linux-kernel - Re: [PATCH v12 07/12] KVM: selftests: Report stacktraces SIGBUS, SIGSEGV, SIGILL, and SIGFPE by default

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Message-ID: <diqzikgnhfm4.fsf@google.com>
Date: Thu, 09 Oct 2025 15:31:15 -0700
From: Ackerley Tng <ackerleytng@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Marc Zyngier <maz@...nel.org>, 
	Oliver Upton <oliver.upton@...ux.dev>, Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	David Hildenbrand <david@...hat.com>, Fuad Tabba <tabba@...gle.com>, Shivank Garg <shivankg@....com>, 
	Ashish Kalra <ashish.kalra@....com>, Vlastimil Babka <vbabka@...e.cz>
Subject: Re: [PATCH v12 07/12] KVM: selftests: Report stacktraces SIGBUS,
 SIGSEGV, SIGILL, and SIGFPE by default

Sean Christopherson <seanjc@...gle.com> writes:

> Register handlers for signals for all selftests that are likely happen due
> to test (or kernel) bugs, and explicitly fail tests on unexpected signals
> so that users get a stack trace, i.e. don't have to go spelunking to do
> basic triage.
>
> Register the handlers as early as possible, to catch as many unexpected
> signals as possible, and also so that the common code doesn't clobber a
> handler that's installed by test (or arch) code.
>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

I tested this with

diff --git i/tools/testing/selftests/kvm/guest_memfd_test.c w/tools/testing/selftests/kvm/guest_memfd_test.c
index 618c937f3c90f..f6de2a678bf99 100644
--- i/tools/testing/selftests/kvm/guest_memfd_test.c
+++ w/tools/testing/selftests/kvm/guest_memfd_test.c
@@ -182,6 +182,8 @@ static void test_fault_sigbus(int fd, size_t accessible_size, size_t map_size)
        TEST_EXPECT_SIGBUS(memset(mem, val, map_size));
        TEST_EXPECT_SIGBUS((void)READ_ONCE(mem[accessible_size]));
 
+       mem[accessible_size] = 0xdd;
+
        for (i = 0; i < accessible_size; i++)
                TEST_ASSERT_EQ(READ_ONCE(mem[i]), val);

And got

==== Test Assertion Failure ====
  lib/kvm_util.c:2299: false
  pid=388 tid=388 errno=29 - Illegal seek
     1  0x000000000040a253: report_unexpected_signal at kvm_util.c:2299
     2  0x000000000042615f: sigaction at ??:?
     3  0x000000000040283f: test_fault_sigbus at guest_memfd_test.c:183 (discriminator 4)
     4  0x0000000000402c1c: test_fault_private at guest_memfd_test.c:200
     5   (inlined by) __test_guest_memfd at guest_memfd_test.c:376
     6  0x0000000000401e15: test_guest_memfd at guest_memfd_test.c:401
     7   (inlined by) main at guest_memfd_test.c:491
     8  0x000000000041ea03: __libc_start_call_main at libc-start.o:?
     9  0x0000000000420bac: __libc_start_main_impl at ??:?
    10  0x0000000000401fe0: _start at ??:?
  Unexpected SIGBUS (7)

I expected the line number to be 185 but the report says 183. Not sure
if this is a compiler issue or something caused by macros, or if it's
because of signals mess with the tracking of instruction execution.

Either way, this is a very useful test feature, thanks!

Tested-by: Ackerley Tng <ackerleytng@...gle.com>
Reviewed-by: Ackerley Tng <ackerleytng@...gle.com>

> ---
>  tools/testing/selftests/kvm/lib/kvm_util.c | 24 ++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
>
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index 8b60b767224b..0c3a6a40d1a9 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -2290,11 +2290,35 @@ __weak void kvm_selftest_arch_init(void)
>  {
>  }
>
> +static void report_unexpected_signal(int signum)
> +{
> +#define KVM_CASE_SIGNUM(sig)					\
> +	case sig: TEST_FAIL("Unexpected " #sig " (%d)\n", signum)
> +
> +	switch (signum) {
> +	KVM_CASE_SIGNUM(SIGBUS);
> +	KVM_CASE_SIGNUM(SIGSEGV);
> +	KVM_CASE_SIGNUM(SIGILL);
> +	KVM_CASE_SIGNUM(SIGFPE);
> +	default:
> +		TEST_FAIL("Unexpected signal %d\n", signum);
> +	}
> +}
> +
>  void __attribute((constructor)) kvm_selftest_init(void)
>  {
> +	struct sigaction sig_sa = {
> +		.sa_handler = report_unexpected_signal,
> +	};
> +
>  	/* Tell stdout not to buffer its content. */
>  	setbuf(stdout, NULL);
>
> +	sigaction(SIGBUS, &sig_sa, NULL);
> +	sigaction(SIGSEGV, &sig_sa, NULL);
> +	sigaction(SIGILL, &sig_sa, NULL);
> +	sigaction(SIGFPE, &sig_sa, NULL);
> +
>  	guest_random_seed = last_guest_seed = random();
>  	pr_info("Random seed: 0x%x\n", guest_random_seed);
>
> --
> 2.51.0.710.ga91ca5db03-goog