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

[Catalin Marinas <catalin.marinas@arm.com>]

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.
> (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.

> Later on, we propose some changes in core layer, so that we can read the elf
> headers of executable binary. ELFCLASS will be able to tell us, whether its a 32
> bit or 64 bit executable. I think, moving "struct uprobe" from
> kernel/events/uprobes.c to a include/linux header file will do the job.  "struct
> arch_uprobe" is part of "struct uprobe". "struct arch_uprobe" is passed in
> arch_uprobe_analyze_insn(). So, we can access struct uprobe's "inode" element
> with this change. 

You can get access to struct linux_binfmt via mm_struct but it doesn't
currently help much since all the members of this structure point to
static functions. Maybe an enum in struct linux_binfmt with format types
exposed to the rest of the kernel?

-- 
Catalin