Messages in this thread | | | Date | Thu, 14 Nov 2019 11:19:52 -0500 | From | Steven Rostedt <> | Subject | Re: [PATCH 09/10] ftrace/x86: Add register_ftrace_direct() for custom trampolines |
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On Thu, 14 Nov 2019 16:34:58 +0100 (CET) Miroslav Benes <mbenes@suse.cz> wrote:
> On Fri, 8 Nov 2019, Steven Rostedt wrote: > > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org> > > > > Enable x86 to allow for register_ftrace_direct(), where a custom trampoline > > may be called directly from an ftrace mcount/fentry location. > > > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org> > > [...] > > > +++ b/arch/x86/kernel/ftrace_64.S > > @@ -88,6 +88,7 @@ EXPORT_SYMBOL(__fentry__) > > movq %rdi, RDI(%rsp) > > movq %r8, R8(%rsp) > > movq %r9, R9(%rsp) > > + movq $0, ORIG_RAX(%rsp) > > /* > > * Save the original RBP. Even though the mcount ABI does not > > * require this, it helps out callers. > > @@ -114,7 +115,8 @@ EXPORT_SYMBOL(__fentry__) > > subq $MCOUNT_INSN_SIZE, %rdi > > .endm > > > > -.macro restore_mcount_regs > > +.macro restore_mcount_regs save=0 > > + > > movq R9(%rsp), %r9 > > movq R8(%rsp), %r8 > > movq RDI(%rsp), %rdi > > @@ -123,10 +125,7 @@ EXPORT_SYMBOL(__fentry__) > > movq RCX(%rsp), %rcx > > movq RAX(%rsp), %rax > > > > - /* ftrace_regs_caller can modify %rbp */ > > - movq RBP(%rsp), %rbp > > - > > - addq $MCOUNT_REG_SIZE, %rsp > > + addq $MCOUNT_REG_SIZE-\save, %rsp > > > > .endm > > > > @@ -228,10 +227,30 @@ GLOBAL(ftrace_regs_call) > > movq R10(%rsp), %r10 > > movq RBX(%rsp), %rbx > > > > - restore_mcount_regs > > + movq RBP(%rsp), %rbp > > + > > + movq ORIG_RAX(%rsp), %rax > > + movq %rax, MCOUNT_REG_SIZE-8(%rsp) > > + > > + /* If ORIG_RAX is anything but zero, make this a call to that */ > > + movq ORIG_RAX(%rsp), %rax > > + cmpq $0, %rax > > + je 1f > > + > > + /* Swap the flags with orig_rax */ > > + movq MCOUNT_REG_SIZE(%rsp), %rdi > > + movq %rdi, MCOUNT_REG_SIZE-8(%rsp) > > + movq %rax, MCOUNT_REG_SIZE(%rsp) > > + > > + restore_mcount_regs 8 > > + > > + jmp 2f > > + > > +1: restore_mcount_regs > > + > > > > /* Restore flags */ > > - popfq > > +2: popfq > > If I am reading the code correctly (and I was confused couple of times, so > maybe I am not), this is what makes the direct fops incompatible with > ipmodify and livepatching for now. Is that correct?
Actually, it's the fact that the return goes to some unknown trampoline that may do something else as well.
> > What are your plans regarding this?
I wanted to see what the eBPF folks were doing, and then perhaps allow the ip modify occur too. I could let it happen as well now, and then we can see what the fallout is later ;-)
> > Moreover, we could replace ftrace_regs_caller with direct fops for live > patching when this is merged with all arch support we need. After all, all
Note, direct call is currently only available for x86_64.
> we need is to change the rip, which we could do easily in the direct > trampoline. On the other hand, it would exclude coexistence of a live > patch and a BPF filter (both direct now) on one function.
It may also end up being more complex, and not much of a performance benefit. I believe the BPF is injecting programs into the start of functions, but your trampoline for live patching may be not much different than what ftrace gives you today.
-- Steve
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