linux-kernel - Re: [PATCH] string: Improve the generic strlcpy() implementation

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Date:	Mon, 5 Oct 2015 16:32:37 +0100
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Chris Metcalf <cmetcalf@...hip.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Borislav Petkov <bp@...en8.de>,
	open list <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] string: Improve the generic strlcpy() implementation

On Mon, Oct 5, 2015 at 3:33 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> So we could solve that by adding a generic range check:
>
>  static inline int range_ok(unsigned long low, unsigned long val, unsigned long high)

So what about the type of the thing you're checking?

Maybe negative values are ok. It's unusual, but it's not unheard of.
We do have cases like

    if ((uch_config < -1) || (uch_config > 31)) {

so we have range checks that actually have signed ranges.

So I don't think a "generic" range check helper can force types like
"unsigned long".

That said, doing a simple

    git grep '<.*||.*>'

does show that the "positive or non-zero ranges with constant range
limits" case is fairly common, and maybe we could have a macro that
does some magic compile-time checking that (a) the range really is a
compile-time constant and (b) that range is valid and (c) avoids the
comparison with zero if the expression to be tested is unsigned.

So it is possible that we could enable type limit checking if we also
introduce a good way to not then create crap patches that actually
make the code more fragile or less readable. I'm not violently against
that. But I *am* violently against introducing that braindead warning
without very clear rules that we don't then have the mindless and
wrong changes to remove proper and obvious range checking and replace
it with "the expression is unsigned so we remove the nice readable
lower bounds check as unnecessary".

            Linus
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