linux-kernel - RE: [PATCH 5/8] ucounts: Handle wrapping in is_ucounts

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Date:   Mon, 14 Feb 2022 17:16:06 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Solar Designer' <solar@...nwall.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Alexey Gladkov" <legion@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Shuah Khan <shuah@...nel.org>,
        Christian Brauner <brauner@...nel.org>,
        Ran Xiaokai <ran.xiaokai@....com.cn>,
        Michal Koutn?? <mkoutny@...e.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH 5/8] ucounts: Handle wrapping in is_ucounts_overlimit

From: Solar Designer
> Sent: 12 February 2022 22:37
...
> bool is_ucounts_overlimit(struct ucounts *ucounts, enum ucount_type type, unsigned long rlimit)
> {
> 	struct ucounts *iter;
> 	long max = rlimit;
> 	if (rlimit > LONG_MAX)
> 		max = LONG_MAX;
> 
> The assignment on "long max = rlimit;" would have already been UB if
> "rlimit > LONG_MAX", which is only checked afterwards.  I think the
> above would be better written as:

I'm pretty sure assignments and casts of negative values to unsigned
types are actually well defined.
Although the actual value may differ for ones-compliment and
sign-overpunch systems.
But I suspect Linux requires twos-compliment negative numbers.

(In much the same way as it requires that NULL be the all zero
bit pattern - although a load of annoying compiler warnings are only
relevant if that isn't the case.)

	David

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