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From: David Laight <David.Laight@ACULAB.COM>
To: 'Pavel Machek' <pavel@ucw.cz>, Justin Stitt <justinstitt@google.com>
CC: Kees Cook <keescook@chromium.org>, Andy Shevchenko <andy@kernel.org>,
	Rasmus Villemoes <linux@rasmusvillemoes.dk>,
	"linux-hardening@vger.kernel.org" <linux-hardening@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
Subject: RE: [RFC] string: Allow 2-argument strscpy()
Thread-Topic: [RFC] string: Allow 2-argument strscpy()
Thread-Index: AQHaWe5TIJbv1x38yUWctvLGNyIGYrEDjCtw
Date: Sat, 10 Feb 2024 13:02:20 +0000
Message-ID: <7060edadcbac4452bd70d7894a50568d@AcuMS.aculab.com>
References: <20240129202901.work.282-kees@kernel.org>
 <20240129215525.4uxchtrywzzsrauc@google.com> <ZcPDMZFPY08S4MGR@duo.ucw.cz>
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From: Pavel Machek
> Sent: 07 February 2024 17:52
>=20
> > > Using sizeof(dst) is the overwhelmingly common case for strscpy().
> > > Instead of requiring this everywhere, allow a 2-argument version to b=
e
> > > used that will use the sizeof() internally.
> >
> > Yeah, this is definitely the case. I have a ton of patches replacing
> > strncpy with strscpy [1] and many of them match the pattern of:
> > | strscpy(dest, src, sizeof(dest))
> >
> > BTW, this hack for function overloading is insane. Never really looked =
into
> > it before.
>=20
> This hack is insane, but this is also highly confusing, please don't
> do this.

An alternative would be to convert xxx(tgt, src, 0) to
xxx(tgt, src, sizeof (tgt) - that is when the specified
length is a compile-time constant zero.

Either with:
=09(__builtin_constat_p(len) && (len) =3D=3D 0 ? sizeof (dst) : (len))
Or, leveraging is_constexpr() and doing (I've probably got the syntax wrong=
):
=09__Generic(0 ? (void *)(len) : (int *)0,
=09=09void *: len,
=09=09int *: sizeof (dst))

That probably needs a helper:
=09is_constzero(value, if_zero, if_non_zero)
to make it more generally useful.

=09David

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