Recover memory mappings before writing dump on ChromeOS

On Linux, breakpad relies on /proc/[pid]/maps to associate symbols from
addresses. ChromeOS' hugepage implementation replaces some segments
with anonymous private pages, which is a restriction of current
implementation in Linux kernel at the time of writing. Thus, breakpad
can no longer symbolize addresses from those text segments replaced by
hugepages.

This patch tries to recover the mappings. Because hugepages are always
inserted in between some .text sections, it tries to infer the names and
offsets of the segments, by looking at segments immediately precede and
succeed them.

For example, a text segment before hugepage optimization
  02001000-03002000 r-xp /opt/google/chrome/chrome

can be broken into
  02001000-02200000 r-xp /opt/google/chrome/chrome
  02200000-03000000 r-xp
  03000000-03002000 r-xp /opt/google/chrome/chrome

BUG=crbug.com/628040
R=mark@chromium.org

Review URL: https://codereview.chromium.org/2161713002 .

Patch from Ting-Yuan (Leo) Huang <laszio@chromium.org>.

src/client/linux/minidump_writer/linux_dumper.cc

@@ -84,6 +84,173 @@ inline static bool IsMappedFileOpenUnsafe(
 
 namespace google_breakpad {
 
+#if defined(__CHROMEOS__)
+
+namespace {
+
+// Recover memory mappings before writing dump on ChromeOS
+//
+// On Linux, breakpad relies on /proc/[pid]/maps to associate symbols from
+// addresses. ChromeOS' hugepage implementation replaces some segments with
+// anonymous private pages, which is a restriction of current implementation
+// in Linux kernel at the time of writing. Thus, breakpad can no longer
+// symbolize addresses from those text segments replaced with hugepages.
+//
+// This postprocess tries to recover the mappings. Because hugepages are always
+// inserted in between some .text sections, it tries to infer the names and
+// offsets of the segments, by looking at segments immediately precede and
+// succeed them.
+//
+// For example, a text segment before hugepage optimization
+//   02001000-03002000 r-xp /opt/google/chrome/chrome
+//
+// can be broken into
+//   02001000-02200000 r-xp /opt/google/chrome/chrome
+//   02200000-03000000 r-xp
+//   03000000-03002000 r-xp /opt/google/chrome/chrome
+//
+// For more details, see:
+// crbug.com/628040 ChromeOS' use of hugepages confuses crash symbolization
+
+// Copied from CrOS' hugepage implementation, which is unlikely to change.
+// The hugepage size is 2M.
+const unsigned int kHpageShift = 21;
+const size_t kHpageSize = (1 << kHpageShift);
+const size_t kHpageMask = (~(kHpageSize - 1));
+
+// Find and merge anonymous r-xp segments with surrounding named segments.
+// There are two cases:
+
+// Case 1: curr, next
+//   curr is anonymous
+//   curr is r-xp
+//   curr.size >= 2M
+//   curr.size is a multiple of 2M.
+//   next is backed by some file.
+//   curr and next are contiguous.
+//   offset(next) == sizeof(curr)
+void TryRecoverMappings(MappingInfo *curr, MappingInfo *next) {
+  // Merged segments are marked with size = 0.
+  if (curr->size == 0 || next->size == 0)
+    return;
+
+  if (curr->size >= kHpageSize &&
+      curr->exec &&
+      (curr->size & kHpageMask) == curr->size &&
+      (curr->start_addr & kHpageMask) == curr->start_addr &&
+      curr->name[0] == '\0' &&
+      next->name[0] != '\0' &&
+      curr->start_addr + curr->size == next->start_addr &&
+      curr->size == next->offset) {
+
+    // matched
+    my_strlcpy(curr->name, next->name, NAME_MAX);
+    if (next->exec) {
+      // (curr, next)
+      curr->size += next->size;
+      next->size = 0;
+    }
+  }
+}
+
+// Case 2: prev, curr, next
+//   curr is anonymous
+//   curr is r-xp
+//   curr.size >= 2M
+//   curr.size is a multiple of 2M.
+//   next and prev are backed by the same file.
+//   prev, curr and next are contiguous.
+//   offset(next) == offset(prev) + sizeof(prev) + sizeof(curr)
+void TryRecoverMappings(MappingInfo *prev, MappingInfo *curr,
+    MappingInfo *next) {
+  // Merged segments are marked with size = 0.
+  if (prev->size == 0 || curr->size == 0 || next->size == 0)
+    return;
+
+  if (curr->size >= kHpageSize &&
+      curr->exec &&
+      (curr->size & kHpageMask) == curr->size &&
+      (curr->start_addr & kHpageMask) == curr->start_addr &&
+      curr->name[0] == '\0' &&
+      next->name[0] != '\0' &&
+      curr->start_addr + curr->size == next->start_addr &&
+      prev->start_addr + prev->size == curr->start_addr &&
+      my_strncmp(prev->name, next->name, NAME_MAX) == 0 &&
+      next->offset == prev->offset + prev->size + curr->size) {
+
+    // matched
+    my_strlcpy(curr->name, prev->name, NAME_MAX);
+    if (prev->exec) {
+      curr->offset = prev->offset;
+      curr->start_addr = prev->start_addr;
+      if (next->exec) {
+        // (prev, curr, next)
+        curr->size += prev->size + next->size;
+        prev->size = 0;
+        next->size = 0;
+      } else {
+        // (prev, curr), next
+        curr->size += prev->size;
+        prev->size = 0;
+      }
+    } else {
+      curr->offset = prev->offset + prev->size;
+      if (next->exec) {
+        // prev, (curr, next)
+        curr->size += next->size;
+        next->size = 0;
+      } else {
+        // prev, curr, next
+      }
+    }
+  }
+}
+
+// mappings_ is sorted excepted for the first entry.
+// This function tries to merge segemnts into the first entry,
+// then check for other sorted entries.
+// See LinuxDumper::EnumerateMappings().
+void CrOSPostProcessMappings(wasteful_vector<MappingInfo*>& mappings) {
+  // Find the candidate "next" to first segment, which is the only one that
+  // could be out-of-order.
+  size_t l = 1;
+  size_t r = mappings.size();
+  size_t next = mappings.size();
+  while (l < r) {
+    int m = (l + r) / 2;
+    if (mappings[m]->start_addr > mappings[0]->start_addr)
+      r = next = m;
+    else
+      l = m + 1;
+  }
+
+  // Try to merge segments into the first.
+  if (next < mappings.size()) {
+    TryRecoverMappings(mappings[0], mappings[next]);
+    if (next - 1 > 0)
+      TryRecoverMappings(mappings[next - 1], mappings[0], mappings[next]);
+  }
+
+  // Iterate through normal, sorted cases.
+  // Normal case 1.
+  for (size_t i = 1; i < mappings.size() - 1; i++)
+    TryRecoverMappings(mappings[i], mappings[i + 1]);
+
+  // Normal case 2.
+  for (size_t i = 1; i < mappings.size() - 2; i++)
+    TryRecoverMappings(mappings[i], mappings[i + 1], mappings[i + 2]);
+
+  // Collect merged (size == 0) segments.
+  size_t f, e;
+  for (f = e = 0; e < mappings.size(); e++)
+    if (mappings[e]->size > 0)
+      mappings[f++] = mappings[e];
+  mappings.resize(f);
+}
+
+}  // namespace
+#endif  // __CHROMEOS__
+
 // All interesting auvx entry types are below AT_SYSINFO_EHDR
 #define AT_MAX AT_SYSINFO_EHDR
 
@@ -113,6 +280,11 @@ bool LinuxDumper::LateInit() {
 #if defined(__ANDROID__)
   LatePostprocessMappings();
 #endif
+
+#if defined(__CHROMEOS__)
+  CrOSPostProcessMappings(mappings_);
+#endif
+
   return true;
 }