Re: [PATCH v4 06/10] x86/alternative: use temporary mm for text poking

[Ingo Molnar]

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Sun, Nov 11, 2018 at 08:53:07PM +0000, Nadav Amit wrote:
> 
> > >> +	/*
> > >> +	 * The lock is not really needed, but this allows to avoid open-coding.
> > >> +	 */
> > >> +	ptep = get_locked_pte(poking_mm, poking_addr, &ptl);
> > >> +
> > >> +	/*
> > >> +	 * If we failed to allocate a PTE, fail. This should *never* happen,
> > >> +	 * since we preallocate the PTE.
> > >> +	 */
> > >> +	if (WARN_ON_ONCE(!ptep))
> > >> +		goto out;
> > > 
> > > Since we hard rely on init getting that right; can't we simply get rid
> > > of this?
> > 
> > This is a repeated complaint of yours, which I do not feel comfortable with.
> > One day someone will run some static analysis tool and start finding that
> > all these checks are missing.
> > 
> > The question is why do you care about them.
> 
> Mostly because they should not be happening, ever.

Since get_locked_pte() might in principle return NULL, it's an entirely 
routine pattern to check the return for NULL. This will save reviewer 
time in the future.

> [...] And if they happen, there really isn't anything sensible we can 
> do about it.

Warning about it is 'something', even if we cash afterwards, isn't it?

> > If it is because they affect the
> > generated code and make it less efficient, I can fully understand and perhaps
> > we should have something like PARANOID_WARN_ON_ONCE() which compiles into nothing
> > unless a certain debug option is set.
> > 
> > If it is about the way the source code looks - I guess it doesnât sore my
> > eyes as hard as some other stuff, and I cannot do much about it (other than
> > removing it as you asked).
> 
> And yes on the above two points. It adds both runtime overhead (albeit
> trivially small) and code complexity.

It's trivially small cycle level overhead in something that will be 
burdened by two TLB flushes anyway is is utterly slow.

> > >> +out:
> > >> +	if (memcmp(addr, opcode, len))
> > >> +		r = -EFAULT;
> > > 
> > > How could this ever fail? And how can we reliably recover from that?
> > 
> > This code has been there before (with slightly uglier code). Before this
> > patch, a BUG_ON() was used here. However, I noticed that kgdb actually
> > checks that text_poke() succeeded after calling it and gracefully fail.
> > However, this was useless, since text_poke() would panic before kgdb gets
> > the chance to do anything (see patch 7).
> 
> Yes, I know it was there before, and I did see kgdb do it too. But aside
> from that out-label case, which we also should never hit, how can we
> realistically ever fail that memcmp()?
> 
> If we fail here, something is _seriously_ buggered.

So wouldn't it be better to just document and verify our assumptions of 
this non-trivial code by using return values intelligently?

I mean, being worried about overhead would be legitimate in the syscall 
entry code. In code patching code, which is essentially a slow path, we 
should be much more worried about *robustness*.

Thanks,

	Ingo