Re: [PATCH bpf RESEND v2 1/2] bpf: Fix memory access flags in helper prototypes

[Eduard Zingerman <eddyz87@gmail.com>]

On Sun, 2026-01-18 at 16:16 +0800, Zesen Liu wrote:
> After commit 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking"),
> the verifier started relying on the access type flags 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 flags. 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.
>
> For bpf_get_stack_proto_raw_tp, the original MEM_RDONLY was incorrect
> since the helper writes to the buffer. Change it to ARG_PTR_TO_UNINIT_MEM
> which correctly indicates write access to potentially uninitialized memory.
>
> 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 flags.
>
> Fixes: 37cce22dbd51 ("bpf: verifier: Refactor helper access type tracking")
> 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>
> ---

I looked trough the helpers annotated with MEM_WRITE in this patch,
indeed the write annotation is missing from these helpers.

In conjunction with the following logic in verifier.c:check_func_arg:

        case ARG_PTR_TO_MEM:
                /* The access to this pointer is only checked when we hit the
                 * next is_mem_size argument below.
                 */
                meta->raw_mode = arg_type & MEM_UNINIT;
                if (arg_type & MEM_FIXED_SIZE) {
                        err = check_helper_mem_access(env, regno, fn->arg_size[arg],
                                                      arg_type & MEM_WRITE ? BPF_WRITE : BPF_READ,
						      ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
						      // arguments considered read-only by default
                                                      false, meta);
                        if (err)
                                return err;
                        if (arg_type & MEM_ALIGNED)
                                err = check_ptr_alignment(env, reg, 0, fn->arg_size[arg], true);
                }
                break;

This patch fixes a real problem.

[...]

> index fe28d86f7c35..59c2394981c7 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c

[...]

> @@ -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,

Q: why ARG_PTR_TO_UNINIT_MEM here, but not for a previous function and
   not for snprintf variants?

>  	.arg3_type	= ARG_CONST_SIZE_OR_ZERO,
>  	.arg4_type	= ARG_ANYTHING,
>  };

[...]