linux-kernel - Re: [PATCH v4 08/17] arm64/mm: Remove needless section quotes

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Date:   Mon, 29 Jun 2020 17:06:19 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Kees Cook <keescook@...omium.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Will Deacon <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Peter Collingbourne <pcc@...gle.com>,
        James Morse <james.morse@....com>,
        Borislav Petkov <bp@...e.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nathan Chancellor <natechancellor@...il.com>,
        Arnd Bergmann <arnd@...db.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Joe Perches <joe@...ches.com>
Subject: Re: [PATCH v4 08/17] arm64/mm: Remove needless section quotes

On Mon, Jun 29, 2020 at 04:54:48PM -0400, Arvind Sankar wrote:
> On Mon, Jun 29, 2020 at 01:04:31PM -0700, Kees Cook wrote:
> > On Mon, Jun 29, 2020 at 12:53:47PM -0700, Nick Desaulniers wrote:
> > > On Sun, Jun 28, 2020 at 11:18 PM Kees Cook <keescook@...omium.org> wrote:
> > > >
> > > > Fix a case of needless quotes in __section(), which Clang doesn't like.
> > > >
> > > > Acked-by: Will Deacon <will@...nel.org>
> > > > Signed-off-by: Kees Cook <keescook@...omium.org>
> > > 
> > > Yep, I remember bugs from this.  Probably should scan the kernel for
> > > other instances of this.  +Joe for checkpatch.pl validation.
> > 
> > I think the others are safe because they're in macros:
> 
> Why does that make it safe -- the commit msg is a bit sparse, but I
> assume the problem is that it generates
> 	__attribute__((__section__("\".foo\"")))
> from
> 	__section(".foo")
> after preprocessing, and clang keeps the quotes in the section name when
> generating assembly, while gcc appears to strip them off.
> 
> It does that even if nested in another macro, no?

Yep, I can see things like:
[25] ".discard.ksym"   PROGBITS         0000000000000000  0000217c
       0000000000000000  0000000000000000  WA       0     0     4

Doesn't seem to cause a build error, but that can't be good.

> 
> > 
> > $ git grep -4 '__section("'
> > include/linux/compiler.h-# define KENTRY(sym)                                           \
> 
> Am I missing something, or is KENTRY unused in the tree?
> 
> > include/linux/compiler.h-       extern typeof(sym) sym;                                 \
> > include/linux/compiler.h-       static const unsigned long __kentry_##sym               \
> > include/linux/compiler.h-       __used                                                  \
> > include/linux/compiler.h:       __section("___kentry" "+" #sym )                        \
> > include/linux/compiler.h-       = (unsigned long)&sym;
> > --
> > include/linux/export.h-#define __ksym_marker(sym)       \
> > include/linux/export.h: static int __ksym_marker_##sym[0] __section(".discard.ksym") __used
> > --
> > include/linux/srcutree.h-# define __DEFINE_SRCU(name, is_static)                                \
> > include/linux/srcutree.h-       is_static struct srcu_struct name;                              \
> > include/linux/srcutree.h-       struct srcu_struct * const __srcu_struct_##name                 \
> > include/linux/srcutree.h:               __section("___srcu_struct_ptrs") = &name
> > 
> > 
> > > Reviewed-by: Nick Desaulniers <ndesaulniers@...gle.com>
> > 
> > Thanks!
> > 
> > -- 
> > Kees Cook