Subject: Re: [PATCH 3/3] isystem: delete global -isystem compile option

From: Segher Boessenkool <[email-masked]>
Date: Mon, 2 Aug 2021 11:47:47 -0500

On Mon, Aug 02, 2021 at 09:42:45AM +0300, Alexey Dobriyan wrote:
> On Sun, Aug 01, 2021 at 04:32:47PM -0500, Segher Boessenkool wrote:
> > On Sun, Aug 01, 2021 at 11:13:36PM +0300, Alexey Dobriyan wrote:
> > > In theory, it enables "leakage" of userspace headers into kernel which
> > > may present licensing problem.
> > 
> > > -NOSTDINC_FLAGS += -nostdinc -isystem $(shell $(CC) -print-file-name=include)
> > > +NOSTDINC_FLAGS += -nostdinc
> > 
> > This is removing the compiler's own include files.  These are required
> > for all kinds of basic features, and required to be compliant to the C
> > standard at all.
> 
> No they are not required.

This is false, they *are* required, whenever you want to use these
features.  If you do not include the required headers you get undefined
behaviour.

> Kernel uses its own bool, uintptr_t and
> static_assert, memset(), CHAR_BIT.

Yes, and it occasionally gets it wrong.  Great fun.  See c46bbf5d2def
for the latest episode in this saga.  (Yes I know this is uapi so maybe
not the best example here, but it isn't like the kernel gets such things
wrong so often these days ;-) )

The kernel *cannot* make up its own types for this.  It has to use the
types it is required to use (by C, by the ABIs, etc.)  So why
reimplement this?

> noreturn, alignas newest C standard
> are next.

What is wrong with <stdalign.h> and <stdnoreturn.h>?

> This version changelog didn't mention but kernel would use
> -ffreestanding too if not other problems with the flag.

It is still true for freestanding C implementations, you just get a
severely reduced standard library,

> > These are not "userspace headers", that is what
> > -nostdinc takes care of already.
> 
> They are userspace headers in the sense they are external to the project
> just like userspace programs are external to the kernel.

So you are going to rewrite all of the rest of GCC inside the kernel
project as well?

> > In the case of GCC all these headers are GPL-with-runtime-exception, so
> > claiming this can cause licensing problems is fearmongering.
> 
> I agree licensing problem doesn't really exist.
> It would take gcc drop-in replacement with authors insane enough to not
> license standard headers properly.

There does still not exist a drop-in replacement for GCC, not if you
look closely and/or rely on details (like the kernel does).  Some of the
differences are hidden by "linux/compiler-*.h", but hardly all.

> > I strongly advise against doing this.
> 
> Kernel chose to be self-contained.

That is largely historical, imo.  Nowadays this is less necessary.

Also, the kernel chose to *do* use the compiler include files.  It is
you who wants to abolish that here.

> -isystem removal makes sense then.

-nostdinc -isystem $(shell $(CC) -print-file-name=include)  makes sense
for that: you do indeed not want the userspace headers.  Maiming the
compiler (by removing some of its functional parts, namely, its generic
headers) does not make sense.

> It will be used for intrinsics where necessary.

Like, everywhere.

Segher