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

[Ihor Solodrai <ihor.solodrai@linux.dev>]

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?

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