Re: [PATCH v8 44/46] KVM: selftests: Make TEST_EXPECT_SIGBUS thread-safe

[Fuad Tabba]

On Fri, 19 Jun 2026 at 01:32, Ackerley Tng via B4 Relay
<devnull+ackerleytng.google.com@xxxxxxxxxx> wrote:
>
> From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
>
> The TEST_EXPECT_SIGBUS macro is not thread-safe as it uses a global
> sigjmp_buf and installs a global SIGBUS signal handler. If multiple threads
> execute the macro concurrently, they will race on installing the signal
> handler and stomp on other threads' jump buffers, leading to incorrect test
> behavior.
>
> Make TEST_EXPECT_SIGBUS thread-safe with the following changes:
>
> Share the KVM tests' global signal handler. sigaction() applies to all
> threads; without sharing a global signal handler, one thread may have
> removed the signal handler that another thread added, hence leading to
> unexpected signals.
>
> The alternative of layering signal handlers was considered, but calling
> sigaction() within TEST_EXPECT_SIGBUS() necessarily creates a race. To
> avoid adding new setup and teardown routines to do sigaction() and keep
> usage of TEST_EXPECT_SIGBUS() simple, share the KVM tests' global signal
> handler.
>
> Opportunistically rename report_unexpected_signal to
> catchall_signal_handler.
>
> To continue to only expect SIGBUS within specific regions of code, use a
> thread-specific variable, expecting_sigbus, to replace installing and
> removing signal handlers.
>
> Make the execution environment for the thread, sigjmp_buf, a
> thread-specific variable.
>
> As part of TEST_EXPECT_SIGBUS(), assert the prerequisite for this setup,
> that the current signal handler is the catchall_signal_handler.
>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>

Reviewed-by: Fuad Tabba <tabba@xxxxxxxxxx>

Cheers,
/fuad

> ---
>  tools/testing/selftests/kvm/include/test_util.h | 32 +++++++++++++------------
>  tools/testing/selftests/kvm/lib/kvm_util.c      | 18 ++++++++++----
>  tools/testing/selftests/kvm/lib/test_util.c     |  7 ------
>  3 files changed, 30 insertions(+), 27 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/include/test_util.h b/tools/testing/selftests/kvm/include/test_util.h
> index 51287fac8138a..bd75162ec868d 100644
> --- a/tools/testing/selftests/kvm/include/test_util.h
> +++ b/tools/testing/selftests/kvm/include/test_util.h
> @@ -82,21 +82,23 @@ do {                                                                        \
>         __builtin_unreachable(); \
>  } while (0)
>
> -extern sigjmp_buf expect_sigbus_jmpbuf;
> -void expect_sigbus_handler(int signum);
> -
> -#define TEST_EXPECT_SIGBUS(action)                                             \
> -do {                                                                           \
> -       struct sigaction sa_old, sa_new = {                                     \
> -               .sa_handler = expect_sigbus_handler,                            \
> -       };                                                                      \
> -                                                                               \
> -       sigaction(SIGBUS, &sa_new, &sa_old);                                    \
> -       if (sigsetjmp(expect_sigbus_jmpbuf, 1) == 0) {                          \
> -               action;                                                         \
> -               TEST_FAIL("'%s' should have triggered SIGBUS", #action);        \
> -       }                                                                       \
> -       sigaction(SIGBUS, &sa_old, NULL);                                       \
> +extern __thread sigjmp_buf expect_sigbus_jmpbuf;
> +extern __thread volatile sig_atomic_t expecting_sigbus;
> +extern void catchall_signal_handler(int signum);
> +
> +#define TEST_EXPECT_SIGBUS(action)                                     \
> +do {                                                                   \
> +       struct sigaction __sa = {};                                     \
> +                                                                       \
> +       TEST_ASSERT_EQ(sigaction(SIGBUS, NULL, &__sa), 0);              \
> +       TEST_ASSERT_EQ(__sa.sa_handler, &catchall_signal_handler);      \
> +                                                                       \
> +       expecting_sigbus = true;                                        \
> +       if (sigsetjmp(expect_sigbus_jmpbuf, 1) == 0) {                  \
> +               action;                                                 \
> +               TEST_FAIL("'%s' should have triggered SIGBUS", #action);\
> +       }                                                               \
> +       expecting_sigbus = false;                                       \
>  } while (0)
>
>  size_t parse_size(const char *size);
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index 6b304e8a0e0d5..b4f104436875b 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -2292,13 +2292,20 @@ __weak void kvm_selftest_arch_init(void)
>  {
>  }
>
> -static void report_unexpected_signal(int signum)
> +__thread sigjmp_buf expect_sigbus_jmpbuf;
> +__thread volatile sig_atomic_t expecting_sigbus;
> +
> +void catchall_signal_handler(int signum)
>  {
> +       switch (signum) {
> +       case SIGBUS: {
> +               if (expecting_sigbus)
> +                       siglongjmp(expect_sigbus_jmpbuf, 1);
> +
> +               TEST_FAIL("Unexpected SIGBUS (%d)\n", 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);
> @@ -2310,12 +2317,13 @@ static void report_unexpected_signal(int signum)
>  void __attribute((constructor)) kvm_selftest_init(void)
>  {
>         struct sigaction sig_sa = {
> -               .sa_handler = report_unexpected_signal,
> +               .sa_handler = catchall_signal_handler,
>         };
>
>         /* Tell stdout not to buffer its content. */
>         setbuf(stdout, NULL);
>
> +       expecting_sigbus = false;
>         sigaction(SIGBUS, &sig_sa, NULL);
>         sigaction(SIGSEGV, &sig_sa, NULL);
>         sigaction(SIGILL, &sig_sa, NULL);
> diff --git a/tools/testing/selftests/kvm/lib/test_util.c b/tools/testing/selftests/kvm/lib/test_util.c
> index bab1bd2b775b6..30eb701e4becd 100644
> --- a/tools/testing/selftests/kvm/lib/test_util.c
> +++ b/tools/testing/selftests/kvm/lib/test_util.c
> @@ -18,13 +18,6 @@
>
>  #include "test_util.h"
>
> -sigjmp_buf expect_sigbus_jmpbuf;
> -
> -void __attribute__((used)) expect_sigbus_handler(int signum)
> -{
> -       siglongjmp(expect_sigbus_jmpbuf, 1);
> -}
> -
>  /*
>   * Random number generator that is usable from guest code. This is the
>   * Park-Miller LCG using standard constants.
>
> --
> 2.55.0.rc0.738.g0c8ab3ebcc-goog
>
>