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

[Peter Zijlstra]

On Mon, Nov 12, 2018 at 04:46:46AM +0100, Ingo Molnar wrote:
> 
> * 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.

The reviewer can read a comment.

> > > 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.

The code complexity not so much.

> > > >> +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?

The thing is, I don't think there is realistically anything the caller
can do; our text is not what we expect it to be, that is a fairly
fundamentally buggered situation to be in.

I'm fine with validating it; I'm as paranoid as the next guy; but
passing along that information seems pointless. At best we can try
poking again, but that's not going to help much if it failed the first
time around.