Message-ID: <CA+rthh-ZH4mhv5s4XUEYLHFe8aFW95K7dGfwQ-dzjUDum-znKw@mail.gmail.com>
Date:	Wed, 25 Jun 2014 10:17:47 +0200
From:	Mathias Krause <minipli@...glemail.com>
To:	Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:	Joe Perches <joe@...ches.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC PATCH 0/3] Mark literal strings in __init / __exit code

On 25 June 2014 09:35, Rasmus Villemoes <linux@...musvillemoes.dk> wrote:
> Mathias Krause <minipli@...glemail.com> writes:
>
>> On 24 June 2014 16:31, Rasmus Villemoes <linux@...musvillemoes.dk> wrote:
>>> gcc already seems to contain infrastructure for this kind of thing, so
>>> maybe it doesn't even require a plugin, but simply a little coordination
>>> with the gcc folks. This snippet from gcc internals seems relevant:
>>>
>>> -- Target Hook: section * TARGET_ASM_FUNCTION_RODATA_SECTION (tree
>>>           DECL)
>>>      Return the readonly data section associated with 'DECL_SECTION_NAME
>>>      (DECL)'.  The default version of this function selects
>>>      '.gnu.linkonce.r.name' if the function's section is
>>>      '.gnu.linkonce.t.name', '.rodata.name' if function is in
>>>      '.text.name', and the normal readonly-data section otherwise.
>>>
>>
>> I don't think it's that easy. You cannot simply put all strings into
>> the .init.rodata section when code currently gets emitted to
>> .init.text. The reason is because strings used in __init code might be
>> referenced later on, too. For example, the name passed to
>> class_create() won't be copied.
>
> Right, didn't think about that, so yes, the source would need
> to be annotated some way or other, or gcc would need to learn the
> semantics of certain kernel functions.

I would rather like to avoid the latter.

>
> Speaking of dangling pointers: A similar disaster would happen if some
> code containing pi_* calls gets copy-pasted to some non-__init
> function. Could checkpatch learn to warn about calling these functions
> from the wrong context?

But the same is already true for code. If an __init function gets
referenced from a non-__init function, the same dangling references
would occur. That's what scripts/mod/modpost is for -- detecting such
cases and warn about them. So this is already handled by the kernel
build system. A checkpatch warning might be a nice addition, anyway.

>
> Mathias Krause <minipli@...glemail.com> writes:
>
>> Merging strings across multiple compilation units does not happen,
>> anyway -- not now, not with the new macros.
>
> Certainly string merging seems to happen, at least at -O1 and higher:
>
> $ grep . *.c
> a.c:const char *a(void) { return "654321"; }
> b.c:const char *b(void) { return "4321"; }
> c.c:const char *c(void) { return "654321"; }
> main.c:#include <stdio.h>
> main.c:const char *a(void);
> main.c:const char *b(void);
> main.c:const char *c(void);
> main.c:int main(void)
> main.c:{
> main.c: printf("%p\n", a());
> main.c: printf("%p\n", b());
> main.c: printf("%p\n", c());
> main.c: return 0;
> main.c:}
> $ gcc -O1 -c a.c && gcc -O1 -c b.c && gcc -O1 -c c.c
> $ gcc -O1 main.c a.o b.o c.o
> $ ./a.out
> 0x400630
> 0x400632
> 0x400630
>
> So not only are identical strings merged; suffixes are also optimized.

Oh, indeed! But they're luckily not merged across sections. So no
problems with dangling pointers here, too.

Thanks,
Mathias
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