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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Sat, 4 Dec 2021 09:31:34 +0100
Message-ID: <CAMj1kXG-+d-ak9od9z-VxOk9Y+fp_KDSbLP=ns_ZfVEXjrzKsg@mail.gmail.com>
Subject: Re: [PATCH v8 05/14] x86: conditionally place regular ASM functions
 into separate sections
To:     Nicolas Pitre <nico@fluxnic.net>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Alexander Lobakin <alexandr.lobakin@intel.com>,
        linux-hardening@vger.kernel.org, X86 ML <x86@kernel.org>,
        Jesse Brandeburg <jesse.brandeburg@intel.com>,
        Kristen Carlson Accardi <kristen@linux.intel.com>,
        Kees Cook <keescook@chromium.org>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Tony Luck <tony.luck@intel.com>,
        Bruce Schlobohm <bruce.schlobohm@intel.com>,
        Jessica Yu <jeyu@kernel.org>,
        kernel test robot <lkp@intel.com>,
        Miroslav Benes <mbenes@suse.cz>,
        Evgenii Shatokhin <eshatokhin@virtuozzo.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Michal Marek <michal.lkml@markovi.net>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        "David S. Miller" <davem@davemloft.net>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>, Ingo Molnar <mingo@redhat.com>,
        Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Marios Pomonis <pomonis@google.com>,
        Sami Tolvanen <samitolvanen@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux Kbuild mailing list <linux-kbuild@vger.kernel.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        live-patching@vger.kernel.org, llvm@lists.linux.dev,
        hjl.tools@gmail.com
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Precedence: bulk

On Fri, 3 Dec 2021 at 20:46, Nicolas Pitre <nico@fluxnic.net> wrote:
>
> On Fri, 3 Dec 2021, Peter Zijlstra wrote:
>
> > On Fri, Dec 03, 2021 at 03:10:51PM +0100, Alexander Lobakin wrote:
> > > From: Peter Zijlstra <peterz@infradead.org>
> > > Date: Fri, 3 Dec 2021 10:44:10 +0100
> > >
> > > > On Thu, Dec 02, 2021 at 11:32:05PM +0100, Alexander Lobakin wrote:
> > > > > Use the newly introduces macros to create unique separate sections
> > > > > for (almost) every "regular" ASM function (i.e. for those which
> > > > > aren't explicitly put into a specific one).
> > > > > There should be no leftovers as input .text will be size-asserted
> > > > > in the LD script generated for FG-KASLR.
> > > >
> > > > *groan*...
> > > >
> > > > Please, can't we do something like:
> > > >
> > > > #define SYM_PUSH_SECTION(name)    \
> > > > .if section == .text              \
> > > > .push_section .text.##name        \
> > > > .else                             \
> > > > .push_section .text               \
> > > > .endif
> > > >
> > > > #define SYM_POP_SECTION() \
> > > > .pop_section
> > > >
> > > > and wrap that inside the existing SYM_FUNC_START*() SYM_FUNC_END()
> > > > macros.
> > >
> > > Ah I see. I asked about this in my previous mail and you replied
> > > already (: Cool stuff, I'll use it, it simplifies things a lot.
> >
> > Note, I've no idea if it works. GAS and me aren't really on speaking
> > terms. It would be my luck for that to be totally impossible, hjl?
>
> Surely this would do it:
>
> http://sourceware.org/git/?p=binutils-gdb.git;a=commitdiff;h=451133cefa839104
>

That seems rather useful, actually. It will also fix a problem with
subsections, which are sometimes difficult to construct from a macro,
as they cannot be created using pushsection/popsection unless you know
the current section name, and the alternative syntax (.subsection /
.previous) does not permit nesting. This makes their use from a macro
risky, given that it may not be obvious to the macro's caller that it
uses a subsection under the hood.