linux-kernel - Re: [RFC PATCH 1/7] static_call: get rid of static_call

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Message-ID: <YYrAHMy8xmPXj4ql@hirez.programming.kicks-ass.net>
Date:   Tue, 9 Nov 2021 19:38:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mark Rutland <mark.rutland@....com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [RFC PATCH 1/7] static_call: get rid of static_call_cond()

On Tue, Nov 09, 2021 at 05:45:43PM +0100, Ard Biesheuvel wrote:
> The main reason for the existence of static_call_cond() seems to be that
> in theory, when using the generic implementation of static calls, it may
> be possible for a compiler to elide the indirect call entirely if the
> target is NULL, while still guaranteeing that all side effects of
> argument evaluation occur as expected.
> 
> This is rather optimistic: as documented by an existing code comment,
> both GCC and Clang (version 10) get this wrong, and even if they ever
> get it right, this is far too subtle to rely on for a code path that is
> expected to be used only by the 'remaining' architectures once all the
> best supported ones implement either the out-of-line or inline optimized
> variety of static calls.
> 
> Given that having static_call_cond() clutters up the API, and puts the
> burden on the caller to go and check what kind of static call they are
> dealing with, let's just get rid of the distinction.

No, static_call_cond() signifies the function can be NULL. Both gcc and
clang generate correct (but wildly ineffecient) code for this. Without
static_call_cond() the generic implementation will do a NULL deref.

That is, static_call_cond() does properly encapuslate:

	func = READ_ONCE(key.func);
	if (func)
		func(ARGS);

You can't take that out.