Re: [PATCH bpf-next 3/3] selftests/bpf: Add tests for arena fault reporting

[Yonghong Song <yonghong.song@linux.dev>]

On 8/6/25 1:58 AM, Puranjay Mohan wrote:
> Add selftests for testing the reporting of arena page faults through BPF
> streams. Two new bpf programs are added that read and write to an
> unmapped arena address and the fault reporting is verified in the
> userspace through streams.
>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> ---
>   .../testing/selftests/bpf/prog_tests/stream.c | 24 ++++++++++++
>   tools/testing/selftests/bpf/progs/stream.c    | 37 +++++++++++++++++++
>   2 files changed, 61 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/stream.c b/tools/testing/selftests/bpf/prog_tests/stream.c
> index d9f0185dca61b..4bdde56de35b1 100644
> --- a/tools/testing/selftests/bpf/prog_tests/stream.c
> +++ b/tools/testing/selftests/bpf/prog_tests/stream.c
> @@ -41,6 +41,22 @@ struct {
>   		"([a-zA-Z_][a-zA-Z0-9_]*\\+0x[0-9a-fA-F]+/0x[0-9a-fA-F]+\n"
>   		"|[ \t]+[^\n]+\n)*",
>   	},
> +	{
> +		offsetof(struct stream, progs.stream_arena_read_fault),
> +		"ERROR: Arena READ access at unmapped address 0x.*\n"
> +		"CPU: [0-9]+ UID: 0 PID: [0-9]+ Comm: .*\n"
> +		"Call trace:\n"
> +		"([a-zA-Z_][a-zA-Z0-9_]*\\+0x[0-9a-fA-F]+/0x[0-9a-fA-F]+\n"
> +		"|[ \t]+[^\n]+\n)*",
> +	},
> +	{
> +		offsetof(struct stream, progs.stream_arena_write_fault),
> +		"ERROR: Arena WRITE access at unmapped address 0x.*\n"
> +		"CPU: [0-9]+ UID: 0 PID: [0-9]+ Comm: .*\n"
> +		"Call trace:\n"
> +		"([a-zA-Z_][a-zA-Z0-9_]*\\+0x[0-9a-fA-F]+/0x[0-9a-fA-F]+\n"
> +		"|[ \t]+[^\n]+\n)*",
> +	},
>   };
>   
>   static int match_regex(const char *pattern, const char *string)
> @@ -85,6 +101,14 @@ void test_stream_errors(void)
>   			continue;
>   		}
>   #endif
> +#if !defined(__x86_64__) && !defined(__aarch64__)
> +		ASSERT_TRUE(1, "Arena fault reporting unsupported, skip.");
> +		if (i == 2 || i == 3) {
> +			ret = bpf_prog_stream_read(prog_fd, 2, buf, sizeof(buf), &ropts);
> +			ASSERT_EQ(ret, 0, "stream read");
> +			continue;
> +		}
> +#endif
>   
>   		ret = bpf_prog_stream_read(prog_fd, BPF_STREAM_STDERR, buf, sizeof(buf), &ropts);
>   		ASSERT_GT(ret, 0, "stream read");
> diff --git a/tools/testing/selftests/bpf/progs/stream.c b/tools/testing/selftests/bpf/progs/stream.c
> index 35790897dc879..58ebff60cd96a 100644
> --- a/tools/testing/selftests/bpf/progs/stream.c
> +++ b/tools/testing/selftests/bpf/progs/stream.c
> @@ -1,10 +1,15 @@
>   // SPDX-License-Identifier: GPL-2.0
>   /* Copyright (c) 2025 Meta Platforms, Inc. and affiliates. */
> +#define BPF_NO_KFUNC_PROTOTYPES

Do we have to defineBPF_NO_KFUNC_PROTOTYPES in the above? Without the above, we do not need 
below extern bpf_res_spin_lock and bpf_res_spin_unlock.

>   #include <vmlinux.h>
>   #include <bpf/bpf_tracing.h>
>   #include <bpf/bpf_helpers.h>
>   #include "bpf_misc.h"
>   #include "bpf_experimental.h"
> +#include "bpf_arena_common.h"
> +
> +extern int bpf_res_spin_lock(struct bpf_res_spin_lock *lock) __weak __ksym;
> +extern void bpf_res_spin_unlock(struct bpf_res_spin_lock *lock) __weak __ksym;
>   
>   struct arr_elem {
>   	struct bpf_res_spin_lock lock;
> @@ -17,6 +22,12 @@ struct {
>   	__type(value, struct arr_elem);
>   } arrmap SEC(".maps");
>   
> +struct {
> +	__uint(type, BPF_MAP_TYPE_ARENA);
> +	__uint(map_flags, BPF_F_MMAPABLE);
> +	__uint(max_entries, 1); /* number of pages */
> +} arena SEC(".maps");
> +
>   #define ENOSPC 28
>   #define _STR "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
>   
> @@ -76,4 +87,30 @@ int stream_syscall(void *ctx)
>   	return 0;
>   }
>   
> +SEC("syscall")
> +__success __retval(0)
> +int stream_arena_write_fault(void *ctx)
> +{
> +	unsigned char __arena *page;
> +
> +	page = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
> +	bpf_arena_free_pages(&arena, page, 1);
> +
> +	*(page + 0xbeef) = 1;
> +
> +	return 0;
> +}
> +
> +SEC("syscall")
> +__success __retval(0)
> +int stream_arena_read_fault(void *ctx)
> +{
> +	unsigned char __arena *page;
> +
> +	page = bpf_arena_alloc_pages(&arena, NULL, 1, NUMA_NO_NODE, 0);
> +	bpf_arena_free_pages(&arena, page, 1);
> +
> +	return *(page + 0xbeef);
> +}
> +
>   char _license[] SEC("license") = "GPL";