RE: [PATCH] compiler.h: avoid escaped section names

From: David Laight <David.Laight@ACULAB.COM>
To: 'Nick Desaulniers' <ndesaulniers@google.com>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>,
	Luc Van Oostenryck <luc.vanoostenryck@gmail.com>,
	Nathan Chancellor <natechancellor@gmail.com>,
	"linux-sparse@vger.kernel.org" <linux-sparse@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"clang-built-linux@googlegroups.com" 
	<clang-built-linux@googlegroups.com>
Subject: RE: [PATCH] compiler.h: avoid escaped section names
Date: Wed, 30 Sep 2020 08:33:20 +0000	[thread overview]
Message-ID: <059f629fcf5a4fe0825287a44fd24f52@AcuMS.aculab.com> (raw)
In-Reply-To: <20200929194318.548707-1-ndesaulniers@google.com>

From: Nick Desaulniers
> Sent: 29 September 2020 20:43
> 
> The stringification operator, `#`, in the preprocessor escapes strings.
> For example, `# "foo"` becomes `"\"foo\""`.  GCC and Clang differ in how
> they treat section names that contain \".
> 
> The portable solution is to not use a string literal with the
> preprocessor stringification operator.
> 
> In this case, since __section unconditionally uses the stringification
> operator, we actually want the more verbose
> __attribute__((__section__())).
> 
> Link: https://bugs.llvm.org/show_bug.cgi?id=42950
> Fixes: commit e04462fb82f8 ("Compiler Attributes: remove uses of __attribute__ from compiler.h")
> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
> ---
>  include/linux/compiler.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> index 92ef163a7479..ac45f6d40d39 100644
> --- a/include/linux/compiler.h
> +++ b/include/linux/compiler.h
> @@ -155,7 +155,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
>  	extern typeof(sym) sym;					\
>  	static const unsigned long __kentry_##sym		\
>  	__used							\
> -	__section("___kentry" "+" #sym )			\
> +	__attribute__((__section__("___kentry+" #sym)))		\
>  	= (unsigned long)&sym;
>  #endif

I guess what this really wants is:
	__section(__kentry+##sym)
but that generates an error because you can only use ## between
variable names.

Perhaps someone shouldn't have tries to be clever and not put
an unusual character in the section name.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)