A patch to help you break in Firefox when and where you want it.

MaybeDebugTrap.patch

# HG changeset patch
# User Tom Ritter <tom@mozilla.com>
# Date 1681485863 14400
#      Fri Apr 14 11:24:23 2023 -0400
# Node ID 45c767e3ec49ef47dec62b0ad94481885c46b564
# Parent  fce9e73861891da48d1a0b85240491fa3e042f8b
MaybeDebugTrap

diff --git a/xpcom/base/nsDebug.h b/xpcom/base/nsDebug.h
--- a/xpcom/base/nsDebug.h
+++ b/xpcom/base/nsDebug.h
@@ -322,9 +322,23 @@ inline void MOZ_PretendNoReturn() MOZ_PR
 #endif
 
 #ifdef MOZILLA_INTERNAL_API
 void NS_ABORT_OOM(size_t aSize);
 #else
 inline void NS_ABORT_OOM(size_t) { MOZ_CRASH(); }
 #endif
 
+
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#if defined(DEBUG) && defined(XP_LINUX)
+void MaybeDebugTrap();
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+
 #endif /* nsDebug_h___ */
diff --git a/xpcom/base/nsDebugImpl.cpp b/xpcom/base/nsDebugImpl.cpp
--- a/xpcom/base/nsDebugImpl.cpp
+++ b/xpcom/base/nsDebugImpl.cpp
@@ -668,8 +668,56 @@ nsresult NS_ErrorAccordingToNSPR() {
       return NS_ERROR_FAILURE;
   }
 }
 
 void NS_ABORT_OOM(size_t aSize) {
   CrashReporter::AnnotateOOMAllocationSize(aSize);
   MOZ_CRASH("OOM");
 }
+
+#if defined(DEBUG) && defined(XP_LINUX)
+#  include <stdio.h>
+#  include <signal.h>
+#  include <sys/mman.h>
+
+static FILE* sDebughelper = nullptr;
+static char* sMyPid = nullptr;
+static char* sShouldBreak = nullptr;
+
+#  define TRAP() raise(SIGTRAP)
+// #define TRAP() __builtin_debugtrap()
+
+void MaybeDebugTrap() {
+  if (sShouldBreak == nullptr) {
+    sDebughelper = fopen("/tmp/mozilla_debughelper", "r");
+    if (!sDebughelper) {
+      fprintf(stderr,
+              "Could not open /tmp/mozilla_debughelper in pid %i, ensure it "
+              "exists with a single byte present.\n",
+              getpid());
+      sShouldBreak = new char[1];
+      sShouldBreak[0] = '\0';
+    } else {
+      sShouldBreak = (char*)mmap(nullptr, 1, PROT_READ, MAP_SHARED,
+                                 fileno(sDebughelper), 0);
+      if (!sShouldBreak) {
+        fprintf(stderr, "Could not mmap /tmp/mozilla_debughelper in pid %i.\n",
+                getpid());
+        TRAP();
+      } else {
+        fprintf(
+            stderr,
+            "Mapped /tmp/mozilla_debughelper in pid %i, doing a test read.\n",
+            getpid());
+      }
+    }
+
+    int num_digits = (int)((ceil(log10(getpid())) + 1) * sizeof(char));
+    sMyPid = new char[num_digits];
+    snprintf(sMyPid, num_digits, "%d", getpid());
+  }
+
+  if (strncmp(sShouldBreak, sMyPid, strlen(sMyPid)) == 0) {
+    raise(SIGTRAP);
+  }
+}
+#endif