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

[Alexey Dobriyan]

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. Kernel uses its own bool, uintptr_t and
static_assert, memset(), CHAR_BIT. noreturn, alignas newest C standard
are next.

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

> 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.

> 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.

> I strongly advise against doing this.

Kernel chose to be self-contained. -isystem removal makes sense then.
It will be used for intrinsics where necessary.