Re: [PATCH] bpf: avoid VLAs in progs/test_xdp_dynptr.c

[Yonghong Song <yonghong.song@linux.dev>]

On 1/23/24 12:17 PM, Jose E. Marchesi wrote:
> VLAs are not supported by either the BPF port of clang nor GCC.  The
> selftest test_xdp_dynptr.c contains the following code:
>
>    const size_t tcphdr_sz = sizeof(struct tcphdr);
>    const size_t udphdr_sz = sizeof(struct udphdr);
>    const size_t ethhdr_sz = sizeof(struct ethhdr);
>    const size_t iphdr_sz = sizeof(struct iphdr);
>    const size_t ipv6hdr_sz = sizeof(struct ipv6hdr);
>
>    [...]
>
>    static __always_inline int handle_ipv4(struct xdp_md *xdp, struct bpf_dynptr *xdp_ptr)
>    {
> 	__u8 eth_buffer[ethhdr_sz + iphdr_sz + ethhdr_sz];
> 	__u8 iph_buffer_tcp[iphdr_sz + tcphdr_sz];
> 	__u8 iph_buffer_udp[iphdr_sz + udphdr_sz];
> 	[...]
>    }
>
> The eth_buffer, iph_buffer_tcp and other automatics are fixed size
> only if the compiler optimizes away the constant global variables.
> clang does this, but GCC does not, turning these automatics into
> variable length arrays.
>
> This patch removes the global variables and turns these values into
> preprocessor constants.  This makes the selftest to build properly
> with GCC.
>
> Tested in bpf-next master.
> No regressions.
>
> Signed-off-by: Jose E. Marchesi <jose.marchesi@oracle.com>
> Cc: Yonghong Song <yhs@meta.com>
> Cc: Eduard Zingerman <eddyz87@gmail.com>
> Cc: david.faust@oracle.com
> Cc: cupertino.miranda@oracle.com

Acked-by: Yonghong Song <yonghong.song@linux.dev>