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Date: Thu, 30 Apr 2020 20:17:39 +0200
From: Alfred Agrell <alfred@...ell.info>
To: musl@...ts.openwall.com
Subject: bug: integer overflow in memmem()
To reproduce: Compile src/string/memmem.c with -fsanitize=undefined, then
int main()
{
char a[4] = { -1,-1,-1,-1 };
memmem(a, 4, a, 3);
memmem(a, 4, a, 4);
}
Expected result: No output
Actual (Ubuntu 18.04 x86_64, gcc 7.5.0, ):
memmem.c:15:20: runtime error: left shift of 255 by 24 places cannot be
represented in type 'int'
memmem.c:16:20: runtime error: left shift of 255 by 24 places cannot be
represented in type 'int'
memmem.c:24:20: runtime error: left shift of 255 by 24 places cannot be
represented in type 'int'
memmem.c:25:20: runtime error: left shift of 255 by 24 places cannot be
represented in type 'int'
C's integer promotion rules are fairly unintuitive for <<; it promotes
unsigned small LHS to signed. To fix, change the two n[0]<<24 to
(uint32_t)n[0]<<24, and similar for h[0]<<24.
I'm not aware of any compiler on any platform where it'll actually
break, so your choice whether this is a real bug. I didn't check if
similar issues exist elsewhere across musl.
I'm not subscribed to the list; I'll read the archives, but if you want
a timely response, please cc me.
-- Alfred Agrell
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