Messages in this thread | | | Date | Tue, 2 Jun 2015 07:44:31 +0200 | From | Ingo Molnar <> | Subject | Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again |
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* Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com> wrote:
> On 2015/06/02 2:04, Andy Lutomirski wrote: > > On Mon, Jun 1, 2015 at 9:32 AM, Eugene Shatokhin > > <eugene.shatokhin@rosalab.ru> wrote: > >> Commit 91e5ed49fca0 ("x86/asm/decoder: Fix and enforce max instruction > >> size in the insn decoder") has changed MAX_INSN_SIZE from 16 to 15 bytes > >> on x86. > >> > >> As a side effect, the slots Kprobes use to store the instructions became > >> 1 byte shorter. This is unfortunate because, for example, the Kprobes' > >> "boost" feature can not be used now for the instructions of length 11, > >> like a quite common kind of MOV: > >> * movq $0xffffffffffffffff,-0x3fe8(%rax) (48 c7 80 18 c0 ff ff ff ff ff ff) > >> * movq $0x0,0x88(%rdi) (48 c7 87 88 00 00 00 00 00 00 00) > >> and so on. > >> > >> This patch makes the insn slots 16 bytes long, like they were before while > >> keeping MAX_INSN_SIZE intact. > >> > >> Other tools may benefit from this change as well. > > > > What is a "slot" and why does this patch make sense? Naively, I'd > > expect that the check you're patching is entirely unnecessary -- I > > don't see what the size of the instruction being probed has to do with > > the safety of executing it out of line and then jumping back. > > > > Is there another magic 16 somewhere that this is enforcing that we > > don't overrun? > > The kprobe-"booster" adds a jump back code (jmp <probed address + insn length>) > right after the instruction in the out-of-code buffer(slot). So we need at least > the insn-length + 5 bytes for the slot, it's the trick of the magic :)
Please at minimum rename it to 'dynamic code buffer' or some other sensible name - the name 'slot' is pretty meaningless at best and misleading at worst.
Thanks,
Ingo
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