linux-kernel - Re: [PATCH v2 02/10] x86: assembly, FUNC_START for fn, DATA

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Date:   Wed, 22 Mar 2017 10:33:36 -0500
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Jiri Slaby <jslaby@...e.cz>
Cc:     Ingo Molnar <mingo@...nel.org>, Pavel Machek <pavel@....cz>,
        mingo@...hat.com, tglx@...utronix.de, hpa@...or.com,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        xen-devel@...ts.xenproject.org,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <len.brown@...el.com>, linux-pm@...r.kernel.org
Subject: Re: [PATCH v2 02/10] x86: assembly, FUNC_START for fn, DATA_START
 for data

On Wed, Mar 22, 2017 at 04:01:08PM +0100, Jiri Slaby wrote:
> On 03/22/2017, 03:11 PM, Josh Poimboeuf wrote:
> > Or, here's a much easier way to do it, without involving objtool:
> > 
> > --- a/include/linux/linkage.h
> > +++ b/include/linux/linkage.h
> > @@ -138,9 +138,17 @@
> >  	name:
> >  #endif
> >  
> > +#ifndef CHECK_DUP_SYM_END
> > +#define CHECK_DUP_SYM_END(name)				\
> > +	.pushsection .discard.sym_func_end ASM_NL	\
> > +	SYM_END_##name: .byte 0 ASM_NL			\
> > +	.popsection
> > +#endif
> > +
> >  /* SYM_END -- use only if you have to */
> >  #ifndef SYM_END
> >  #define SYM_END(name, sym_type)				\
> > +	CHECK_DUP_SYM_END(name) ASM_NL			\
> >  	.type name sym_type ASM_NL			\
> >  	.size name, .-name
> >  #endif
> 
> I tried this approach and it didn't work for me inside .macros. Oh,
> well, the name cannot be first, so now, we can have a check for both
> correct pairing _and_ duplicate ends in one:
> 
> #define SYM_CHECK_START(name)                           \
>         .pushsection .rodata.bubak ASM_NL               \
>         .long has_no_SYM_END_##name - . ASM_NL          \
>         .popsection
> 
> #define SYM_CHECK_END(name)                             \
>         has_no_SYM_END_##name:
> 
> /* SYM_START -- use only if you have to */
> #ifndef SYM_START
> #define SYM_START(name, align, visibility, entry)       \
>         SYM_CHECK_START(name) ASM_NL                    \
>         visibility(name) ASM_NL                         \
>         align ASM_NL                                    \
>         name: ASM_NL                                    \
>         entry
> #endif
> 
> /* SYM_END -- use only if you have to */
> #ifndef SYM_END
> #define SYM_END(name, sym_type, exit)                   \
>         exit ASM_NL                                     \
>         SYM_CHECK_END(name) ASM_NL                      \
>         .type name sym_type ASM_NL                      \
>         .size name, .-name
> #endif
> 
> 
> So for the ftrace mistake I did:
> 
>   AS      arch/x86/kernel/mcount_64.o
> /home/latest/linux/arch/x86/kernel/mcount_64.S: Assembler messages:
> /home/latest/linux/arch/x86/kernel/mcount_64.S:192: Error: symbol
> `has_no_SYM_END_ftrace_caller' is already defined
> 
> 
> or if I remove SYM_END_FUNC completely:
>   LD      vmlinux.o
>   MODPOST vmlinux.o
> arch/x86/built-in.o:(.rodata.bubak+0x130): undefined reference to
> `has_no_SYM_END_ftrace_stub'
> 
> 
> Sad is that this occurs only during linking, so I cannot put it in the
> .discard section -- ideas?

Ah, interesting idea but I can't think of a way to do the missing end
check before link time.

But it would be easy for objtool to check for a missing end because the
symbol would have a zero size.

-- 
Josh