netdev - Re: [PATCH bpf-next] bpf: fix bpf_prog_pack build for ppc64

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Date:   Fri, 11 Feb 2022 15:54:56 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Song Liu <song@...nel.org>
Cc:     <bpf@...r.kernel.org>, <netdev@...r.kernel.org>, <ast@...nel.org>,
        <daniel@...earbox.net>, <andrii@...nel.org>, <kernel-team@...com>,
        <sfr@...b.auug.org.au>
Subject: Re: [PATCH bpf-next] bpf: fix bpf_prog_pack build for
 ppc64_defconfig

On Thu, 10 Feb 2022 17:49:15 -0800 Song Liu wrote:
> bpf_prog_pack causes build error with powerpc ppc64_defconfig:
> 
> kernel/bpf/core.c:830:23: error: variably modified 'bitmap' at file scope
>   830 |         unsigned long bitmap[BITS_TO_LONGS(BPF_PROG_CHUNK_COUNT)];
>       |                       ^~~~~~

No idea what this error means but...

> Fix it by turning bitmap into a 0-length array.
> 
> Fixes: 57631054fae6 ("bpf: Introduce bpf_prog_pack allocator")
> Reported-by: Stephen Rothwell <sfr@...b.auug.org.au>
> Signed-off-by: Song Liu <song@...nel.org>
> ---
>  kernel/bpf/core.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
> index 42d96549a804..44623c9b5bb1 100644
> --- a/kernel/bpf/core.c
> +++ b/kernel/bpf/core.c
> @@ -827,7 +827,7 @@ int bpf_jit_add_poke_descriptor(struct bpf_prog *prog,
>  struct bpf_prog_pack {
>  	struct list_head list;
>  	void *ptr;
> -	unsigned long bitmap[BITS_TO_LONGS(BPF_PROG_CHUNK_COUNT)];
> +	unsigned long bitmap[];

This is really asking to be DECLARE_BITMAP(), does that fix the issue?

>  };
>  
>  #define BPF_PROG_MAX_PACK_PROG_SIZE	BPF_PROG_PACK_SIZE
> @@ -840,7 +840,7 @@ static struct bpf_prog_pack *alloc_new_pack(void)
>  {
>  	struct bpf_prog_pack *pack;
>  
> -	pack = kzalloc(sizeof(*pack), GFP_KERNEL);
> +	pack = kzalloc(sizeof(*pack) + BITS_TO_BYTES(BPF_PROG_CHUNK_COUNT), GFP_KERNEL);

Otherwise you may want to use struct_size(pack, bitmap, BITS...).
One of the bots will soon send us a patch to do that.

>  	if (!pack)
>  		return NULL;
>  	pack->ptr = module_alloc(BPF_PROG_PACK_SIZE);