From aa5ff205cbe6ce5be97e032b50e5d3cf4b7c3d0a Mon Sep 17 00:00:00 2001
From: Lei Zhang <thestig@google.com>
Date: Thu, 5 Nov 2015 15:45:01 -0800
Subject: [PATCH] Android: Workaround for ftruncate() issues.

This works around a bug in M that prevents Breakpad from using
ftruncate() in the renderer process.

To do this, skip the calls to ftruncate() when allocating bigger
minidump files and strictly depends on write() to append to the end.

It might be less efficient but this is probably less of an issue on
SD cards. It is much better than not getting crash reports.

BUG=542840

Original CL: https://codereview.appspot.com/273880044/
Original CL Author: acleung@chromium.org

Review URL: https://codereview.chromium.org/1407233016 .
---
 src/client/minidump_file_writer.cc | 78 +++++++++++++++++++++++++++---
 1 file changed, 72 insertions(+), 6 deletions(-)

diff --git a/src/client/minidump_file_writer.cc b/src/client/minidump_file_writer.cc
index 9e905335..a1957f32 100644
--- a/src/client/minidump_file_writer.cc
+++ b/src/client/minidump_file_writer.cc
@@ -44,6 +44,47 @@
 #include "third_party/lss/linux_syscall_support.h"
 #endif
 
+#if defined(__ANDROID__)
+#include <errno.h>
+
+namespace {
+
+bool g_need_ftruncate_workaround = false;
+bool g_checked_need_ftruncate_workaround = false;
+
+void CheckNeedsFTruncateWorkAround(int file) {
+  if (g_checked_need_ftruncate_workaround) {
+    return;
+  }
+  g_checked_need_ftruncate_workaround = true;
+
+  // Attempt an idempotent truncate that chops off nothing and see if we
+  // run into any sort of errors.
+  off_t offset = sys_lseek(file, 0, SEEK_END);
+  if (offset == -1) {
+    // lseek failed. Don't apply work around. It's unlikely that we can write
+    // to a minidump with either method.
+    return;
+  }
+
+  int result = ftruncate(file, offset);
+  if (result == -1 && errno == EACCES) {
+    // It very likely that we are running into the kernel bug in M devices.
+    // We are going to deploy the workaround for writing minidump files
+    // without uses of ftruncate(). This workaround should be fine even
+    // for kernels without the bug.
+    // See http://crbug.com/542840 for more details.
+    g_need_ftruncate_workaround = true;
+  }
+}
+
+bool NeedsFTruncateWorkAround() {
+  return g_need_ftruncate_workaround;
+}
+
+}  // namespace
+#endif  // defined(__ANDROID__)
+
 namespace google_breakpad {
 
 const MDRVA MinidumpFileWriter::kInvalidMDRVA = static_cast<MDRVA>(-1);
@@ -75,15 +116,24 @@ void MinidumpFileWriter::SetFile(const int file) {
   assert(file_ == -1);
   file_ = file;
   close_file_when_destroyed_ = false;
+#if defined(__ANDROID__)
+  CheckNeedsFTruncateWorkAround(file);
+#endif
 }
 
 bool MinidumpFileWriter::Close() {
   bool result = true;
 
   if (file_ != -1) {
-    if (-1 == ftruncate(file_, position_)) {
+#if defined(__ANDROID__)
+    if (!NeedsFTruncateWorkAround() && ftruncate(file_, position_)) {
        return false;
     }
+#else
+    if (ftruncate(file_, position_)) {
+       return false;
+    }
+#endif
 #if defined(__linux__) && __linux__
     result = (sys_close(file_) == 0);
 #else
@@ -220,6 +270,20 @@ bool MinidumpFileWriter::WriteMemory(const void *src, size_t size,
 MDRVA MinidumpFileWriter::Allocate(size_t size) {
   assert(size);
   assert(file_ != -1);
+#if defined(__ANDROID__)
+  if (NeedsFTruncateWorkAround()) {
+    // If ftruncate() is not available. We simply increase the size beyond the
+    // current file size. sys_write() will expand the file when data is written
+    // to it. Because we did not over allocate to fit memory pages, we also
+    // do not need to ftruncate() the file once we are done.
+    size_ += size;
+
+    // We don't need to seek since the file is unchanged.
+    MDRVA current_position = position_;
+    position_ += static_cast<MDRVA>(size);
+    return current_position;
+  }
+#endif
   size_t aligned_size = (size + 7) & ~7;  // 64-bit alignment
 
   if (position_ + aligned_size > size_) {
@@ -256,14 +320,16 @@ bool MinidumpFileWriter::Copy(MDRVA position, const void *src, ssize_t size) {
 #if defined(__linux__) && __linux__
   if (sys_lseek(file_, position, SEEK_SET) == static_cast<off_t>(position)) {
     if (sys_write(file_, src, size) == size) {
-#else
-  if (lseek(file_, position, SEEK_SET) == static_cast<off_t>(position)) {
-    if (write(file_, src, size) == size) {
-#endif
       return true;
     }
   }
-
+#else
+  if (lseek(file_, position, SEEK_SET) == static_cast<off_t>(position)) {
+    if (write(file_, src, size) == size) {
+      return true;
+    }
+  }
+#endif
   return false;
 }