Re: [PATCH bpf-next] bpf: pack struct bpf_fib_lookup

[Alexander Lobakin <aleksander.lobakin@intel.com>]

From: Anton Protopopov <aspsk@isovalent.com>
Date: Wed,  3 Apr 2024 14:33:03 +0200

> The struct bpf_fib_lookup is supposed to be of size 64. A recent commit
> 59b418c7063d ("bpf: Add a check for struct bpf_fib_lookup size") added
> a static assertion to check this property so that future changes to the
> structure will not accidentally break this assumption.
> 
> As it immediately turned out, on some 32-bit arm systems, when AEABI=n,
> the total size of the structure was equal to 68, see [1]. This happened
> because the bpf_fib_lookup structure contains a union of two 16-bit
> fields:
> 
>     union {
>             __u16 tot_len;
>             __u16 mtu_result;
>     };
> 
> which was supposed to compile to a 16-bit-aligned 16-bit field. On the
> aforementioned setups it was instead both aligned and padded to 32-bits.
> 
> Declare this inner union as __attribute__((packed, aligned(2))) such
> that it always is of size 2 and is aligned to 16 bits.
> 
>   [1] https://lore.kernel.org/all/CA+G9fYtsoP51f-oP_Sp5MOq-Ffv8La2RztNpwvE6+R1VtFiLrw@mail.gmail.com/#t
> 
> Signed-off-by: Anton Protopopov <aspsk@isovalent.com>

Reviewed-by: Alexander Lobakin <aleksander.lobakin@intel.com>

> ---
>  include/uapi/linux/bpf.h       | 2 +-
>  tools/include/uapi/linux/bpf.h | 2 +-
>  2 files changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> index 79c548276b6b..6fe9f11c8abe 100644
> --- a/include/uapi/linux/bpf.h
> +++ b/include/uapi/linux/bpf.h
> @@ -7157,7 +7157,7 @@ struct bpf_fib_lookup {
>  
>  		/* output: MTU value */
>  		__u16	mtu_result;
> -	};
> +	} __attribute__((packed, aligned(2)));
>  	/* input: L3 device index for lookup
>  	 * output: device index from FIB lookup
>  	 */
> diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
> index 79c548276b6b..6fe9f11c8abe 100644
> --- a/tools/include/uapi/linux/bpf.h
> +++ b/tools/include/uapi/linux/bpf.h
> @@ -7157,7 +7157,7 @@ struct bpf_fib_lookup {
>  
>  		/* output: MTU value */
>  		__u16	mtu_result;
> -	};
> +	} __attribute__((packed, aligned(2)));
>  	/* input: L3 device index for lookup
>  	 * output: device index from FIB lookup
>  	 */

Thanks,
Olek