Re: [PATCH v2 0/7] Allow disabling HW_BREAKPOINTS, PERF_EVENTS, INSTRUCTION_DECODER, IRQ_WORK, ANON_INODES

[Ingo Molnar <mingo@kernel.org>]

* Josh Triplett <josh@joshtriplett.org> wrote:

> On Mon, Mar 10, 2014 at 08:43:54AM +0100, Ingo Molnar wrote:
> > 
> > * Josh Triplett <josh@joshtriplett.org> wrote:
> > 
> > > This patch series makes it possible to disable HW_BREAKPOINTS, 
> > > PERF_EVENTS, INSTRUCTION_DECODER, IRQ_WORK, and ANON_INODES.  
> > > Without this patch series, all of these config options get 
> > > automatically selected on x86, making them impossible to 
> > > disable.
> > >
> > > This is a revival of a previous patch series from Andi Kleen 
> > > sent in October 2013. [...]
> > 
> > So my main problem with those patches was that if HW_BREAKPOINTS 
> > is disabled then GDB 'hbreak' isn't simply disabled but fails in 
> > various non-obvious ways, taking down the rest of GDB with it, so 
> > it's in essence an ABI and tool breaker which is not very useful.
> 
> It's hidden behind EXPERT for a reason, [...]

With many distros enabling CONFIG_EXPERT=y that's a distinction 
without much meaning.

> [...] and non-hardware breakpoints still work just fine. [...]

A lot of other stuff will work just fine as well. Yet my point was 
that 'hbreak' breaks in ugly ways.

> [...] This is the kind of thing enabled on an embedded system, where 
> you're not going to be running GDB at all, let alone using "hbreak".

So that is why I suggested making ptrace configurable. Perhaps.

> Given that other options depending on EXPERT let you disable things 
> as critical as futexes or ELF binary support...

Both of which will break in pretty clear ways. The granularity of how 
we can disable ABIs largely depends on how well actual user-space code 
handles the failures, and what I'm saying here is that the disabling 
of hardware breakpoints is too finegrained.

Disabling ptrace for embedded OTOH makes sense and gives us even more 
savings and security advantage.

Thanks,

	Ingo

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