Messages in this thread | | | Date | Tue, 02 Jun 2015 06:49:50 +0900 | From | Masami Hiramatsu <> | Subject | Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again |
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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 :)
Thank you,
> > --Andy >
-- Masami HIRAMATSU Linux Technology Research Center, System Productivity Research Dept. Center for Technology Innovation - Systems Engineering Hitachi, Ltd., Research & Development Group E-mail: masami.hiramatsu.pt@hitachi.com
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