[PATCH 3/4] selftests/bpf: Fix lsm_bdev dev_t encoding mismatch

["Ricardo B. Marlière" <rbm@suse.com>]

progs/lsm_bdev.c keys its verity_devices hashmap with the raw kernel dev_t
read straight off bdev->bd_dev, i.e. MKDEV(major, minor) = (major << 20) |
minor. prog_tests/lsm_bdev.c instead builds its lookup key with dev_key =
(__u32)st.st_rdev from stat(2), but the stat(2) syscall fills st_rdev via
the kernel's new_encode_dev(), a different bit layout: (minor & 0xff) |
(major << 8) | ((minor & ~0xff) << 12).

For any device with a non-trivial major these two values differ, so the
lookup can never find what the BPF program stored, and test_lsm_bdev()
always fails with:

  test_lsm_bdev:FAIL:map lookup unexpected error: -2 (errno 2)

Reconstruct the raw kernel dev_t from the decoded major/minor instead of
casting st_rdev directly, restoring the layout the BPF program actually
reads.

Fixes: 96f4c251a087 ("selftests/bpf: add block device management selftests")
Signed-off-by: Ricardo B. Marlière <rbm@suse.com>
---
 tools/testing/selftests/bpf/prog_tests/lsm_bdev.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/lsm_bdev.c b/tools/testing/selftests/bpf/prog_tests/lsm_bdev.c
index a970798e1173..28bc4b117f41 100644
--- a/tools/testing/selftests/bpf/prog_tests/lsm_bdev.c
+++ b/tools/testing/selftests/bpf/prog_tests/lsm_bdev.c
@@ -17,6 +17,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <sys/stat.h>
+#include <sys/sysmacros.h>
 #include <sys/types.h>
 #include <unistd.h>
 #include "lsm_bdev.skel.h"
@@ -172,7 +173,7 @@ void test_lsm_bdev(void)
 	if (!ASSERT_OK(stat(DM_DEV_PATH, &st), "stat dm dev"))
 		goto remove_dm;
 
-	dev_key = (__u32)st.st_rdev;
+	dev_key = (major(st.st_rdev) << 20) | minor(st.st_rdev);
 
 	/* Look up the device in the BPF map and verify. */
 	err = bpf_map__lookup_elem(skel->maps.verity_devices,

-- 
2.54.0