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

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Date:   Sat, 12 Feb 2022 01:23:02 +0000
From:   Song Liu <songliubraving@...com>
To:     Jakub Kicinski <kuba@...nel.org>
CC:     Song Liu <song@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "sfr@...b.auug.org.au" <sfr@...b.auug.org.au>
Subject: Re: [PATCH bpf-next] bpf: fix bpf_prog_pack build for ppc64_defconfig



> On Feb 11, 2022, at 3:54 PM, Jakub Kicinski <kuba@...nel.org> wrote:
> 
> 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...
v2 (which has applied to bpf-next) has more information about this. 
Basically, BPF_PROG_CHUNK_COUNT contains a global variable for ppc. 

> 
>> 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?

I am afraid DECLARE_BITMAP() will hit the same error here. 

> 
>> };
>> 
>> #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.

Since v2 has applied to bpf-next, I will send another follow-up patch to
use struct_size, unless some bot get it first. 

Thanks,
Song