Re: [PATCH 1/6] xen/build: Allow the use of C freestanding headers

From: Tim Deegan <tim@xen.org>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Ian Jackson <ian.jackson@eu.citrix.com>,
	Xen-devel <xen-devel@lists.xen.org>,
	Jan Beulich <JBeulich@suse.com>
Subject: Re: [PATCH 1/6] xen/build: Allow the use of C freestanding headers
Date: Wed, 13 Jul 2016 16:17:38 +0100	[thread overview]
Message-ID: <20160713151738.GB71226@deinos.phlegethon.org> (raw)
In-Reply-To: <f4b8c3c2-afdc-56d3-837a-58fecf041c63@citrix.com>

At 14:46 +0100 on 13 Jul (1468421211), Andrew Cooper wrote:
> On 22/06/16 14:12, Tim Deegan wrote:
> > At 12:24 +0100 on 22 Jun (1466598248), Andrew Cooper wrote:
> >> The C standard defines two types of conforming implementation; hosted and
> >> freestanding.  A subset of the standard headers are the freestanding headers,
> >> requiring no library support at all to use, and therefore usable by Xen.
> >>
> >> Unfortunately, -nostdinc is an overly large switch, and there is no
> >> alternative to only permit inclusion of the freestanding headers.  Removing it
> >> is unfortunate, as we lose the protection it offers, but anyone who does try
> >> to use other parts of the standard library will still fail to link.
> > I'm afraid I don't think this is a good idea:
> >  - Leaving the standard include path around in the Xen build means
> >    that the build may differ based on what (unrelated) libraries are
> >    installed on the build machines.
> >  - There are plenty of ways for an unexpected header to break things
> >    that don't fail at link time, e.g. macros and inlines.
> >  - "Freestanding" headers can bring in quite a lot of unrelated cruft.
> >    See Jan's email about linux/glibc, and I remember seeing similar
> >    things on solaris and *BSD when I tidied up stdarg.h. E.g. looking
> >    at two machines I'm working on today, on one of them,
> >    #include <limits.h> defines __packed, and on the other it does not.
> >
> > Since what we have already works fine for all the compilers we
> > support, I think it ain't broke and we shouldn't fix it.
> 
> Except it is broken.

Broken in theory or actually causing a problem right now?

> We cannot expect to use -Wformat and not the compiler provided
> inttypes.h.

Since we're not using the compiler-provided printf(), I think that
we're on pretty thin ice with -Wformat anyway.  But as long as our
own PRI* macros match our type definitions, all should be well, right?

> The sizes of constructs like "long long" are implementation
> defined, not spec defined.  The compiler is perfectly free to choose
> something which doesn't match our inttypes.h, and we would be in the
> wrong when it fails to compile.

If a compiler we support does that, we can update our integer type
definitions, like we do for attribute annotations, &c.

BTW, I can absolutely see the argument for deferring to the compiler
in C implementation details.  I think that would have to include
removing all the implementation-reserved names from Xen so we don't
clash with compiler headers.

But in practice it seems to be messy enough to be better avoided --
having /usr/include on the search path is pretty silly.

Tim.

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