[PATCH] bpf, arm64: start flushing icache range from header

[Daniel Borkmann]

On 11/16/2015 12:39 PM, Mark Rutland wrote:
> On Sat, Nov 14, 2015 at 01:16:18AM +0100, Daniel Borkmann wrote:
>> While recently going over ARM64's BPF code, I noticed that the icache
>> range we're flushing should start at header already and not at ctx.image.
>>
>> Reason is that after b569c1c622c5 ("net: bpf: arm64: address randomize
>> and write protect JIT code"), we also want to make sure to flush the
>> random-sized trap in front of the start of the actual program (analogous
>> to x86). No operational differences from user side.
>>
>> Signed-off-by: Daniel Borkmann <daniel at iogearbox.net>
>> Acked-by: Zi Shen Lim <zlim.lnx at gmail.com>
>> Cc: Alexei Starovoitov <ast at kernel.org>
>> ---
>>   ( As arm64 jit fixes seem to go via arm64 tree, sending them here. )
>>
>>   arch/arm64/net/bpf_jit_comp.c | 2 +-
>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
>> index a44e529..ee06570 100644
>> --- a/arch/arm64/net/bpf_jit_comp.c
>> +++ b/arch/arm64/net/bpf_jit_comp.c
>> @@ -740,7 +740,7 @@ void bpf_int_jit_compile(struct bpf_prog *prog)
>>   	if (bpf_jit_enable > 1)
>>   		bpf_jit_dump(prog->len, image_size, 2, ctx.image);
>>
>> -	bpf_flush_icache(ctx.image, ctx.image + ctx.idx);
>> +	bpf_flush_icache(header, ctx.image + ctx.idx);
>
> As far as I can see, ctx.idx doesn't take into account the size of the
> header, given we zero it after bpf_jit_binary_alloc, and increment it
> for each instruction.
>
> So won't this prevent us from flushing the end of the image? Or did I
> miss something?

Nope, bpf_flush_icache() takes start and end pointer ... header starts
before ctx.image on the linear buffer. Why should this prevent us from
flushing the end of the image?