On 2015/06/02 14:44, Ingo Molnar wrote: > > * Masami Hiramatsu <masami.hiramatsu...@hitachi.com> wrote: > >> On 2015/06/02 2:04, Andy Lutomirski wrote: >>> On Mon, Jun 1, 2015 at 9:32 AM, Eugene Shatokhin >>> <eugene.shatok...@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.
OK, would 'exec_buffer' is sensible? or just a 'code_buffer' is better? Thank you, -- 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...@hitachi.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/