libbrillo: add retries for attach device

While it is unlikely, loopdev->AttachDevice should not
indefinitely try to attach a loop device. Instead,
set an arbitrary retry limit (10).

BUG=chromium:968826
TEST=unittests.

Change-Id: I124c11c241cfb2351a87681973f1f1d97e62ce36
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform2/+/1644362
Tested-by: Sarthak Kukreti <sarthakkukreti@chromium.org>
Commit-Queue: Sarthak Kukreti <sarthakkukreti@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Gwendal Grignou <gwendal@chromium.org>
Cr-Mirrored-From: https://chromium.googlesource.com/chromiumos/platform2
Cr-Mirrored-Commit: 3066f10027f9b8e4547a0ff352228eca60318128

1 files changed, 17 insertions, 1 deletions

diff --git a/brillo/blkdev_utils/loop_device.cc b/brillo/blkdev_utils/loop_device.cc
index 2b2219d..0ba485a 100644
--- a/brillo/blkdev_utils/loop_device.cc
+++ b/brillo/blkdev_utils/loop_device.cc
@@ -35,6 +35,8 @@ constexpr char kDeviceIdPath[] = "dev";
 constexpr char kLoopBackingFile[] = "loop/backing_file";
 constexpr int kLoopDeviceIoctlFlags = O_RDWR | O_NOFOLLOW | O_CLOEXEC;
 constexpr int kLoopControlIoctlFlags = O_RDONLY | O_NOFOLLOW | O_CLOEXEC;
+// Arbitrary retry limit for attempting to attach a loop device.
+constexpr int kMaxLoopDeviceAttachTries = 10;
 
 // ioctl runner for LoopDevice and LoopDeviceManager
 int LoopDeviceIoctl(const base::FilePath& device,
@@ -165,7 +167,9 @@ LoopDeviceManager::LoopDeviceManager(LoopIoctl ioctl_runner)
 std::unique_ptr<LoopDevice> LoopDeviceManager::AttachDeviceToFile(
     const base::FilePath& backing_file) {
   int device_number = -1;
-  while (true) {
+  int retries = kMaxLoopDeviceAttachTries;
+
+  while (retries >= 0) {
     device_number =
         loop_ioctl_.Run(base::FilePath(kLoopControl), LOOP_CTL_GET_FREE, 0,
                         kLoopControlIoctlFlags);
@@ -193,7 +197,19 @@ std::unique_ptr<LoopDevice> LoopDeviceManager::AttachDeviceToFile(
       LOG(ERROR) << "ioctl(LOOP_SET_FD) failed";
       return CreateLoopDevice(-1, base::FilePath());
     }
+
+    // Other users could have set the file descriptor between get a valid
+    // loop device number and using it. Continue to loop until the device is
+    // created.
+    retries--;
+  }
+
+  if (retries < 0) {
+    LOG(ERROR) << "Failed to set up loop device after "
+               << kMaxLoopDeviceAttachTries << " retries.";
+    return CreateLoopDevice(-1, base::FilePath());
   }
+
   // All steps of setting up the loop device succeeded.
   return CreateLoopDevice(device_number, backing_file);
 }