Re: [LTP] [PATCH] syscalls/bpf: zero-initialize bpf_attr including padding bits

[Cyril Hrubis <chrubis@suse.cz>]

Hi!
> However kernel bpf syscall checks that all unused fields for a command
> are set to zero in CHECK_ATTR() macro, which causes tests to fail with
> EINVAL.
> 
> Signed-off-by: Jan Stancek <jstancek@redhat.com>
> ---
>  testcases/kernel/syscalls/bpf/bpf_common.c | 32 ++++++++++++----------
>  1 file changed, 18 insertions(+), 14 deletions(-)
> 
> diff --git a/testcases/kernel/syscalls/bpf/bpf_common.c b/testcases/kernel/syscalls/bpf/bpf_common.c
> index 95b5bc12eaa4..d765c4e32936 100644
> --- a/testcases/kernel/syscalls/bpf/bpf_common.c
> +++ b/testcases/kernel/syscalls/bpf/bpf_common.c
> @@ -49,13 +49,14 @@ int bpf_map_create(union bpf_attr *const attr)
>  
>  int bpf_map_array_create(const uint32_t max_entries)
>  {
> -	union bpf_attr map_attr = {
> -		.map_type = BPF_MAP_TYPE_ARRAY,
> -		.key_size = 4,
> -		.value_size = 8,
> -		.max_entries = max_entries,
> -		.map_flags = 0
> -	};
> +	/* zero-initialize entire struct including padding bits */
> +	union bpf_attr map_attr = {};
> +
> +	map_attr.map_type = BPF_MAP_TYPE_ARRAY;
> +	map_attr.key_size = 4;
> +	map_attr.value_size = 8;
> +	map_attr.max_entries = max_entries;
> +	map_attr.map_flags = 0;

I had a closer look here, the map_attr is an union with anonymous
structures and I suppose that the problem here is that the padding after
the union is no longer cleared and that there have been some new fields
added, at least compared to the lapi fallback structures we have and we
possibly pass random mess in flags.

Maybe slightly better version would be:

diff --git a/testcases/kernel/syscalls/bpf/bpf_common.c b/testcases/kernel/syscalls/bpf/bpf_common.c
index 95b5bc12e..a8289e106 100644
--- a/testcases/kernel/syscalls/bpf/bpf_common.c
+++ b/testcases/kernel/syscalls/bpf/bpf_common.c
@@ -49,7 +49,9 @@ int bpf_map_create(union bpf_attr *const attr)

 int bpf_map_array_create(const uint32_t max_entries)
 {
-       union bpf_attr map_attr = {
+       union bpf_attr map_attr = {};
+
+       map_attr = (union bpf_attr) {
                .map_type = BPF_MAP_TYPE_ARRAY,
                .key_size = 4,
                .value_size = 8,


-- 
Cyril Hrubis
chrubis@suse.cz

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