* Peter Zijlstra <pet...@infradead.org> 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 doesn’t 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
