Skip FUSE request from /dev/fuse if unique=0

APCT log shows that we got FUSE request unique=0 and replying to such
request causes a EINVAL.

The possible reasons of getting unique=0 here are:

 * /dev/fuse actually submits such requests. In this case, not replying
   to such request probabbly safe as the kernel cannot wait corresponding
   response without a unique number. We can observing the kernel code to
   find out what unique=0 actually means.
 * Memory corruption happens and unique number are cleared with zero.
   In this case, if we skip unique=0 request, libappfuse does not reply
   to the kernel request and APCT result will become timeout .

To see which case happens, the CL ScopedLogSeverity to output
verbose logs and lets FuseBridgeLoop skip a request from /dev/fuse if unique=0.

Bug: 62429763
Test: libappfuse_tests
Change-Id: I8c4d532564b690d55573b92260170b0cd68150ab

2 files changed, 9 insertions, 1 deletions

diff --git a/libappfuse/FuseBridgeLoop.cc b/libappfuse/FuseBridgeLoop.cc
index 07923071b..5a897a472 100644
--- a/libappfuse/FuseBridgeLoop.cc
+++ b/libappfuse/FuseBridgeLoop.cc
@@ -173,13 +173,20 @@ class FuseBridgeEntry {
     }
 
     FuseBridgeState ReadFromDevice(FuseBridgeLoopCallback* callback) {
+        // To observe APCT failures.
+        base::ScopedLogSeverity log_severity(base::VERBOSE);
+
         LOG(VERBOSE) << "ReadFromDevice";
         if (!buffer_.request.Read(device_fd_)) {
             return FuseBridgeState::kClosing;
         }
 
         const uint32_t opcode = buffer_.request.header.opcode;
-        LOG(VERBOSE) << "Read a fuse packet, opcode=" << opcode;
+        const uint64_t unique = buffer_.request.header.unique;
+        LOG(VERBOSE) << "Read a fuse packet, opcode=" << opcode << " unique=" << unique;
+        if (unique == 0) {
+            return FuseBridgeState::kWaitToReadEither;
+        }
         switch (opcode) {
             case FUSE_FORGET:
                 // Do not reply to FUSE_FORGET.
diff --git a/libappfuse/tests/FuseBridgeLoopTest.cc b/libappfuse/tests/FuseBridgeLoopTest.cc
index 51d605136..0a28451bf 100644
--- a/libappfuse/tests/FuseBridgeLoopTest.cc
+++ b/libappfuse/tests/FuseBridgeLoopTest.cc
@@ -67,6 +67,7 @@ class FuseBridgeLoopTest : public ::testing::Test {
     memset(&request_, 0, sizeof(FuseRequest));
     request_.header.opcode = opcode;
     request_.header.len = sizeof(fuse_in_header);
+    request_.header.unique = 1;
     ASSERT_TRUE(request_.Write(dev_sockets_[0]));
 
     memset(&response_, 0, sizeof(FuseResponse));