Re: [PATCH 5/5] arm64: Add uprobe support

[Pratyush Anand]

On 22/09/2016:05:50:30 PM, Catalin Marinas wrote:
> On Thu, Sep 22, 2016 at 08:53:28AM +0530, Pratyush Anand wrote:
> > On 21/09/2016:06:04:04 PM, Catalin Marinas wrote:
> > > On Wed, Sep 21, 2016 at 04:30:47PM +0530, Pratyush Anand wrote:
> > > > On 20/09/2016:05:59:46 PM, Catalin Marinas wrote:
> > > > > > +int arch_uprobe_analyze_insn(struct arch_uprobe *auprobe, struct mm_struct *mm,
> > > > > > +		unsigned long addr)
> > > > > > +{
> > > > > > +	probe_opcode_t insn;
> > > > > > +
> > > > > > +	/* TODO: Currently we do not support AARCH32 instruction probing */
> > > > > 
> > > > > Is there a way to check (not necessarily in this file) that we don't
> > > > > probe 32-bit tasks?
> > > > 
> > > > - Well, I do not have complete idea about it that, how it can be done. I think
> > > >   we can not check that just by looking a single bit in an instruction.
> > > >   My understanding is that, we can only know about it when we are executing the
> > > >   instruction, by reading pstate, but that would not be useful for uprobe
> > > >   instruction analysis.
> > > >   
> > > >   I hope, instruction encoding for aarch32 and aarch64 are different, and by
> > > >   analyzing for all types of aarch32 instructions, we will be able to decide
> > > >   that whether instruction is 32 bit trace-able or not.  Accordingly, we can use
> > > >   either BRK or BKPT instruction for breakpoint generation.
> > > 
> > > We may have some unrelated instruction encoding overlapping but I
> > > haven't checked. I was more thinking about whether we know which task is
> > > being probed and check is_compat_task() or maybe using
> > > compat_user_mode(regs).
> > 
> > I had thought of this, but problem is that we might not have task in existence
> > when we enable uprobes.  For example: Lets say we are inserting a trace probe at
> > offset 0x690 in a executable binary.
> > 
> > echo "p test:0x690" > /sys/kernel/debug/tracing/uprobe_events
> > echo 1 > /sys/kernel/debug/tracing/events/uprobes/enable
> > 
> > In the 'enable' step, it is decided that whether instruction is traceable or
> > not. 
> > 
> > (1) But at this point 'test' executable might not be running.

Let me correct myself first here. When executable is not running, then,
arch_uprobe_analyze_insn() is not called while uprobes enabling (ie writing '1'
to 'enable'). In that case, it is called when binary is executed and task is
created.

> > (2) Even if it is running, is_compat_task() or compat_user_mode() might not be
> > usable, as they work with 'current' task.
> 
> What I find strange is that uprobes allows you to insert a breakpoint
> instruction that's not even compatible with the task (so it would
> SIGILL rather than generate a debug exception).
> 
> > What I was thinking that, let it go with 'TODO' as of now. 
> 
> Only that I don't have any guarantee that someone is going to fix it ;).
> 
> As a quick workaround you could check mm->task_size > TASK_SIZE_32 in
> the arch_uprobe_analyze_insn() function.

It would be doable. TASK_SIZE_32 is defined only for COMPAT. So, may be I can
return -EINVAL when mm->task_size < TASK_SIZE_64.

Thanks for your input. 

~Pratyush