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Date: Sun, 21 Jan 2024 12:03:03 -0500
From: Rich Felker <dalias@...c.org>
To: julien.voisin@...tri.org
Cc: musl@...ts.openwall.com
Subject: Re: Protect pthreads' mutexes against use-after-destroy
On Sun, Jan 21, 2024 at 12:06:14PM +0000, julien.voisin@...tri.org wrote:
> > Draft attached in case anyone wants to play with it. This could
> > probably be something we could consider to adopt.
>
> Couldn't a macro like `#define mutex_is_destroyed (!(m->_m_type & 8) && (m->_m_lock == 0x3fffffff)` be
> used instead? Or at least named constants instead of `8` and `0x3fffffff`..
Maybe something like that, but that's a change I'd like to make in a
consistent uniform way for all of the uses of magic numbers in the
mutex implementation. Just introducing it in a single place like this
doesn't really help readability; in some ways it makes it less
readable since you can't see how it's interacting with the other
tests.
If doing it, I think it would probably make more sense not to have
that predicate macro, but instead something like:
if (own == M_UNRECOVERABLE && !(m->_m_type & MT_ROBUST))
because seeing the individual parts is relevant to understanding:
> Also, the code-style seems inconsistent:
>
> ```
> + if (own == 0x3fffffff) {
> + /* Catch use-after-destroy */
> + if (!(type & 8)) a_crash();
> + return ENOTRECOVERABLE;
> + }
> ```
>
> vs
>
> ```
> + /* Catch use-after-destroy */
> + if (own == 0x3fffffff && !(type & 8)) a_crash();
> return EPERM;
> ```
>
> Both are the same check, yet only one has both conditions in a single `if`.
These are decision trees on what to do in exceptional cases. The aim
is not to present a consistent "style" between two functions that do
different things and have different decision trees for how to act.
Rich
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