linux-kernel - Re: [PATCH 1/2] module: Overwrite st_size instead of st

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Date:   Thu, 22 Nov 2018 12:01:54 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Vincent Whitchurch <vincent.whitchurch@...s.com>
Cc:     linux@...linux.org.uk, jeyu@...nel.org,
        Vincent Whitchurch <rabinv@...s.com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] module: Overwrite st_size instead of st_info

On Mon, Nov 19, 2018 at 05:25:12PM +0100, Vincent Whitchurch wrote:
> st_info is currently overwritten after relocation and used to store the
> elf_type().  However, we're going to need it fix kallsyms on ARM's
> Thumb-2 kernels, so preserve st_info and overwrite the st_size field
> instead.  st_size is neither used by the module core nor by any
> architecture.
> 
> Signed-off-by: Vincent Whitchurch <vincent.whitchurch@...s.com>
> ---
> v4: Split out to separate patch.  Use st_size instead of st_other.
> v1-v3: See PATCH 2/2
> 
>  kernel/module.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/module.c b/kernel/module.c
> index 49a405891587..3d86a38b580c 100644
> --- a/kernel/module.c
> +++ b/kernel/module.c
> @@ -2682,7 +2682,7 @@ static void add_kallsyms(struct module *mod, const struct load_info *info)
>  
>  	/* Set types up while we still have access to sections. */
>  	for (i = 0; i < mod->kallsyms->num_symtab; i++)
> -		mod->kallsyms->symtab[i].st_info
> +		mod->kallsyms->symtab[i].st_size
>  			= elf_type(&mod->kallsyms->symtab[i], info);
>  
>  	/* Now populate the cut down core kallsyms for after init. */
> @@ -4061,7 +4061,7 @@ int module_get_kallsym(unsigned int symnum, unsigned long *value, char *type,
>  		kallsyms = rcu_dereference_sched(mod->kallsyms);
>  		if (symnum < kallsyms->num_symtab) {
>  			*value = kallsyms->symtab[symnum].st_value;
> -			*type = kallsyms->symtab[symnum].st_info;
> +			*type = kallsyms->symtab[symnum].st_size;
>  			strlcpy(name, symname(kallsyms, symnum), KSYM_NAME_LEN);
>  			strlcpy(module_name, mod->name, MODULE_NAME_LEN);
>  			*exported = is_exported(name, *value, mod);

This is fine if st_size is really unused, but how sure are you of that?

grepping for st_size throws up some hits that appear ELF-related, but
I've not investigated them in detail.

(The fact that struct stat has an identically named field throws up
a load of false positives too.)

Cheers
---Dave