Subject: Re: [RFC PATCH bpf-next v2 02/11] selftests/bpf: Test passing CONST_PTR_TO_DYNPTR to kfunc that may mutate dynptr

From: Mykyta Yatsenko <[email-masked]>
Date: Wed, 11 Mar 2026 15:26:33 +0000

Amery Hung <[email-masked]> writes:

> Make sure for kfunc that takes mutable dynptr argument, verifier rejects
> passing CONST_PTR_TO_DYNPTR to it.
>
> Rename struct sample to test_sample to avoid a conflict with the
> definition in vmlinux.h
>
> In test_kfunc_dynptr_param.c, initialize dynptr to 0 to avoid
> -Wuninitialized-const-pointer warning.
>
> Signed-off-by: Amery Hung <[email-masked]>
> ---
>  .../testing/selftests/bpf/progs/dynptr_fail.c | 37 +++++++++++++++----
>  .../bpf/progs/test_kfunc_dynptr_param.c       |  2 +-
>  2 files changed, 30 insertions(+), 9 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/progs/dynptr_fail.c b/tools/testing/selftests/bpf/progs/dynptr_fail.c
> index 8f2ae9640886..5e1b1cf4ea8e 100644
> --- a/tools/testing/selftests/bpf/progs/dynptr_fail.c
> +++ b/tools/testing/selftests/bpf/progs/dynptr_fail.c
> @@ -1,15 +1,14 @@
>  // SPDX-License-Identifier: GPL-2.0
>  /* Copyright (c) 2022 Facebook */
>  
> +#include <vmlinux.h>
>  #include <errno.h>
>  #include <string.h>
> -#include <stdbool.h>
> -#include <linux/bpf.h>
>  #include <bpf/bpf_helpers.h>
>  #include <bpf/bpf_tracing.h>
> -#include <linux/if_ether.h>
>  #include "bpf_misc.h"
>  #include "bpf_kfuncs.h"
> +#include "../test_kmods/bpf_testmod_kfunc.h"
>  
>  char _license[] SEC("license") = "GPL";
>  
> @@ -46,7 +45,7 @@ struct {
>  	__type(value, __u64);
>  } array_map4 SEC(".maps");
>  
> -struct sample {
> +struct test_sample {
>  	int pid;
>  	long value;
>  	char comm[16];
> @@ -95,7 +94,7 @@ __failure __msg("Unreleased reference id=4")
>  int ringbuf_missing_release2(void *ctx)
>  {
>  	struct bpf_dynptr ptr1, ptr2;
> -	struct sample *sample;
> +	struct test_sample *sample;
>  
>  	bpf_ringbuf_reserve_dynptr(&ringbuf, sizeof(*sample), 0, &ptr1);
>  	bpf_ringbuf_reserve_dynptr(&ringbuf, sizeof(*sample), 0, &ptr2);
> @@ -173,7 +172,7 @@ __failure __msg("type=mem expected=ringbuf_mem")
>  int ringbuf_invalid_api(void *ctx)
>  {
>  	struct bpf_dynptr ptr;
> -	struct sample *sample;
> +	struct test_sample *sample;
>  
>  	bpf_ringbuf_reserve_dynptr(&ringbuf, sizeof(*sample), 0, &ptr);
>  	sample = bpf_dynptr_data(&ptr, 0, sizeof(*sample));
> @@ -315,7 +314,7 @@ __failure __msg("invalid mem access 'scalar'")
>  int data_slice_use_after_release1(void *ctx)
>  {
>  	struct bpf_dynptr ptr;
> -	struct sample *sample;
> +	struct test_sample *sample;
>  
>  	bpf_ringbuf_reserve_dynptr(&ringbuf, sizeof(*sample), 0, &ptr);
>  	sample = bpf_dynptr_data(&ptr, 0, sizeof(*sample));
> @@ -347,7 +346,7 @@ __failure __msg("invalid mem access 'scalar'")
>  int data_slice_use_after_release2(void *ctx)
>  {
>  	struct bpf_dynptr ptr1, ptr2;
> -	struct sample *sample;
> +	struct test_sample *sample;
>  
>  	bpf_ringbuf_reserve_dynptr(&ringbuf, 64, 0, &ptr1);
>  	bpf_ringbuf_reserve_dynptr(&ringbuf, sizeof(*sample), 0, &ptr2);
> @@ -1993,3 +1992,25 @@ int test_dynptr_reg_type(void *ctx)
>  	global_call_bpf_dynptr((const struct bpf_dynptr *)current);
>  	return 0;
>  }
> +
> +/* Cannot pass CONST_PTR_TO_DYNPTR to bpf_kfunc_dynptr_test() that may mutate the dynptr */
> +__noinline int global_subprog_dynptr_mutable(const struct bpf_dynptr *dynptr)
> +{
> +	long ret = 0;
Why do we need this long ret? Do we even need this function at all, why
not calling bpf_kfunc_dynptr_test() directly from the
kfunc_dynptr_const_to_mutable()?
> +
> +	/* this should fail */
> +	bpf_kfunc_dynptr_test((struct bpf_dynptr *)dynptr, NULL);
> +	__sink(ret);
> +	return ret;
> +}
> +
> +SEC("tc")
nit: it looks like most of the programs in this file are optional:
SEC("?tc").
> +__failure __msg("cannot pass pointer to const bpf_dynptr, the helper mutates it")
> +int kfunc_dynptr_const_to_mutable(struct __sk_buff *skb)
> +{
> +	struct bpf_dynptr data;
> +
> +	bpf_dynptr_from_skb(skb, 0, &data);
> +	global_subprog_dynptr_mutable(&data);
> +	return 0;
> +}
> diff --git a/tools/testing/selftests/bpf/progs/test_kfunc_dynptr_param.c b/tools/testing/selftests/bpf/progs/test_kfunc_dynptr_param.c
> index c3631fd41977..1c6cfd0888ba 100644
> --- a/tools/testing/selftests/bpf/progs/test_kfunc_dynptr_param.c
> +++ b/tools/testing/selftests/bpf/progs/test_kfunc_dynptr_param.c
> @@ -33,7 +33,7 @@ SEC("?lsm.s/bpf")
>  __failure __msg("cannot pass in dynptr at an offset=-8")
>  int BPF_PROG(not_valid_dynptr, int cmd, union bpf_attr *attr, unsigned int size, bool kernel)
>  {
> -	unsigned long val;
> +	unsigned long val = 0;
>  
>  	return bpf_verify_pkcs7_signature((struct bpf_dynptr *)&val,
>  					  (struct bpf_dynptr *)&val, NULL);
> -- 
> 2.47.3