Messages in this thread | | | From | Andy Lutomirski <> | Date | Mon, 1 Jun 2015 14:57:55 -0700 | Subject | Re: [PATCH 2/2] kprobes/x86: Use 16 bytes for each instruction slot again |
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On Mon, Jun 1, 2015 at 2:49 PM, 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 :)
This still doesn't explain what a "slot" is.
I broke (?) something because I didn't see anything that looked relevant that I was changing. But now I see it:
- .insn_size = MAX_INSN_SIZE, + .insn_size = KPROBE_INSN_SLOT_SIZE,
Would it make sense to clean this up? insn_size isn't the size of an instruction at all -- it's the size of a kprobe jump target in units of sizeof(kprobe_opcode_t).
How about renaming insn_size to something sensible (and maybe specifying the size in *bytes*)?
--Andy
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