fix(profiler): guard line-number-table copy against stale jmethodID

fillJavaMethodInfo() memcpy'd the JVMTI-returned line-number table
without probing it first, unlike the sibling class/method name+sig
strings hardened in #537. Crash reported in production as SIGSEGV in
Lookup::resolveMethod on stock HotSpot JDK 11.

Author: jbachorik

ddprof-lib/src/main/cpp/counters.h

@@ -73,6 +73,7 @@
   X(UNWINDING_TIME_JVMTI, "unwinding_ticks_jvmti")                             \
   X(CALLTRACE_STORAGE_DROPPED, "calltrace_storage_dropped_traces")             \
   X(LINE_NUMBER_TABLES, "line_number_tables")                                  \
+  X(LINE_NUMBER_TABLE_UNREADABLE, "line_number_table_unreadable")             \
   X(REMOTE_SYMBOLICATION_FRAMES, "remote_symbolication_frames")                \
   X(REMOTE_SYMBOLICATION_LIBS_WITH_BUILD_ID, "remote_symbolication_libs_with_build_id") \
   X(REMOTE_SYMBOLICATION_BUILD_ID_CACHE_HITS, "remote_symbolication_build_id_cache_hits") \

ddprof-lib/src/main/cpp/flightRecorder.cpp

@@ -348,11 +348,25 @@ void Lookup::fillJavaMethodInfo(MethodInfo *mi, jmethodID method,
       void *owned_table = nullptr;
       if (line_number_table_size > 0) {
         size_t bytes = (size_t)line_number_table_size * sizeof(jvmtiLineNumberEntry);
-        owned_table = malloc(bytes);
-        if (owned_table != nullptr) {
-          memcpy(owned_table, line_number_table, bytes);
+        // GetLineNumberTable() is called on the same possibly-stale jmethodID
+        // that GetMethodDeclaringClass/GetClassSignature/GetMethodName above
+        // were probed for -- the TOCTOU race documented above (class
+        // unloaded between sample capture and dump) applies here just as
+        // much as to those calls, and crash telemetry already showed those
+        // sibling calls returning JVMTI_ERROR_NONE with unmapped string
+        // pointers despite the spec saying the returned array should be a
+        // fresh, caller-owned allocation. Nothing about this call guarantees
+        // it is exempt from the same failure mode, so probe before copying
+        // rather than assume the pointer is safe to dereference.
+        if (SafeAccess::isReadableRange(line_number_table, bytes)) {
+          owned_table = malloc(bytes);
+          if (owned_table != nullptr) {
+            memcpy(owned_table, line_number_table, bytes);
+          } else {
+            TEST_LOG("Failed to allocate %zu bytes for line number table copy", bytes);
+          }
         } else {
-          TEST_LOG("Failed to allocate %zu bytes for line number table copy", bytes);
+          Counters::increment(LINE_NUMBER_TABLE_UNREADABLE);
         }
       }
       jvmtiError dealloc_err = jvmti->Deallocate((unsigned char *)line_number_table);

ddprof-lib/src/test/cpp/lineNumberTableCopy_ut.cpp

@@ -0,0 +1,183 @@
+/*
+ * Copyright 2026, Datadog, Inc.
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+// Reproduces a crash reported in production:
+//
+//   __memcpy_evex_unaligned_erms
+//   Lookup::resolveMethod(_asgct_callframe&)
+//   ...
+//   Recording::writeStackTraces(Buffer*, Lookup*)
+//   ...
+//   Profiler::dump / FlightRecorder::dump
+//
+// Lookup::fillJavaMethodInfo() (inlined into resolveMethod() under -O2/-O3)
+// calls jvmtiEnv::GetLineNumberTable(method, ...) and then copied the
+// result with an unguarded:
+//
+//   memcpy(owned_table, line_number_table, bytes);
+//
+// (flightRecorder.cpp now guards this copy with SafeAccess::isReadableRange()
+// -- see below -- so the line numbers of the unguarded version are no longer
+// present in the file; this file's tests reproduce that removed pattern.)
+//
+// Unlike the class_name/method_name/method_sig strings returned by the
+// preceding JVMTI calls (which ARE probed with SafeAccess::isReadableRange
+// before use, see PR #537 / commit ef13aa29f), the line_number_table pointer
+// is used directly.
+//
+// Per the JVMTI spec, GetLineNumberTable() hands back a freshly-allocated,
+// caller-owned array (hence the Deallocate() call after the copy) that is
+// decoupled from the Method's lifetime -- a spec-compliant implementation
+// cannot invalidate it out from under the caller. The actual risk, per the
+// comments already in fillJavaMethodInfo, is the TOCTOU race documented
+// right there: "GetMethodDeclaringClass may return a jclass wrapping a
+// stale/garbage oop when the class was unloaded between sample capture and
+// dump" -- a race against class unloading, not tied to any one JVM vendor.
+// That same comment block notes crash telemetry showed GetClassSignature/
+// GetMethodName returning JVMTI_ERROR_NONE with unmapped string pointers
+// despite the spec saying they should be valid (a separate OpenJ9-specific
+// jmethodID-corruption mode is also handled a few lines above, but is not
+// the only source of this). The production crash motivating this file was
+// on a stock HotSpot JDK 11, confirming the race is not OpenJ9-specific.
+// GetLineNumberTable() is called on the exact same jmethodID as
+// GetMethodDeclaringClass/GetClassSignature/GetMethodName, with nothing
+// about it that would exempt it from that same observed failure mode.
+//
+// These tests isolate the copy step from JVMTI/JNI (which fillJavaMethodInfo
+// requires and which is impractical to fake in a plain gtest) by exercising
+// the exact copy pattern against a pointer whose backing memory is gone --
+// standing in for "GetLineNumberTable() returned a bad pointer" regardless
+// of the precise reason. The result at the memcpy call site is identical
+// either way, so this is sufficient to prove both the crash and the fix.
+
+#include <gtest/gtest.h>
+#include <cstring>
+#include <signal.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+#include "flightRecorder.h"
+#include "os.h"
+#include "safeAccess.h"
+
+namespace {
+
+// SafeAccess::isReadableRange()/isReadable() rely on a SIGSEGV handler
+// registered via OS::replaceSigsegvHandler() to catch the fault at a known
+// trampoline address and turn it into a safe return value (see
+// safefetch_ut.cpp for the same pattern). Without it, a fault inside the
+// safefetch trampoline is an ordinary unhandled SIGSEGV.
+void (*orig_segvHandler)(int signo, siginfo_t *siginfo, void *ucontext);
+
+void lineNumberTableSegvHandler(int signo, siginfo_t *siginfo, void *context) {
+  if (!SafeAccess::handle_safefetch(signo, context)) {
+    if (orig_segvHandler != nullptr) {
+      orig_segvHandler(signo, siginfo, context);
+    }
+  }
+}
+
+class LineNumberTableCopyTest : public ::testing::Test {
+protected:
+  void SetUp() override {
+    orig_segvHandler = OS::replaceSigsegvHandler(lineNumberTableSegvHandler);
+  }
+
+  void TearDown() override { OS::replaceSigsegvHandler(orig_segvHandler); }
+};
+
+// Allocates a page-sized region seeded with `count` jvmtiLineNumberEntry
+// records, standing in for a buffer jvmtiEnv::GetLineNumberTable() would
+// have returned.
+jvmtiLineNumberEntry *makeFakeLineNumberTable(void *page, int count) {
+  jvmtiLineNumberEntry *table = (jvmtiLineNumberEntry *)page;
+  for (int i = 0; i < count; i++) {
+    table[i].start_location = i * 4;
+    table[i].line_number = i + 1;
+  }
+  return table;
+}
+
+} // namespace
+
+// Reproducer: the code shape that used to be in flightRecorder.cpp's
+// fillJavaMethodInfo() before the fix, with no readability guard before the
+// memcpy. Demonstrates that once the source page is gone, the copy step used
+// by resolveMethod() crashes the process rather than failing gracefully.
+TEST(LineNumberTableCopyRawTest, UnguardedCopyCrashesWhenSourceUnmapped) {
+  EXPECT_DEATH(
+      {
+        long page_size = sysconf(_SC_PAGESIZE);
+        void *page = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
+                          MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+        if (page == MAP_FAILED) {
+          _exit(1); // treat as death via non-zero exit if mmap itself fails
+        }
+        jint line_number_table_size = 4;
+        jvmtiLineNumberEntry *line_number_table =
+            makeFakeLineNumberTable(page, line_number_table_size);
+
+        // Stand-in for GetLineNumberTable() handing back a bad pointer for a
+        // corrupted/stale jmethodID: the backing memory is simply gone by
+        // the time the copy runs.
+        munmap(page, page_size);
+
+        // This mirrors the pre-fix fillJavaMethodInfo() shape verbatim: no
+        // readability check on line_number_table before the copy.
+        size_t bytes =
+            (size_t)line_number_table_size * sizeof(jvmtiLineNumberEntry);
+        void *owned_table = malloc(bytes);
+        memcpy(owned_table, line_number_table, bytes); // <-- crashes here
+        free(owned_table);
+      },
+      "");
+}
+
+// Documents the fix: guarding the same copy with
+// SafeAccess::isReadableRange() (the same primitive already used to probe
+// class_name/method_name/method_sig in fillJavaMethodInfo) turns the crash
+// into a clean, detectable failure with no memory touched past the guard.
+TEST_F(LineNumberTableCopyTest, GuardedCopySkipsSafelyWhenSourceUnmapped) {
+  long page_size = sysconf(_SC_PAGESIZE);
+  void *page = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
+                    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
+  ASSERT_NE(page, MAP_FAILED);
+
+  jint line_number_table_size = 4;
+  jvmtiLineNumberEntry *line_number_table =
+      makeFakeLineNumberTable(page, line_number_table_size);
+
+  ASSERT_EQ(0, munmap(page, page_size));
+
+  size_t bytes = (size_t)line_number_table_size * sizeof(jvmtiLineNumberEntry);
+  void *owned_table = nullptr;
+  if (SafeAccess::isReadableRange(line_number_table, bytes)) {
+    owned_table = malloc(bytes);
+    memcpy(owned_table, line_number_table, bytes);
+  }
+
+  EXPECT_EQ(nullptr, owned_table);
+  free(owned_table);
+}
+
+// Sanity check: the guard must not reject a genuinely valid table, or every
+// real dump would silently lose line-number info.
+TEST_F(LineNumberTableCopyTest, GuardedCopyStillWorksForValidSource) {
+  jint line_number_table_size = 8;
+  jvmtiLineNumberEntry stack_table[8];
+  jvmtiLineNumberEntry *line_number_table =
+      makeFakeLineNumberTable(stack_table, line_number_table_size);
+
+  size_t bytes = (size_t)line_number_table_size * sizeof(jvmtiLineNumberEntry);
+  void *owned_table = nullptr;
+  if (SafeAccess::isReadableRange(line_number_table, bytes)) {
+    owned_table = malloc(bytes);
+    memcpy(owned_table, line_number_table, bytes);
+  }
+
+  ASSERT_NE(nullptr, owned_table);
+  EXPECT_EQ(0, memcmp(owned_table, line_number_table, bytes));
+  free(owned_table);
+}