From: Anton Protopopov <aspsk@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxxx/#t
>
> Signed-off-by: Anton Protopopov <aspsk@xxxxxxxxxxxxx>
Reviewed-by: Alexander Lobakin <aleksander.lobakin@xxxxxxxxx>
> ---
> 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
