SIZE,
>         .arg3_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
>         .arg4_type      = ARG_CONST_SIZE,
> @@ -1526,7 +1526,7 @@ static const struct bpf_func_proto bpf_read_branch_records_proto = {
>         .gpl_only       = true,
>         .ret_type       = RET_INTEGER,
>         .arg1_type      = ARG_PTR_TO_CTX,
> -       .arg2_type      = ARG_PTR_TO_MEM_OR_NULL,
> +       .arg2_type      = ARG_PTR_TO_MEM_OR_NULL | MEM_WRITE,
>         .arg3_type      = ARG_CONST_SIZE_OR_ZERO,
>         .arg4_type      = ARG_ANYTHING,
>  };
> @@ -1661,7 +1661,7 @@ static const struct bpf_func_proto bpf_get_stack_proto_raw_tp = {
>         .gpl_only       = true,
>         .ret_type       = RET_INTEGER,
>         .arg1_type      = ARG_PTR_TO_CTX,
> -       .arg2_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
> +       .arg2_type      = ARG_PTR_TO_UNINIT_MEM,
>         .arg3_type      = ARG_CONST_SIZE_OR_ZERO,
>         .arg4_type      = ARG_ANYTHING,
>  };
> diff --git a/net/core/filter.c b/net/core/filter.c
> index 616e0520a0bb..6e07bb994aa7 100644
> --- a/net/core/filter.c
> +++ b/net/core/filter.c
> @@ -6399,7 +6399,7 @@ static const struct bpf_func_proto bpf_xdp_fib_lookup_proto = {
>         .gpl_only       = true,
>         .ret_type       = RET_INTEGER,
>         .arg1_type      = ARG_PTR_TO_CTX,
> -       .arg2_type      = ARG_PTR_TO_MEM,
> +       .arg2_type      = ARG_PTR_TO_MEM | MEM_WRITE,
>         .arg3_type      = ARG_CONST_SIZE,
>         .arg4_type      = ARG_ANYTHING,
>  };
> @@ -6454,7 +6454,7 @@ static const struct bpf_func_proto bpf_skb_fib_lookup_proto = {
>         .gpl_only       = true,
>         .ret_type       = RET_INTEGER,
>         .arg1_type      = ARG_PTR_TO_CTX,
> -       .arg2_type      = ARG_PTR_TO_MEM,
> +       .arg2_type      = ARG_PTR_TO_MEM | MEM_WRITE,
>         .arg3_type      = ARG_CONST_SIZE,
>         .arg4_type      = ARG_ANYTHING,
>  };
> @@ -8010,7 +8010,7 @@ static const struct bpf_func_proto bpf_tcp_raw_gen_syncookie_ipv4_proto = {
>         .ret_type       = RET_INTEGER,
>         .arg1_type      = ARG_PTR_TO_FIXED_SIZE_MEM,
>         .arg1_size      = sizeof(struct iphdr),
> -       .arg2_type      = ARG_PTR_TO_MEM,
> +       .arg2_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
>         .arg3_type      = ARG_CONST_SIZE_OR_ZERO,
>  };
>
> @@ -8042,7 +8042,7 @@ static const struct bpf_func_proto bpf_tcp_raw_gen_syncookie_ipv6_proto = {
>         .ret_type       = RET_INTEGER,
>         .arg1_type      = ARG_PTR_TO_FIXED_SIZE_MEM,
>         .arg1_size      = sizeof(struct ipv6hdr),
> -       .arg2_type      = ARG_PTR_TO_MEM,
> +       .arg2_type      = ARG_PTR_TO_MEM | MEM_RDONLY,
>         .arg3_type      = ARG_CONST_SIZE_OR_ZERO,
>  };
>

I did a quick search and there is one helper slipping. We should also
annotate arg1_type of bpf_kallsyms_lookup_name with MEM_RDONLY.


>
> --
> 2.43.0
>
>