netdev - Re: [PATCH bpf-next 12/13] bpf: arm64: add JIT support for multi-function programs

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Message-ID: <78484175-eba0-0847-5f2f-44d1c06b1951@fb.com>
Date:   Mon, 18 Dec 2017 09:55:28 -0800
From:   Alexei Starovoitov <ast@...com>
To:     Daniel Borkmann <daniel@...earbox.net>,
        Arnd Bergmann <arnd@...db.de>,
        Alexei Starovoitov <ast@...nel.org>
CC:     "David S . Miller" <davem@...emloft.net>,
        John Fastabend <john.fastabend@...il.com>,
        Edward Cree <ecree@...arflare.com>,
        Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Networking <netdev@...r.kernel.org>, <kernel-team@...com>
Subject: Re: [PATCH bpf-next 12/13] bpf: arm64: add JIT support for
 multi-function programs

On 12/18/17 7:51 AM, Daniel Borkmann wrote:
> On 12/18/2017 04:29 PM, Arnd Bergmann wrote:
>> On Fri, Dec 15, 2017 at 2:55 AM, Alexei Starovoitov <ast@...nel.org> wrote:
>>
>>
>>> +       if (jit_data->ctx.offset) {
>>> +               ctx = jit_data->ctx;
>>> +               image_ptr = jit_data->image;
>>> +               header = jit_data->header;
>>> +               extra_pass = true;
>>> +               goto skip_init_ctx;
>>> +       }
>>>         memset(&ctx, 0, sizeof(ctx));
>>>         ctx.prog = prog;
>>
>> The 'goto' jumps over the 'image_size' initialization
>>
>>>         prog->bpf_func = (void *)ctx.image;
>>>         prog->jited = 1;
>>>         prog->jited_len = image_size;
>>
>> so we now get a warning here, starting with linux-next-20171218:
>>
>> arch/arm64/net/bpf_jit_comp.c: In function 'bpf_int_jit_compile':
>> arch/arm64/net/bpf_jit_comp.c:982:18: error: 'image_size' may be used
>> uninitialized in this function [-Werror=maybe-uninitialized]
>>
>> I could not figure out what the code should be doing instead, or if it is
>> indeed safe and the warning is a false-positive.
>
> Good catch, it's buggy indeed. Fix like below is needed; I can submit
> it properly a bit later today:

good catch. My arm gcc 4.8.5 didn't warn about it.

the following would be better fix:
diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
index 396490cf7316..acaa935ed977 100644
--- a/arch/arm64/net/bpf_jit_comp.c
+++ b/arch/arm64/net/bpf_jit_comp.c
@@ -897,6 +897,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog 
*prog)
                 image_ptr = jit_data->image;
                 header = jit_data->header;
                 extra_pass = true;
+               image_size = sizeof(u32) * ctx.idx;
                 goto skip_init_ctx;
         }
         memset(&ctx, 0, sizeof(ctx));

> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
> index 396490c..a6fd585 100644
> --- a/arch/arm64/net/bpf_jit_comp.c
> +++ b/arch/arm64/net/bpf_jit_comp.c
> @@ -855,6 +855,7 @@ static inline void bpf_flush_icache(void *start, void *end)
>  struct arm64_jit_data {
>  	struct bpf_binary_header *header;
>  	u8 *image;
> +	int image_size;
>  	struct jit_ctx ctx;
>  };
>
> @@ -895,6 +896,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>  	if (jit_data->ctx.offset) {
>  		ctx = jit_data->ctx;
>  		image_ptr = jit_data->image;
> +		image_size = jit_data->image_size;
>  		header = jit_data->header;
>  		extra_pass = true;
>  		goto skip_init_ctx;
> @@ -975,6 +977,7 @@ struct bpf_prog *bpf_int_jit_compile(struct bpf_prog *prog)
>  	} else {
>  		jit_data->ctx = ctx;
>  		jit_data->image = image_ptr;
> +		jit_data->image_size = image_size;
>  		jit_data->header = header;
>  	}
>  	prog->bpf_func = (void *)ctx.image;
>