Re: [PATCH] powerpc: Use aligned stores in memset

[Zack Weinberg <zackw@panix.com>]

On Tue, Sep 12, 2017 at 6:30 AM, Florian Weimer <fweimer@redhat.com> wrote:
>
> I could not find the manual which has the requirement that the mem*
> functions do not use unaligned accesses.  Unless they are worded in a
> very peculiar way, right now, the GCC/glibc combination does not comply
> with a requirement that memset & Co. can be used for device memory access.

mem* are required to behave as-if they access memory as an array of
unsigned char.  Therefore it is valid to give them arbitrarily
(un)aligned pointers.  The C abstract machine doesn't specifically
contemplate the possibility of a CPU that can do unaligned word reads
but maybe not to all memory addresses, but I would argue that if there
is such a CPU, then mem* are obliged to cope with it.

> ...the current glibc
> implementation accesses locations which are outside the specified object
> boundaries.

I think that's technically a defect.  Nothing in the C standard
licenses it to do that; we just get away with it because, on the
implementations to date, it's not observable (unless you go past the
end of a page, which you'll note there are a bunch of tests to ensure
we don't do).  If an over-read by a single byte is observable, then
mem* is not allowed to do that.

zw