BpfSyscallWrappers: grab shared lock on writable map open
and add an accessor to grab an exclusive lock on a R/W map open
(such a map could be accessed with a write-through cache)
Note: we can't grab a flock as that would occupy the full inode,
and all bpfmaps are actually (currently) the *same* anonymous inode.
As such we actually grab a lock on a range (a single byte),
the offset being determined by the unique bpf map id.
We include some very simple, but sufficient, correctness tests
in the critical boot path: this is to prevent any surprises
caused by kernel implementation changes.
$ adb root && sleep 1 && adb wait-for-device shell grep OFDLCK /proc/locks
id: OFDLCK ADVISORY [WRITE|READ] pid blkmaj:min:inode min_offset max_offset
11: OFDLCK ADVISORY READ -1 00:0e:1048 36 36
14: OFDLCK ADVISORY READ -1 00:0e:1048 35 35
15: OFDLCK ADVISORY READ -1 00:0e:1048 41 41
16: OFDLCK ADVISORY READ -1 00:0e:1048 40 40
22: OFDLCK ADVISORY READ -1 00:0e:1048 24 24
23: OFDLCK ADVISORY READ -1 00:0e:1048 17 17
24: OFDLCK ADVISORY READ -1 00:0e:1048 16 16
25: OFDLCK ADVISORY READ -1 00:0e:1048 13 13
OFDLCK probably means 'Open File Descriptor LoCK' since an OFDLCK
is associated with (held by) the file descriptor and not a process/pid,
on the given (anonymous in this case) block device + inode.
Where READ=shared and WRITE=exclusive.
There are (as yet) no exclusive locks held post boot.
The pid field is unfortunately always -1 (and cannot be manually set).
The 00:0e:1048 (or at least the inode portion) is random
(likely depends on boot ordering)
The final two fields (min and max offset) are the bpf map id.
Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I208e3450da3fe4689ad5fd578539f401f25a4fef
diff --git a/netd/BpfHandler.cpp b/netd/BpfHandler.cpp
index 925ee50..aa84089 100644
--- a/netd/BpfHandler.cpp
+++ b/netd/BpfHandler.cpp
@@ -181,7 +181,27 @@
return netdutils::status::ok;
}
+static void mapLockTest(void) {
+ // The maps must be R/W, and as yet unopened (or more specifically not yet lock'ed).
+ const char * const m1 = BPF_NETD_PATH "map_netd_lock_array_test_map";
+ const char * const m2 = BPF_NETD_PATH "map_netd_lock_hash_test_map";
+
+ unique_fd fd0(bpf::mapRetrieveExclusiveRW(m1)); if (!fd0.ok()) abort();
+
+ unique_fd fd1(bpf::mapRetrieveExclusiveRW(m2)); if (!fd1.ok()) abort(); // no conflict with fd0
+ unique_fd fd2(bpf::mapRetrieveExclusiveRW(m2)); if ( fd2.ok()) abort(); // busy due to fd1
+ unique_fd fd3(bpf::mapRetrieveRO(m2)); if (!fd3.ok()) abort(); // no lock taken
+ unique_fd fd4(bpf::mapRetrieveRW(m2)); if ( fd4.ok()) abort(); // busy due to fd1
+ fd1.reset(); // releases exclusive lock
+ unique_fd fd5(bpf::mapRetrieveRO(m2)); if (!fd5.ok()) abort(); // no lock taken
+ unique_fd fd6(bpf::mapRetrieveRW(m2)); if (!fd6.ok()) abort(); // now ok
+ unique_fd fd7(bpf::mapRetrieveRO(m2)); if (!fd7.ok()) abort(); // no lock taken
+ unique_fd fd8(bpf::mapRetrieveExclusiveRW(m2)); if ( fd8.ok()) abort(); // busy due to fd6
+}
+
Status BpfHandler::initMaps() {
+ mapLockTest();
+
RETURN_IF_NOT_OK(mStatsMapA.init(STATS_MAP_A_PATH));
RETURN_IF_NOT_OK(mStatsMapB.init(STATS_MAP_B_PATH));
RETURN_IF_NOT_OK(mConfigurationMap.init(CONFIGURATION_MAP_PATH));