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Date: Sat, 16 Sep 2023 14:53:35 +0800
From: James R T <>
To: Rich Felker <>, Joakim Sindholt <>
Subject: Re: [PATCH] Add a safe dequeue integrity check for mallocng

On Thu, Sep 14, 2023 at 5:23 PM Joakim Sindholt <> wrote:
> It's a little confusing but assert() in mallocng is not real assert():
> The issue is that if memory is under control of an attacker then doing
> anything at all, especially running the stdio machinery, is unsafe. To
> that end musl uses a_crash() here which expands to a minimal set of
> instructions to crash the process:
> Furthermore, musl doesn't use any of thosed tagged branch tricks and I
> personally doubt it would make any difference.

Ah okay, got it.

On Thu, Sep 14, 2023 at 8:18 PM Rich Felker <> wrote:
> Yes. mallocng is written such that you could use the normal assert
> with it, but presently it's just expanding to a_crash(). At some point
> this might be revamped to crash with a message string in a particular
> register or argument slot or something so that you get a bit more
> meaningful information if looking at it in a debugger. And indeed, the
> reason not to do any message printing, etc. is that you're running in
> a known-compromised process state where any further complex execution
> is unsafe (e.g. if the out-of-band malloc metadata was clobbered, the
> function pointers in stderr might also have been clobbered, since the
> latter are *easier to reach* than the OOB metadata).

Hmm sure, that makes sense.

> Yes, the only reason libm.h has them is because nsz is using the code
> in other environments that want them, and it made sense to avoid
> gratuitous differences. We don't generally use those in musl. If the
> compiler isn't generating good code and puts the failure path as a hot
> path, we probably should explore whether the compiler is missing that
> it's a does-not-return thing (which should always be treated as cold).
> But indeed I doubt it makes a difference.

Got it. I will send in a new patch to simply use an assertion instead then.

Thank you for the detailed explanations!

Best regards,
James Raphael Tiovalen

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