i#4425: handle unspecified-by-the-app sigaction restorer for AArch64 (#…

…4840)

Prevents a seg fault in the burst_aarch64_sys test that was caused by reading an unspecified
sigaction restorer in sig_has_restorer() in unix/signal.c. Does so by returning false early
in sig_has_restorer() for AArch64 when the SA_RESTORER flag is not set.

By preventing the seg fault, it also prevents the nested signal handling and consequently
the stack overflow in burst_aarch64_sys test when the -signal_stack_size is not specified.

Issue: #4425

clients/drcachesim/tests/burst_aarch64_sys.cpp

@@ -176,11 +176,7 @@ post_process()
 static std::string
 gather_trace()
 {
-    if (!my_setenv("DYNAMORIO_OPTIONS",
-                   // XXX i#4425: Fix debug-build stack overflow issue and
-                   // remove custom signal_stack_size below.
-                   "-stderr_mask 0xc -signal_stack_size 64K "
-                   "-client_lib ';;-offline'"))
+    if (!my_setenv("DYNAMORIO_OPTIONS", "-stderr_mask 0xc -client_lib ';;-offline'"))
         std::cerr << "failed to set env var!\n";
 
     std::cerr << "pre-DR init\n";

core/unix/signal.c

@@ -2879,6 +2879,17 @@ sig_has_restorer(thread_sig_info_t *info, int sig)
         return false;
     if (TEST(SA_RESTORER, info->app_sigaction[sig]->flags))
         return true;
+#    ifdef AARCH64
+    /* In AArch64 either the app or the kernel defines a restorer, not glibc, contrary
+     * to x86/ARM where glibc defines a restorer if the app did not define one. Thus,
+     * reading info->app_sigaction[sig]->restorer when SA_RESTORER is not specified by
+     * the app was never an issue for x86/ARM, but for AArch64 if the SA_RESTORER is
+     * not specified DR will read garbage leading to a seg fault later when
+     * safe_reading the restorer. To avoid this issue return false early for AArch64 if
+     * SA_RESTORER is not specified.
+     */
+    return false;
+#    endif
     if (info->app_sigaction[sig]->restorer == NULL)
         return false;
     /* we cache the result due to the safe_read cost */