On 02/22/2018 04:53 AM, Eric Dumazet wrote:
> On Wed, 2018-02-21 at 19:43 -0800, Alexei Starovoitov wrote:
>> On Wed, Feb 21, 2018 at 07:04:02PM -0800, Eric Dumazet wrote:
>>> On Thu, 2018-02-22 at 01:05 +0100, Daniel Borkmann wrote:
>>>
>>> ...
>>>
>>>> +/* Instead of plain jmp %rax, we emit a retpoline to control
>>>> + * speculative execution for the indirect branch.
>>>> + */
>>>> +static void emit_retpoline_rax_trampoline(u8 **pprog)
>>>> +{
>>>> + u8 *prog = *pprog;
>>>> + int cnt = 0;
>>>> +
>>>> + EMIT1_off32(0xE8, 7); /* callq <set_up_target> */
>>>> + /* capture_spec: */
>>>> + EMIT2(0xF3, 0x90); /* pause */
>>>> + EMIT3(0x0F, 0xAE, 0xE8); /* lfence */
>>>> + EMIT2(0xEB, 0xF9); /* jmp <capture_spec> */
>>>> + /* set_up_target: */
>>>> + EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */
>>>> + EMIT1(0xC3); /* retq */
>>>> +
>>>> + BUILD_BUG_ON(cnt != RETPOLINE_SIZE);
>>>> + *pprog = prog;
>>>
>>> You might define the actual code sequence (and length) in
>>> arch/x86/include/asm/nospec-branch.h
>>>
>>> If we need to adjust code sequences for RETPOLINE, then we wont
>>> forget/miss that arch/x86/net/bpf_jit_comp.c had it hard-coded.
>>
>> like adding a comment to asm/nospec-branch.h that says
>> "dont forget to adjust bpf_jit_comp.c" ?
>> but clang/gcc generate slightly different sequences for
>> retpoline anyway, so even if '.macro RETPOLINE_JMP' in
>> nospec-branch.h changes it doesn't mean that x64 jit has to change.
>> So what kinda comment there would make sense?
>
> I was thinking of something very explicit :
>
> /* byte sequence for following assembly code used by eBPF
> call ...
> ...
> retq
> */
> #define RETPOLINE_RAX_DIRECT_FOR_EBPF \
> EMIT1_off32(0xE8, 7); /* callq <set_up_target> */ \
> /* capture_spec: */ \
> EMIT2(0xF3, 0x90); /* pause */ \
> EMIT3(0x0F, 0xAE, 0xE8); /* lfence */ \
> EMIT2(0xEB, 0xF9); /* jmp <capture_spec> */ \
> /* set_up_target: */ \
> EMIT4(0x48, 0x89, 0x04, 0x24); /* mov %rax,(%rsp) */ \
> EMIT1(0xC3); /* retq */ \
>
> Might be simply byte encoded, (array of 17 bytes)
>
> Well, something like that anyway...
Okay, sounds fine. Will respin, thanks Eric!
