linux-kernel - Re: [RFC PATCH 1/2] Kbuild: kallsyms: ignore veneers emitted by the ARM linker

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Date:	Mon, 30 Mar 2015 14:11:00 +0200
From:	Michal Marek <mmarek@...e.cz>
To:	Ard Biesheuvel <ard.biesheuvel@...aro.org>,
	linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, arnd@...db.de,
	linux@....linux.org.uk
Subject: Re: [RFC PATCH 1/2] Kbuild: kallsyms: ignore veneers emitted by the
 ARM linker

On 2015-03-30 13:49, Ard Biesheuvel wrote:
> When linking large kernels on ARM, the linker will insert veneers
> (i.e., PLT like stubs) when function symbols are out of reach for
> the ordinary relative branch/branch-and-link instructions.
> 
> However, due to the fact that the kallsyms region sits in .rodata,
> which is between .text and .init.text, additional veneers may be
> emitted in the second pass due to the fact that the size of the
> kallsyms region itself has pushed the .init.text section further
> down, resulting in additional veneers to be emitted.
> 
> So ignore the veneers when generating the symbol table. Veneers
> have no corresponding source code, and they will not turn up in
> backtraces anyway.
> 
> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> ---
>  scripts/kallsyms.c | 17 ++++++++++++++---
>  1 file changed, 14 insertions(+), 3 deletions(-)
> 
> diff --git a/scripts/kallsyms.c b/scripts/kallsyms.c
> index c6d33bd15b04..6668c87d599f 100644
> --- a/scripts/kallsyms.c
> +++ b/scripts/kallsyms.c
> @@ -212,6 +212,12 @@ static int symbol_valid(struct sym_entry *s)
>  		"_SDA_BASE_",		/* ppc */
>  		"_SDA2_BASE_",		/* ppc */
>  		NULL };
> +
> +	static char *special_suffixes[] = {
> +		"_compiled.",		/* gcc < 3.0: "gcc[0-9]_compiled." */

Your newly added comment suggests that this can go away, since we have
been requiring at least gcc 3.2 since some time. But it can be removed
in a follow-up cleanup.


> +		"_veneer",		/* arm */
> +		NULL };
> +
>  	int i;
>  	int offset = 1;
>  
> @@ -244,13 +250,18 @@ static int symbol_valid(struct sym_entry *s)
>  	}
>  
>  	/* Exclude symbols which vary between passes. */
> -	if (strstr((char *)s->sym + offset, "_compiled."))
> -		return 0;
> -
>  	for (i = 0; special_symbols[i]; i++)
>  		if( strcmp((char *)s->sym + offset, special_symbols[i]) == 0 )
>  			return 0;
>  
> +	for (i = 0; special_suffixes[i]; i++) {
> +		char *sym_name = (char *)s->sym + offset;

You should declare and set this useful helper a few lines earlier and
use it in the other statements that test the symbol name.

Michal
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