Re: [PATCH bpf-next v1 14/18] selftests/bpf: Add load conditions on the program stack limit

["Kumar Kartikeya Dwivedi" <memxor@gmail.com>]

On Wed Sep 23, 2026 at 10:12 PM CEST, bot+bpf-ci wrote:
>> diff --git a/tools/testing/selftests/bpf/progs/bpf_misc.h b/tools/testing/selftests/bpf/progs/bpf_misc.h
>> index 2ced1d751ace..f3dbc3b59bff 100644
>> --- a/tools/testing/selftests/bpf/progs/bpf_misc.h
>> +++ b/tools/testing/selftests/bpf/progs/bpf_misc.h
>> @@ -175,6 +175,9 @@
>>  #define __prepare_priv		__test_tag("test_prepare_priv")
>>  #define __load_if_JITed()	__test_tag("load_mode=jited")
>>  #define __load_if_no_JITed()	__test_tag("load_mode=no_jited")
>> +/* Whether programs may use more than 512 bytes of stack on this kernel and JIT */
>> +#define __load_if_large_stack()		__test_tag("stack_mode=large")
>> +#define __load_if_no_large_stack()	__test_tag("stack_mode=small")
>>  #define __stderr(msg)		__test_tag("test_expect_stderr=" msg)
>>  #define __stderr_unpriv(msg)	__test_tag("test_expect_stderr_unpriv=" msg)
>>  #define __stdout(msg)		__test_tag("test_expect_stdout=" msg)
>> diff --git a/tools/testing/selftests/bpf/test_loader.c b/tools/testing/selftests/bpf/test_loader.c
>> index a6e3fcc1079c..25eeb1c1248b 100644
>> --- a/tools/testing/selftests/bpf/test_loader.c
>> +++ b/tools/testing/selftests/bpf/test_loader.c
>
> [ ... ]
>
>> @@ -1331,6 +1349,7 @@ void run_subtest(struct test_loader *tester,
>>  {
>>  	struct test_subspec *subspec = unpriv ? &spec->unpriv : &spec->priv;
>>  	int current_runtime = is_jit_enabled() ? JITED : NO_JITED;
>> +	int current_stack = is_large_stack_supported() ? LARGE_STACK : SMALL_STACK;
>>  	struct bpf_program *tprog = NULL, *tprog_iter;
>>  	struct bpf_link *link, *links[32] = {};
>>  	struct test_spec *spec_iter;
>
> [ ... ]
>
>> @@ -1360,6 +1379,11 @@ void run_subtest(struct test_loader *tester,
>>  		return;
>>  	}
>>
>> +	if ((current_stack & spec->stack_mask) == 0) {
>> +		test__skip();
>> +		return;
>> +	}
>> +
>
> A subsystem pattern flags this as potentially concerning: the new
> stack-limit load condition skips the subtest with a bare test__skip() that
> prints no reason. When a user sees a verifier_large_stack or
> verifier_stack_ptr subtest reported as SKIP, they get no hint whether it
> was skipped because is_large_stack_supported() found the 2 KiB budget
> missing or present.
>
> This matters more because the probe can also fall back to "assuming 512
> bytes" when it fails for an unrelated reason. That fallback message goes to
> stderr before test__start_subtest_with_desc() runs, so it is not attached
> to the subtest that gets skipped.
>
> Does the skip need a message like the existing skip_reason and
> jited-disassembly skips in run_subtest do? Or is it acceptable for these
> stack-mode skips to match the arch_mask and load_mask pattern right above
> this check, which also skip without a message?
>

Won't fix.

>> diff --git a/tools/testing/selftests/bpf/testing_helpers.c b/tools/testing/selftests/bpf/testing_helpers.c
>> index d1d60451c5bc..47fe61a1ebff 100644
>> --- a/tools/testing/selftests/bpf/testing_helpers.c
>> +++ b/tools/testing/selftests/bpf/testing_helpers.c
>> @@ -517,6 +517,47 @@ bool is_jit_enabled(void)
>>  	return enabled;
>>  }
>>
>> +/*
>> + * Whether the kernel accepts a program using more than 512 bytes of stack,
>> + * which depends on the JIT in use. Probed once with a program that stores
>> + * at the 2 KiB depth. Only the verifier's verdict on that store is cached:
>> + * a load that fails for another reason, such as a missing capability, is
>> + * reported and probed again on the next call.
>> + */
>> +bool is_large_stack_supported(void)
>> +{
>> +	static int supported = -1;
>> +	struct bpf_insn insns[] = {
>> +		BPF_ST_MEM(BPF_DW, BPF_REG_10, -2048, 0),
>> +		BPF_MOV64_IMM(BPF_REG_0, 0),
>> +		BPF_EXIT_INSN(),
>> +	};
>> +	char log[1024] = {};
>> +	LIBBPF_OPTS(bpf_prog_load_opts, opts,
>> +		.log_buf = log,
>> +		.log_size = sizeof(log),
>> +		.log_level = 1,
>> +	);
>> +	int fd;
>> +
>> +	if (supported >= 0)
>> +		return supported;
>> +
>> +	fd = bpf_prog_load(BPF_PROG_TYPE_SOCKET_FILTER, NULL, "GPL", insns, ARRAY_SIZE(insns),
>> +			   &opts);
>> +	if (fd >= 0) {
>> +		close(fd);
>> +		supported = 1;
>> +	} else if (strstr(log, "invalid write to stack")) {
>> +		supported = 0;
>> +	} else {
>> +		fprintf(stderr, "%s: probe failed with errno %d, assuming 512 bytes:\n%s",
>> +			__func__, errno, log);
>> +		return false;
>> +	}
>> +	return supported;
>> +}
>> +
>
> [ ... ]
>
>
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
>
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/35909853278