Subject: Re: [Bug] hook: -Wanalyzer-deref-before-check warning in run_hooks_opt

From: "brian m. carlson" <[email-masked]>
Date: Fri, 9 Jan 2026 22:18:05 +0000

On 2026-01-09 at 11:31:10, Patrick Steinhardt wrote:
> It's not a real bug. If you take a look at the the `if (!options)`
> check, you'll see:
> 
> 	if (!options)
> 		BUG("a struct run_hooks_opt must be provided to run_hooks");
> 
> So we'd abort immediatly with an error message in case the pointer was
> `NULL`. Which clarifies that this is a case that shouldn't ever happen
> in the first place.

You might think that we'd abort, but that's not what modern compilers
do. Dereferencing `options` if it is NULL is undefined behaviour.
Compilers are free to assume that undefined behaviour never happens, so
what most modern compilers do is say, "Oh, we've dereferenced `options`,
so it can never be NULL," and then use that to omit the check
altogether.

This sounds bizarre and like it might actually lead to security bugs,
and you're right.  However, compilers keep wanting to make code go
faster, so they keep relying on eliminating undefined behaviour to make
more assumptions about the code to optimize it, even if that results in
code that doesn't do what the programmer intended.

This is one of the reasons why I'm in favour of writing more Rust, since
safe Rust doesn't have undefined behaviour and therefore doesn't suffer
from these problems.

In any event, this is almost certainly a bug because it almost certainly
does not do what it looks like it does and the compiler is right to warn
about it.
-- 
brian m. carlson (they/them)
Toronto, Ontario, CA