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

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Date:   Wed, 22 Mar 2017 16:01:08 +0100
From:   Jiri Slaby <jslaby@...e.cz>
To:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Ingo Molnar <mingo@...nel.org>
Cc:     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 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?

thanks,
-- 
js
suse labs