Re: [PATCH v3] arm64: Improve kprobes test for atomic sequence

[David Long]

On 09/12/2016 12:29 PM, Masami Hiramatsu wrote:
> On Sun, 11 Sep 2016 21:53:43 -0400
> David Long <dave.long@xxxxxxxxxx> wrote:
>
> > On 09/10/2016 01:48 AM, Masami Hiramatsu wrote:
> > > On Fri,  9 Sep 2016 15:26:09 -0400
> > > David Long <dave.long@xxxxxxxxxx> wrote:
> > >
> > > > From: "David A. Long" <dave.long@xxxxxxxxxx>
> > > >
> > > > Kprobes searches backwards a finite number of instructions to determine if
> > > > there is an attempt to probe a load/store exclusive sequence. It stops when
> > > > it hits the maximum number of instructions or a load or store exclusive.
> > > > However this means it can run up past the beginning of the function and
> > > > start looking at literal constants. This has been shown to cause a false
> > > > positive and blocks insertion of the probe. To fix this, further limit the
> > > > backwards search to stop if it hits a symbol address from kallsyms. The
> > > > presumption is that this is the entry point to this code (particularly for
> > > > the common case of placing probes at the beginning of functions).
> > > >
> > > > This also improves efficiency by not searching code that is not part of the
> > > > function. There may be some possibility that the label might not denote the
> > > > entry path to the probed instruction but the likelihood seems low and this
> > > > is just another example of how the kprobes user really needs to be
> > > > careful about what they are doing.
> > >
> > > Of course user should be careful, but also, in such case, kernel can reject
> > > to probe it.
> >
> > I'm not exactly sure what you mean.  I'm just saying when everything
> > goes right we still cannot promise perfection in detecting a probe
> > within an atomic sequence.  This patch will reject a probe that is after
> > a ldx and has no intervening kallsyms label (and assuming it's within
> > the defined maximum count of subsequent instructions).
>
> Hmm, what I meant was the below code.
>
> > > > +	/*
> > > > +	 * If there's a symbol defined in front of and near enough to
> > > > +	 * the probe address assume it is the entry point to this
> > > > +	 * code and use it to further limit how far back we search
> > > > +	 * when determining if we're in an atomic sequence. If we could
> > > > +	 * not find any symbol skip the atomic test altogether as we
> > > > +	 * could otherwise end up searching irrelevant text/literals.
> > > > +	 * KPROBES depends on KALLSYMS so this last case should never
> > > > +	 * happen.
> > > > +	 */
> > > > +	if (kallsyms_lookup_size_offset((unsigned long) addr, &size, &offset)) {
> > > > +		if (offset < (MAX_ATOMIC_CONTEXT_SIZE*sizeof(kprobe_opcode_t)))
> > > > +			scan_end = addr - (offset / sizeof(kprobe_opcode_t));
> > > > +		else
> > > > +			scan_end = addr - MAX_ATOMIC_CONTEXT_SIZE;
> > >
> > >           } else
> > >                  return INSN_REJECTED;
> > >
> > >     that is what I expected...
>
> As you said above,
>
> > > > +	 * KPROBES depends on KALLSYMS so this last case should never
> > > > +	 * happen.
>
> If it should never happen, it also would be better to reject it because
> it is unexpected result.
>
> Thank you,

OK, cool.  Sounds like we're on the same page.

-dl