Re: [PATCH bpf-next v5 4/4] selftests/bpf: Add tests for arena fault reporting

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

On 9/1/25 12:37 PM, 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.
>
> The added bpf programs need to access the user_vm_start in struct
> bpf_arena, this is done by casting &arena to struct bpf_arena *, but
> barrier_var() is used on this ptr before accessing ptr->user_vm_start;
> to stop GCC from issuing an out-of-bound access due to the cast from
> smaller map struct to larger "struct bpf_arena"
>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>

LGTM with one nit below.

Acked-by: Yonghong Song <yonghong.song@linux.dev>

> ---
>   .../testing/selftests/bpf/prog_tests/stream.c | 34 ++++++++++-
>   tools/testing/selftests/bpf/progs/stream.c    | 61 +++++++++++++++++++
>   2 files changed, 94 insertions(+), 1 deletion(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/stream.c b/tools/testing/selftests/bpf/prog_tests/stream.c
> index 9d0e5d93edee7..b2a85364e3c4f 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)
> @@ -63,6 +79,7 @@ void test_stream_errors(void)
>   	struct stream *skel;
>   	int ret, prog_fd;
>   	char buf[1024];
> +	char fault_addr[64] = {0};

Looks like the above '= {0}' is not necessary as the only usage
is below:

+			sprintf(fault_addr, "0x%lx", skel->bss->fault_addr);
+			ret = match_regex(fault_addr, buf);

>   
>   	skel = stream__open_and_load();
>   	if (!ASSERT_OK_PTR(skel, "stream__open_and_load"))
> @@ -85,6 +102,14 @@ 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");
> @@ -92,8 +117,15 @@ void test_stream_errors(void)
>   		buf[ret] = '\0';
>   
>   		ret = match_regex(stream_error_arr[i].errstr, buf);
> -		if (!ASSERT_TRUE(ret == 1, "regex match"))
> +		if (ret && (i == 2 || i == 3)) {
> +			sprintf(fault_addr, "0x%lx", skel->bss->fault_addr);
> +			ret = match_regex(fault_addr, buf);
> +		}
> +		if (!ASSERT_TRUE(ret == 1, "regex match")) {
>   			fprintf(stderr, "Output from stream:\n%s\n", buf);
> +			if (i == 2 || i == 3)
> +				fprintf(stderr, "Fault Addr: 0x%lx\n", skel->bss->fault_addr);
> +		}
>   	}
>   
>   	stream__destroy(skel);
>
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