netdev - Re: [PATCH net-next 1/3] net: bpf: consolidate JIT binary allocator

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Date:	Sun, 7 Sep 2014 17:17:04 -0700
From:	Alexei Starovoitov <ast@...mgrid.com>
To:	David Miller <davem@...emloft.net>
Cc:	Daniel Borkmann <dborkman@...hat.com>,
	Network Development <netdev@...r.kernel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH net-next 1/3] net: bpf: consolidate JIT binary allocator

On Sun, Sep 7, 2014 at 4:15 PM, David Miller <davem@...emloft.net> wrote:
> From: Daniel Borkmann <dborkman@...hat.com>
> Date: Sat,  6 Sep 2014 11:42:45 +0200
>
>> While reviewing the code, I think on s390, the alignment masking
>> seems not to be correct in it's current form, that is, we make sure
>> the first instruction starts at an even address as stated by commit
>> aa2d2c73c21f but masks the start with '& -2' while 2 byte-alignment
>> should rather be '& ~1'.
>
> Both -2 and ~1 are the same value.

oops. you're right. commit log is incorrect.

The new code makes the logic more clear:
in s390:
- *image_ptr = &header->image[(prandom_u32() % hole) & -2];
+ header = bpf_jit_binary_alloc(size, &jit.start, 2, bpf_jit_fill_hole);

and in bpf/core.c:
+struct bpf_binary_header *
+bpf_jit_binary_alloc(unsigned int proglen, u8 **image_ptr,
+                    unsigned int alignment,
+                    bpf_jit_fill_hole_t bpf_fill_ill_insns)
+{
...
+       start = (prandom_u32() % hole) & ~(alignment - 1);

we'll fix the commit log and resubmit.
Thx
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