linux-kernel - Re: [PATCH 2/2] modpost: don't emit section mismatch warnings for compiler optimizations

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Message-ID: <87wq17pnff.fsf@rustcorp.com.au>
Date:	Mon, 20 Apr 2015 11:32:12 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Paul Gortmaker <paul.gortmaker@...driver.com>
Cc:	linux-kernel@...r.kernel.org,
	Paul Gortmaker <paul.gortmaker@...driver.com>
Subject: Re: [PATCH 2/2] modpost: don't emit section mismatch warnings for compiler optimizations

Paul Gortmaker <paul.gortmaker@...driver.com> writes:
> Currently an allyesconfig build [gcc-4.9.1] can generate the following:
>
>    WARNING: vmlinux.o(.text.unlikely+0x3864): Section mismatch in
>    reference from the function cpumask_empty.constprop.3() to the
>    variable .init.data:nmi_ipi_mask
>
> which comes from the cpumask_empty usage in arch/x86/kernel/nmi_selftest.c.

Thanks for the excellent explanation and patches.

Applied!
Rusty.

>
> Normally we would not see a symbol entry for cpumask_empty since it is:
>
> 	static inline bool cpumask_empty(const struct cpumask *srcp)
>
> however in this case, the variant of the symbol gets emitted when GCC does
> constant propagation optimization.
>
> Fix things up so that any locally optimized constprop variants don't warn
> when accessing variables that live in the __init sections.
>
> Signed-off-by: Paul Gortmaker <paul.gortmaker@...driver.com>
> ---
>  scripts/mod/modpost.c | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
>
> diff --git a/scripts/mod/modpost.c b/scripts/mod/modpost.c
> index b18cecb6bf5a..572f1e4a2626 100644
> --- a/scripts/mod/modpost.c
> +++ b/scripts/mod/modpost.c
> @@ -908,6 +908,9 @@ static const char *const init_sections[] = { ALL_INIT_SECTIONS, NULL };
>  static const char *const init_exit_sections[] =
>  	{ALL_INIT_SECTIONS, ALL_EXIT_SECTIONS, NULL };
>  
> +/* all text sections */
> +static const char *const text_sections[] = { ALL_TEXT_SECTIONS, NULL };
> +
>  /* data section */
>  static const char *const data_sections[] = { DATA_SECTIONS, NULL };
>  
> @@ -926,6 +929,7 @@ static const char *const data_sections[] = { DATA_SECTIONS, NULL };
>  static const char *const head_sections[] = { ".head.text*", NULL };
>  static const char *const linker_symbols[] =
>  	{ "__init_begin", "_sinittext", "_einittext", NULL };
> +static const char *const optim_symbols[] = { "*.constprop.*", NULL };
>  
>  enum mismatch {
>  	TEXT_TO_ANY_INIT,
> @@ -1127,6 +1131,17 @@ static const struct sectioncheck *section_mismatch(
>   *   This pattern is identified by
>   *   refsymname = __init_begin, _sinittext, _einittext
>   *
> + * Pattern 5:
> + *   GCC may optimize static inlines when fed constant arg(s) resulting
> + *   in functions like cpumask_empty() -- generating an associated symbol
> + *   cpumask_empty.constprop.3 that appears in the audit.  If the const that
> + *   is passed in comes from __init, like say nmi_ipi_mask, we get a
> + *   meaningless section warning.  May need to add isra symbols too...
> + *   This pattern is identified by
> + *   tosec   = init section
> + *   fromsec = text section
> + *   refsymname = *.constprop.*
> + *
>   **/
>  static int secref_whitelist(const struct sectioncheck *mismatch,
>  			    const char *fromsec, const char *fromsym,
> @@ -1159,6 +1174,12 @@ static int secref_whitelist(const struct sectioncheck *mismatch,
>  	if (match(tosym, linker_symbols))
>  		return 0;
>  
> +	/* Check for pattern 5 */
> +	if (match(fromsec, text_sections) &&
> +	    match(tosec, init_sections) &&
> +	    match(fromsym, optim_symbols))
> +		return 0;
> +
>  	return 1;
>  }
>  
> -- 
> 2.2.1
>
> --
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