Re: [dev] [st] [PATCH 4/3] tresize: remove unnecessary if

From: Dimitris Papastamos <sin_AT_2f30.org>
Date: Tue, 14 Apr 2015 16:39:44 +0100

On Tue, Apr 14, 2015 at 04:33:56PM +0100, Connor Lane Smith wrote:
> On 14 April 2015 at 13:50, Dimitris Papastamos <sin_AT_2f30.org> wrote:
> > dst and src are required to be valid objects even if n is 0, otherwise
> > this is undefined behaviour.
>
> I looked this up in C11. Seems to be the case:
>
> > 7.24.2.1.2. The memcpy function copies n characters from the object
> > pointed to by s2 into the object pointed to by s1.
>
> > 6.2.6.1.2. Except for bit-fields, objects are composed of contiguous
> > sequences of one or more bytes, ...
>
> > 124) ... there are no pointers to or arrays of bit-field objects.
>
> An object pointed to by s1 or s2 is guaranteed to be at least 1 byte
> long, so memcpy is free to dereference the first byte of either,
> whether or not n > 0.
>
> I think it's a mistake that a libc actually go ahead and do this, but
> the C standard suggests that it is permitted, so we should certainly
> do the check.

Yup, well it happens only with the optimized version of memmove() in OpenBSD.
Not sure if this was intended or not. In any case, it is valid for libc
to assume that src/dst is at least 1 byte long even if n is 0 as you say.
Received on Tue Apr 14 2015 - 17:39:44 CEST