On 2018-11-09, Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote: > On Thu, 8 Nov 2018 08:44:37 -0600 > Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote: > > > On Thu, Nov 08, 2018 at 07:04:48PM +1100, Aleksa Sarai wrote: > > > On 2018-11-08, Aleksa Sarai <cyphar@xxxxxxxxxx> wrote: > > > > I will attach what I have at the moment to hopefully explain what the > > > > issue I've found is (re-using the kretprobe architecture but with the > > > > shadow-stack idea). > > > > > > Here is the patch I have at the moment (it works, except for the > > > question I have about how to handle the top-level pt_regs -- I've marked > > > that code with XXX). > > > > > > -- > > > Aleksa Sarai > > > Senior Software Engineer (Containers) > > > SUSE Linux GmbH > > > <https://www.cyphar.com/> > > > > > > --8<--------------------------------------------------------------------- > > > > > > Since the return address is modified by kretprobe, the various unwinders > > > can produce invalid and confusing stack traces. ftrace mostly solved > > > this problem by teaching each unwinder how to find the original return > > > address for stack trace purposes. This same technique can be applied to > > > kretprobes by simply adding a pointer to where the return address was > > > replaced in the stack, and then looking up the relevant > > > kretprobe_instance when a stack trace is requested. > > > > > > [WIP: This is currently broken because the *first entry* will not be > > > overwritten since it looks like the stack pointer is different > > > when we are provided pt_regs. All other addresses are correctly > > > handled.] > > > > When you see this problem, what does regs->ip point to? If it's > > pointing to generated code, then we don't _currently_ have a way of > > dealing with that. If it's pointing to a real function, we can fix that > > with unwind hints. > > As I replied, If the stackdump is called from kretprobe event, regs->ip > always points trampoline function. Otherwise (maybe from kprobe event, > or panic, BUG etc.) it always be the address which the event occurs. > > So fixing regs->ip is correct. The problem is that the pointer to the *return address* is wrong (kernel_stack_pointer() gives you a different result than the function entry), it's not that regs->ip is wrong. And I'm sure that it's "wrong" because it's not possible for "regs->ip == kretprobe_trampoline" unless you are in a stack frame that has been modified by the kretprobe core. I will take a closer look at this over the weekend -- I posted the patch to try to help explain what the underlying issue I was trying to solve with this patch series is (and why I don't think the ftrace changes proposed in the thread will completely fix them). -- Aleksa Sarai Senior Software Engineer (Containers) SUSE Linux GmbH <https://www.cyphar.com/>
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