Re: [PATCH RFC v4 net-next 00/26] BPF syscall, maps, verifier, samples, llvm

[Alexei Starovoitov]

On Wed, Aug 13, 2014 at 4:34 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Wed, Aug 13, 2014 at 4:25 PM, David Miller <davem@xxxxxxxxxxxxx> wrote:
>> From: David Laight <David.Laight@xxxxxxxxxx>
>> Date: Wed, 13 Aug 2014 08:52:30 +0000
>>
>>> From: Of Alexei Starovoitov
>>>> one more RFC...
>>>>
>>>> Major difference vs previous set is a new 'load 64-bit immediate' eBPF insn.
>>>> Which is first 16-byte instruction. It shows how eBPF ISA can be extended
>>>> while maintaining backward compatibility, but mainly it cleans up eBPF
>>>> program access to maps and improves run-time performance.
>>>
>>> Wouldn't it be more sensible to follow the scheme used by a lot of cpus
>>> and add a 'load high' instruction (follow with 'add' or 'or').
>>> It still takes 16 bytes to load a 64bit immediate value, but the instruction
>>> size remains constant.
>>> There is nothing to stop any JIT software detecting the instruction pair.
>>
>> The opposite argument is that JITs can expand the IMM64 load into whatever
>> sequence of instructions is most optimal.
>>
>> My only real gripe with IMM64 loads is that it's not mainly for
>> loading an immediate, it's for loading a pointer.  And this
>> distinction is important for some JITs.
>>
>> For example, on sparc64 all symbol based addresses are actually 32-bit
>> because of the code model we use to compile the kernel and all modules.
>> So if we knew this is a pointer load and it's to a symbol in a kernel
>> or module image, we could do a 32-bit load.
>
> This is true for x86_64 as well, I think.
>
> (Almost.  For x86_64 we have a choice between a sign-extended load of
> a value in the top 2GB of the address space and lea reg,offset(%rip).)

That would be an interesting optimization. I did movabsq just
because it was straightforward. JITs can play interesting tricks here.
Since it's really a constant value, there is no difference whether
it's a pointer or a constant. If JIT can use $rip trick on x64 or reduce
number of sethi insns on sparc, it should try to do it regardless of
how value in dst_reg will be used later on by the program.
JITs can also allocate some read-only area for constants and
do a relative load from there. Not sure that it will be faster though.
JITs can get more complex and smarter as time goes by. They can
even randomly do some ld_imm64 via movabsq and some via a
sequence of mov, shift, or. That will through away JIT spraying attacks.
If JITed code itself is random, that would be nice defense.
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