linux: add fallback-modes for memfd_create

Bug: chromium:1051354
Change-Id: I5dbbb3b264c09060429db199aa9f046c2f317c48
Reviewed-on: https://chromium-review.googlesource.com/c/crashpad/crashpad/+/2080651
Reviewed-by: Mark Mentovai <[email protected]>

handler/linux/cros_crash_report_exception_handler.cc

@@ -225,7 +225,7 @@ bool CrosCrashReportExceptionHandler::HandleExceptionWithConnection(
   AddUserExtensionStreams(user_stream_data_sources_, snapshot, &minidump);
 
   FileWriter file_writer;
-  if (!file_writer.OpenMemfd(base::FilePath("/tmp/minidump"))) {
+  if (!file_writer.OpenMemfd(base::FilePath("minidump"))) {
     Metrics::ExceptionCaptureResult(Metrics::CaptureResult::kOpenMemfdFailed);
     return false;
   }

util/file/file_io.h

@@ -399,16 +399,27 @@ FileHandle LoggingOpenFileForWrite(const base::FilePath& path,
                                    FilePermissions permissions);
 
 #if defined(OS_LINUX)
-//! \brief Wraps memfd_create(), logging an error if the operation fails.
-//!     Unlike other file open operations, this doesn't set `O_CLOEXEC`.
+//! \brief Opens an in-memory file for input and output.
 //!
-//! \return The newly opened FileHandle, or an invalid FileHandle on failure.
+//! This function first attempts to open the file with `memfd_create()`. If
+//! `memfd_create()` isn't supported by the kernel, this function next attempts
+//! to open a file using `O_TMPFILE`. If `O_TMPFILE` isn't supported, this
+//! function finally falls back to creating a file with a randomized name in
+//! `/tmp` and immediately `unlink()`ing it.
+//!
+//! Unlike other file open operations, this function doesn't set `O_CLOEXEC`.
+//!
+//! \param name A name associated with the file. This name does not indicate any
+//!     exact path and may not be used at all, depending on the strategy used to
+//!     create the file. The name should not contain any '/' characters.
+//! \return The newly opened FileHandle, or an invalid FileHandle on failure,
+//!     with a message logged.
 //!
 //! \sa ScopedFileHandle
 //! \sa LoggingOpenFileForRead
 //! \sa LoggingOpenFileForWrite
 //! \sa LoggingOpenFileForReadAndWrite
-FileHandle LoggingOpenMemFileForWrite(const base::FilePath& path);
+FileHandle LoggingOpenMemoryFileForReadAndWrite(const base::FilePath& name);
 #endif  // OS_LINUX
 
 //! \brief Wraps OpenFileForReadAndWrite(), logging an error if the operation

util/file/file_io_posix.cc

@@ -14,6 +14,7 @@
 
 #include "util/file/file_io.h"
 
+#include <errno.h>
 #include <fcntl.h>
 #include <sys/file.h>
 #include <sys/mman.h>
@@ -26,7 +27,9 @@
 #include "base/files/file_path.h"
 #include "base/logging.h"
 #include "base/posix/eintr_wrapper.h"
+#include "base/strings/stringprintf.h"
 #include "build/build_config.h"
+#include "util/misc/random_string.h"
 
 namespace crashpad {
 
@@ -98,13 +101,6 @@ FileHandle OpenFileForOutput(int rdwr_or_wronly,
            flags,
            permissions == FilePermissions::kWorldReadable ? 0644 : 0600));
 }
-
-#if defined(OS_LINUX)
-FileHandle OpenMemFileForOutput(const base::FilePath& path) {
-  return HANDLE_EINTR(memfd_create(path.value().c_str(), 0));
-}
-#endif
-
 }  // namespace
 
 namespace internal {
@@ -157,10 +153,49 @@ FileHandle LoggingOpenFileForWrite(const base::FilePath& path,
 }
 
 #if defined(OS_LINUX)
-FileHandle LoggingOpenMemFileForWrite(const base::FilePath& path) {
-  FileHandle fd = OpenMemFileForOutput(path);
-  PLOG_IF(ERROR, fd < 0) << "memfd_create " << path.value();
-  return fd;
+FileHandle LoggingOpenMemoryFileForReadAndWrite(const base::FilePath& name) {
+  DCHECK(name.value().find('/') == std::string::npos);
+
+  int result = HANDLE_EINTR(memfd_create(name.value().c_str(), 0));
+  if (result >= 0 || errno != ENOSYS) {
+    PLOG_IF(ERROR, result < 0) << "memfd_create";
+    return result;
+  }
+
+  const char* tmp = getenv("TMPDIR");
+  tmp = tmp ? tmp : "/tmp";
+
+  result = HANDLE_EINTR(open(tmp, O_RDWR | O_EXCL | O_TMPFILE, 0600));
+  if (result >= 0 ||
+      // These are the expected possible error codes indicating that O_TMPFILE
+      // doesn't have kernel or filesystem support. O_TMPFILE was added in Linux
+      // 3.11. Experimentation confirms that at least Linux 2.6.29 and Linux
+      // 3.10 set errno to EISDIR. EOPNOTSUPP is returned when the filesystem
+      // doesn't support O_TMPFILE. The man pages also mention ENOENT as an
+      // error code to check, but the language implies it would only occur when
+      // |tmp| is also an invalid directory. EINVAL is mentioned as a possible
+      // error code for any invalid values in flags, but O_TMPFILE isn't
+      // mentioned explicitly in this context and hasn't been observed in
+      // practice.
+      (errno != EISDIR && errno != EOPNOTSUPP)) {
+    PLOG_IF(ERROR, result < 0) << "open";
+    return result;
+  }
+
+  std::string path = base::StringPrintf("%s/%s.%d.%s",
+                                        tmp,
+                                        name.value().c_str(),
+                                        getpid(),
+                                        RandomString().c_str());
+  result = HANDLE_EINTR(open(path.c_str(), O_RDWR | O_CREAT | O_EXCL, 0600));
+  if (result < 0) {
+    PLOG(ERROR) << "open";
+    return result;
+  }
+  if (unlink(path.c_str()) != 0) {
+    PLOG(WARNING) << "unlink";
+  }
+  return result;
 }
 #endif
 

util/file/file_io_test.cc

@@ -473,6 +473,23 @@ TEST(FileIO, LoggingOpenFileForReadAndWrite) {
   TestOpenFileForWrite(LoggingOpenFileForReadAndWrite);
 }
 
+#if defined(OS_LINUX)
+TEST(FileIO, LoggingOpenMemoryFileForReadAndWrite) {
+  ScopedFileHandle handle(
+      LoggingOpenMemoryFileForReadAndWrite(base::FilePath("memfile")));
+  ASSERT_TRUE(handle.is_valid());
+
+  static constexpr char kTestData[] = "somedata";
+  ASSERT_TRUE(LoggingWriteFile(handle.get(), kTestData, sizeof(kTestData)));
+
+  ASSERT_EQ(LoggingSeekFile(handle.get(), 0, SEEK_SET), 0);
+
+  char buffer[sizeof(kTestData)];
+  ASSERT_TRUE(LoggingReadFileExactly(handle.get(), buffer, sizeof(buffer)));
+  EXPECT_EQ(memcmp(buffer, kTestData, sizeof(buffer)), 0);
+}
+#endif  // OS_LINUX
+
 enum class ReadOrWrite : bool {
   kRead,
   kWrite,

util/file/file_writer.cc

@@ -174,7 +174,7 @@ bool FileWriter::Open(const base::FilePath& path,
 #if defined(OS_LINUX)
 bool FileWriter::OpenMemfd(const base::FilePath& path) {
   CHECK(!file_.is_valid());
-  file_.reset(LoggingOpenMemFileForWrite(path));
+  file_.reset(LoggingOpenMemoryFileForReadAndWrite(path));
   if (!file_.is_valid()) {
     return false;
   }

util/file/file_writer.h

@@ -132,7 +132,7 @@ class FileWriter : public FileWriterInterface {
             FilePermissions permissions);
 
 #if defined(OS_LINUX)
-  //! \brief Wraps LoggingOpenMemFileForWrite().
+  //! \brief Wraps LoggingOpenMemoryFileForWrite().
   //!
   //! \return `true` if the operation succeeded, `false` if it failed, with an
   //!     error message logged.