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Date: Tue, 22 Oct 2019 03:57:09 -0500
From: Segher Boessenkool <segher@kernel.crashing.org>
To: Nathan Chancellor <natechancellor@gmail.com>
Subject: Re: [PATCH v4 3/3] powerpc/prom_init: Use -ffreestanding to avoid a
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Cc: Nick Desaulniers <ndesaulniers@google.com>,
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On Mon, Oct 21, 2019 at 10:15:29PM -0700, Nathan Chancellor wrote:
> On Fri, Oct 18, 2019 at 03:02:10PM -0500, Segher Boessenkool wrote:
> > I think the proper solution is for the kernel to *do* use -ffreestanding,
> > and then somehow tell the kernel that memcpy etc. are the standard
> > functions.  A freestanding GCC already requires memcpy, memmove, memset,
> > memcmp, and sometimes abort to exist and do the standard thing; why cannot
> > programs then also rely on it to be the standard functions.
> > 
> > What exact functions are the reason the kernel does not use -ffreestanding?
> > Is it just memcpy?  Is more wanted?
> 
> I think Linus summarized it pretty well here:
> 
> https://lore.kernel.org/lkml/CAHk-=wi-epJZfBHDbKKDZ64us7WkF=LpUfhvYBmZSteO8Q0RAg@mail.gmail.com/

GCC recognises __builtin_memcpy (or any other __builtin) just fine even
with -ffreestanding.

So the kernel wants a warning (or error) whenever a call to one of these
library functions is generated by the compiler without the user asking
for it directly (via a __builtin)?  And that is all that is needed for
the kernel to use -ffreestanding?

That shouldn't be hard.  Anything missing here?


Segher