From: "H. Peter Anvin" <hpa@zytor.com>
To: Borislav Petkov <bp@suse.de>, Nick Desaulniers <ndesaulniers@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
mingo@redhat.com, x86@kernel.org,
"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
Masahiro Yamada <yamada.masahiro@socionext.com>,
Matthias Kaehlcke <mka@chromium.org>,
Kees Cook <keescook@chromium.org>,
Cao jin <caoj.fnst@cn.fujitsu.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86/boot: define CC_HAVE_ASM_GOTO
Date: Fri, 28 Sep 2018 13:41:46 -0700 [thread overview]
Message-ID: <56442061-7f55-878d-5b26-7cdd14e901d2@zytor.com> (raw)
In-Reply-To: <20180926090841.GC5745@zn.tnic>
On 09/26/18 02:08, Borislav Petkov wrote:
> On Tue, Sep 25, 2018 at 02:13:16PM -0700, Nick Desaulniers wrote:
>> Orthogonally, do you know *why* this Makefile overwrites
>> KBUILD_CFLAGS? We take great care to set various compiler flags in
>> the top level Makefile, so to reset them lower in the tree seems
>> troublesome on first glance. Take for instance the fact that GCC
>> changed the default C standard from gnu89 to gnu11 in GCC 5. So a
>> kernel dev with gcc 5+ is now building a kernel with some translation
>> units built as gnu89, and some as gnu11.
>
> Right, so I'd defer to hpa for a definitive answer here but AFAICT, the
> reasoning is that because this is the special relocatable compressed
> kernel and as such, it is redefining build flags completely so that
> nothing leaks from kernel proper.
>
> Which is funny because what happens is, that something *does* leak,
> and what you're trying to fix is one incarnation. Because we do use
> facilities from include/linux/. And looking at your error:
>
> #warning "Compiler lacks ASM_GOTO support. Add -D __BPF_TRACING__ to your compiler arguments"
>
> I don't think it does matter to the compressed kernel whether the
> compiler has asm goto or not.
>
> And looking at arch/x86/boot/Makefile, it does:
>
> KBUILD_CFLAGS := $(REALMODE_CFLAGS) -D_SETUP
>
> and that _SETUP guard is kinda used to separate that early boot code
> from kernel proper.
>
> And I wonder if adding such guard to arch/x86/boot/compressed/Makefile
> and then protect the asm goto check in arch/x86/include/asm/cpufeature.h
> with it, would be more in line with how this whole machinery should
> work...
>
It overrides CFLAGS because it is by necessity a 32-bit compile (16
bits, really, but that's done at the assembly level only.) Some 64-bit
flags cause the 32-bit compile to error out.
-hpa
next prev parent reply other threads:[~2018-09-28 20:42 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-09-07 20:26 [PATCH] x86/boot: define CC_HAVE_ASM_GOTO Nick Desaulniers
2018-09-25 18:41 ` Nick Desaulniers
2018-09-25 19:08 ` Borislav Petkov
2018-09-25 21:08 ` Nick Desaulniers
2018-09-25 21:13 ` Nick Desaulniers
2018-09-26 9:08 ` Borislav Petkov
2018-09-28 20:41 ` H. Peter Anvin [this message]
2018-09-26 8:56 ` Borislav Petkov
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=56442061-7f55-878d-5b26-7cdd14e901d2@zytor.com \
--to=hpa@zytor.com \
--cc=bp@suse.de \
--cc=caoj.fnst@cn.fujitsu.com \
--cc=keescook@chromium.org \
--cc=kirill.shutemov@linux.intel.com \
--cc=linux-kernel@vger.kernel.org \
--cc=mingo@redhat.com \
--cc=mka@chromium.org \
--cc=ndesaulniers@google.com \
--cc=tglx@linutronix.de \
--cc=x86@kernel.org \
--cc=yamada.masahiro@socionext.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).