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Message-ID: <6D4B315A-2D77-426B-99DC-29BC2B723396@trust-in-soft.com>
Date: Sun, 23 Sep 2018 03:10:14 +0000
From: Pascal Cuoq <cuoq@...st-in-soft.com>
To: "musl@...ts.openwall.com" <musl@...ts.openwall.com>
Subject: Re: un-UBify-strings
On 23 Sep 2018, at 04:45, Rich Felker <dalias@...c.org<mailto:dalias@...c.org>> wrote:
I'm also trying to fix the UB in
address range checks for implementing memmove as memcpy, etc. Is this
correct:
if ((uintptr_t)s-(uintptr_t)d-n <= -2*n) return memcpy(d, s, n);
?
It looks okay to me. You want to test whether (uintptr_t)s-(uintptr_t)d, computed as a mathematical integer, is between -n and n, and since uintptr_t is unsigned, you are using the well-known trick of aligning one of the bounds with 0 so that both inequalities can be tested in one instruction.
It would seen more natural to me to work on the right-hand side of zero, that it, to compute (uintptr_t)s-(uintptr_t)d+n and to check whether that is <= 2*n (overlap) or > 2*n (no overlap). The generated code may even be one instruction shorter. Apart from that, as long as we have the hypothesis that n <= UINTPTR_MAX/2, I cannot immediately see any reason why it would not work.
Pascal
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