Re: [PATCH v13 05/10] arm64: Kprobes with single stepping support

[Masami Hiramatsu]

On Mon, 13 Jun 2016 00:10:29 -0400
David Long <dave.long@xxxxxxxxxx> wrote:

> >> ---
> >>   arch/arm64/Kconfig                      |   1 +
> >>   arch/arm64/include/asm/debug-monitors.h |   5 +
> >>   arch/arm64/include/asm/insn.h           |   4 +-
> >>   arch/arm64/include/asm/kprobes.h        |  60 ++++
> >>   arch/arm64/include/asm/probes.h         |  44 +++
> >>   arch/arm64/kernel/Makefile              |   1 +
> >>   arch/arm64/kernel/debug-monitors.c      |  18 +-
> >>   arch/arm64/kernel/kprobes-arm64.c       | 144 +++++++++
> >>   arch/arm64/kernel/kprobes-arm64.h       |  35 +++
> >>   arch/arm64/kernel/kprobes.c             | 526 ++++++++++++++++++++++++++++++++
> >
> > Not sure why kprobes.c and kprobes-arm64.c are splitted.
> >
> >
> 
> This comes from the model of the arm32 kprobes code where handling of 
> the low-level instruction simulation is implemented in separate files 
> for 32-bit vs. thumb instructions.  It should make a little more sense 
> in the future when additional instruction simulation code will hopefully 
> be added for those instructions we cannot currently single-step 
> out-of-line.  It also probably *could* be merged into one file.

Hmm, at least the name of arch/arm64/kernel/kprobes-arm64.c is
meaningless. As we've done in x86, I think we can make it 
arch/arm64/kernel/kprobes/decode-insn.{c,h}


[..]
> >> +
> >> +/* Return:
> >> + *   INSN_REJECTED     If instruction is one not allowed to kprobe,
> >> + *   INSN_GOOD         If instruction is supported and uses instruction slot,
> >> + *   INSN_GOOD_NO_SLOT If instruction is supported but doesn't use its slot.
> >
> > Is there any chance to return INSN_GOOD_NO_SLOT?
> >
> 
> Ah, that gets used later when simulation support is added.  I've removed 
> this enum value from this commit and will add it to the later one. 
> Please no one complain about using an enum instead of a bool, it will 
> eventually have three possible values.

OK :)

[..]
> >> +enum kprobe_insn __kprobes
> >> +arm_kprobe_decode_insn(kprobe_opcode_t *addr, struct arch_specific_insn *asi)
> >> +{
> >> +	enum kprobe_insn decoded;
> >> +	kprobe_opcode_t insn = le32_to_cpu(*addr);
> >> +	kprobe_opcode_t *scan_start = addr - 1;
> >> +	kprobe_opcode_t *scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE;
> >> +#if defined(CONFIG_MODULES) && defined(MODULES_VADDR)
> >> +	struct module *mod;
> >> +#endif
> >> +
> >> +	if (addr >= (kprobe_opcode_t *)_text &&
> >> +	    scan_end < (kprobe_opcode_t *)_text)
> >> +		scan_end = (kprobe_opcode_t *)_text;
> >> +#if defined(CONFIG_MODULES) && defined(MODULES_VADDR)
> >> +	else {
> >> +		preempt_disable();
> >> +		mod = __module_address((unsigned long)addr);
> >> +		if (mod && within_module_init((unsigned long)addr, mod) &&
> >> +			!within_module_init((unsigned long)scan_end, mod))
> >> +			scan_end = (kprobe_opcode_t *)mod->init_layout.base;
> >> +		else if (mod && within_module_core((unsigned long)addr, mod) &&
> >> +			!within_module_core((unsigned long)scan_end, mod))
> >> +			scan_end = (kprobe_opcode_t *)mod->core_layout.base;
> >
> > What happen if mod == NULL? it should be return error, isn't it?
> >
> 
> No, it should be fine.  It just means it didn't have to do either of the 
> extra checks to limit the end of the search through the code to the 
> boundary of one of the corresponding module text sections. It means the 
> instruction is in the regular kernel (non-module) text segment.

Ah, I see. It is OK then. :)

Thank you,


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>