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

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

On Thu Jul 16, 2026 at 6:11 PM -03, Ihor Solodrai wrote:
> On 7/10/26 3:09 PM, Ricardo B. Marlière wrote:
>> 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);
>
> Acked-by: Ihor Solodrai <ihor.solodrai@linux.dev>
>
> nit: I wonder if there are other tests with this bug. Have you checked?

I don't think there are.

grepping for "map lookup unexpected error" in
https://openqa.opensuse.org/tests/6106285/logfile?filename=test_progs.tap.txt
shows only lsm_bdev.

Tumbleweed is 7.1.3

>
>>  
>>  	/* Look up the device in the BPF map and verify. */
>>  	err = bpf_map__lookup_elem(skel->maps.verity_devices,
>>