Re: [PATCH v7 3/3] arm64: implement KPROBES_ON_FTRACE

[Jisheng Zhang <Jisheng.Zhang@synaptics.com>]

Hi

On Thu, 26 Dec 2019 11:57:07 +0900 Masami Hiramatsu wrote:

> 
> Hi Jisheng,
> 
> On Wed, 25 Dec 2019 09:44:21 +0000
> Jisheng Zhang <Jisheng.Zhang@synaptics.com> wrote:
> 
> > KPROBES_ON_FTRACE avoids much of the overhead with regular kprobes as it
> > eliminates the need for a trap, as well as the need to emulate or
> > single-step instructions.
> >
> > Tested on berlin arm64 platform.
> >
> > ~ # mount -t debugfs debugfs /sys/kernel/debug/
> > ~ # cd /sys/kernel/debug/
> > /sys/kernel/debug # echo 'p _do_fork' > tracing/kprobe_events
> >
> > before the patch:
> >
> > /sys/kernel/debug # cat kprobes/list
> > ffffff801009fe28  k  _do_fork+0x0    [DISABLED]
> >
> > after the patch:
> >
> > /sys/kernel/debug # cat kprobes/list
> > ffffff801009ff54  k  _do_fork+0x0    [DISABLED][FTRACE]  
> 
> What happens if user puts a probe on _do_fork+4?
> Is that return -EILSEQ correctly?

_do_fork+4 can be probed successfully.

> 
> >
> > Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
> > ---
> >  .../debug/kprobes-on-ftrace/arch-support.txt  |  2 +-
> >  arch/arm64/Kconfig                            |  1 +
> >  arch/arm64/include/asm/ftrace.h               |  1 +
> >  arch/arm64/kernel/probes/Makefile             |  1 +
> >  arch/arm64/kernel/probes/ftrace.c             | 78 +++++++++++++++++++
> >  5 files changed, 82 insertions(+), 1 deletion(-)
> >  create mode 100644 arch/arm64/kernel/probes/ftrace.c
> >
> > diff --git a/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt b/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt
> > index 4fae0464ddff..f9dd9dd91e0c 100644
> > --- a/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt
> > +++ b/Documentation/features/debug/kprobes-on-ftrace/arch-support.txt
> > @@ -9,7 +9,7 @@
> >      |       alpha: | TODO |
> >      |         arc: | TODO |
> >      |         arm: | TODO |
> > -    |       arm64: | TODO |
> > +    |       arm64: |  ok  |
> >      |         c6x: | TODO |
> >      |        csky: | TODO |
> >      |       h8300: | TODO |
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index b1b4476ddb83..92b9882889ac 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -166,6 +166,7 @@ config ARM64
> >       select HAVE_STACKPROTECTOR
> >       select HAVE_SYSCALL_TRACEPOINTS
> >       select HAVE_KPROBES
> > +     select HAVE_KPROBES_ON_FTRACE
> >       select HAVE_KRETPROBES
> >       select HAVE_GENERIC_VDSO
> >       select IOMMU_DMA if IOMMU_SUPPORT
> > diff --git a/arch/arm64/include/asm/ftrace.h b/arch/arm64/include/asm/ftrace.h
> > index 91fa4baa1a93..875aeb839654 100644
> > --- a/arch/arm64/include/asm/ftrace.h
> > +++ b/arch/arm64/include/asm/ftrace.h
> > @@ -20,6 +20,7 @@
> >
> >  /* The BL at the callsite's adjusted rec->ip */
> >  #define MCOUNT_INSN_SIZE     AARCH64_INSN_SIZE
> > +#define FTRACE_IP_EXTENSION  MCOUNT_INSN_SIZE
> >
> >  #define FTRACE_PLT_IDX               0
> >  #define FTRACE_REGS_PLT_IDX  1
> > diff --git a/arch/arm64/kernel/probes/Makefile b/arch/arm64/kernel/probes/Makefile
> > index 8e4be92e25b1..4020cfc66564 100644
> > --- a/arch/arm64/kernel/probes/Makefile
> > +++ b/arch/arm64/kernel/probes/Makefile
> > @@ -4,3 +4,4 @@ obj-$(CONFIG_KPROBES)         += kprobes.o decode-insn.o      \
> >                                  simulate-insn.o
> >  obj-$(CONFIG_UPROBES)                += uprobes.o decode-insn.o      \
> >                                  simulate-insn.o
> > +obj-$(CONFIG_KPROBES_ON_FTRACE)      += ftrace.o
> > diff --git a/arch/arm64/kernel/probes/ftrace.c b/arch/arm64/kernel/probes/ftrace.c
> > new file mode 100644
> > index 000000000000..0643aa2dacdb
> > --- /dev/null
> > +++ b/arch/arm64/kernel/probes/ftrace.c
> > @@ -0,0 +1,78 @@
> > +// SPDX-License-Identifier: GPL-2.0-or-later
> > +/*
> > + * Dynamic Ftrace based Kprobes Optimization
> > + *
> > + * Copyright (C) Hitachi Ltd., 2012
> > + * Copyright (C) 2019 Jisheng Zhang <jszhang@kernel.org>
> > + *                 Synaptics Incorporated
> > + */
> > +
> > +#include <linux/kprobes.h>
> > +
> > +/*
> > + * In arm64 FTRACE_WITH_REGS implementation, we patch two nop instructions:
> > + * the lr saver and bl ftrace-entry. Both these instructions are claimed
> > + * by ftrace and we should allow probing on either instruction.  
> 
> No, the 2nd bl ftrace-entry must not be probed.
> The pair of lr-saver and bl ftrace-entry is tightly coupled. You can not
> decouple it.

This is the key. different viewing of this results in different implementation.
I'm just wondering why are the two instructions considered as coupled. I think
here we met similar situation as powerpc: https://lkml.org/lkml/2019/6/18/646
the "mflr r0" equals to lr-saver here, branch to _mcount equals to bl ftrace-entry
could you please kindly comment more?

Thanks in advance



> 
> > + */
> > +int arch_check_ftrace_location(struct kprobe *p)
> > +{
> > +     if (ftrace_location((unsigned long)p->addr))
> > +             p->flags |= KPROBE_FLAG_FTRACE;
> > +     return 0;
> > +}  
> 
> Thus, this must return -EILSEQ if user puts a probe on the bl.
> 
> > +
> > +/* Ftrace callback handler for kprobes -- called under preepmt disabed */
> > +void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
> > +                        struct ftrace_ops *ops, struct pt_regs *regs)
> > +{
> > +     bool lr_saver = false;
> > +     struct kprobe *p;
> > +     struct kprobe_ctlblk *kcb;
> > +
> > +     /* Preempt is disabled by ftrace */
> > +     p = get_kprobe((kprobe_opcode_t *)ip);
> > +     if (!p) {
> > +             p = get_kprobe((kprobe_opcode_t *)(ip - MCOUNT_INSN_SIZE));
> > +             if (unlikely(!p) || kprobe_disabled(p))
> > +                     return;
> > +             lr_saver = true;  
> 
> Then, this can be removed.
> 
> > +     }
> > +
> > +     kcb = get_kprobe_ctlblk();
> > +     if (kprobe_running()) {
> > +             kprobes_inc_nmissed_count(p);
> > +     } else {
> > +             unsigned long orig_ip = instruction_pointer(regs);
> > +
> > +             if (lr_saver)
> > +                     ip -= MCOUNT_INSN_SIZE;  
> 
> Ditto.
> 
> Thank you,
> 
> > +             instruction_pointer_set(regs, ip);
> > +             __this_cpu_write(current_kprobe, p);
> > +             kcb->kprobe_status = KPROBE_HIT_ACTIVE;
> > +             if (!p->pre_handler || !p->pre_handler(p, regs)) {
> > +                     /*
> > +                      * Emulate singlestep (and also recover regs->pc)
> > +                      * as if there is a nop
> > +                      */
> > +                     instruction_pointer_set(regs,
> > +                             (unsigned long)p->addr + MCOUNT_INSN_SIZE);
> > +                     if (unlikely(p->post_handler)) {
> > +                             kcb->kprobe_status = KPROBE_HIT_SSDONE;
> > +                             p->post_handler(p, regs, 0);
> > +                     }
> > +                     instruction_pointer_set(regs, orig_ip);
> > +             }
> > +             /*
> > +              * If pre_handler returns !0, it changes regs->pc. We have to
> > +              * skip emulating post_handler.
> > +              */
> > +             __this_cpu_write(current_kprobe, NULL);
> > +     }
> > +}
> > +NOKPROBE_SYMBOL(kprobe_ftrace_handler);
> > +
> > +int arch_prepare_kprobe_ftrace(struct kprobe *p)
> > +{
> > +     p->ainsn.api.insn = NULL;
> > +     return 0;
> > +}
> > --
> > 2.24.1
> >  
> 
> 
> --
> Masami Hiramatsu <mhiramat@kernel.org>