linux-kernel - Re: [PATCH] compiler.h: avoid escaped section names

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Message-ID: <20200929200801.GA2668747@rani.riverdale.lan>
Date:   Tue, 29 Sep 2020 16:08:01 -0400
From:   Arvind Sankar <nivedita@...m.mit.edu>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        Luc Van Oostenryck <luc.vanoostenryck@...il.com>,
        Nathan Chancellor <natechancellor@...il.com>,
        linux-sparse@...r.kernel.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com
Subject: Re: [PATCH] compiler.h: avoid escaped section names

On Tue, Sep 29, 2020 at 12:43:18PM -0700, Nick Desaulniers wrote:
> 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@...gle.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
>  
> -- 
> 2.28.0.709.gb0816b6eb0-goog
> 

There was this previous mini-thread:
https://lore.kernel.org/lkml/20200629205448.GA1474367@rani.riverdale.lan/
and this older one:
https://lore.kernel.org/lkml/20190904181740.GA19688@gmail.com/

Just for my own curiosity: how does KENTRY actually get used? grep
doesn't show any hits, and the thread from 2019 was actually going to
drop it if I read it right, and also just remove stringification from
the __section macro.

There are still other instances that need to be fixed, right?

Thanks.