Re: [RESEND][PATCH v3 14/17] static_call: Add static_cond_call()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Mar 24, 2020 at 06:03:06PM +0100, Peter Zijlstra wrote:
> On Tue, Mar 24, 2020 at 09:33:21AM -0700, Linus Torvalds wrote:

> > Of course, one alternative is to just say "instead of using NOP, use
> > 'xorl %eax,%eax'", and then we'd have the rule that a NULL conditional
> > function returns zero (or NULL).
> > 
> > I _think_ a "xorl %eax,%eax ; retq" is just three bytes and would fit
> > in the tailcall slot too.
> 
> Correct. The only problem is that our text patching machinery can't
> replace multiple instructions :/

To clarify; the problem is a task getting preempted with its RIP at the
RET. Then when we rewrite the text to be a CALL/JMP.d32 it will read
garbage (1 byte into the displacement of the instruction) instead of a
RET when it resumes.

Now, there are ways to fix this, the easiest being calling
synchronize_rcu_tasks() just like optprobes does (see also commit
5c02ece81848 ("x86/kprobes: Fix ordering while text-patching")).

It would mean patching a call away from NULL will be 'expensive' but it
ought to work.

I'll try and do the patch, see what it looks like.