linux-kernel - Re: [PATCH v2] module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT

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Date:   Wed, 6 Jul 2022 10:58:45 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Aaron Tomlin <atomlin@...hat.com>
Cc:     rostedt@...dmis.org, cl@...ux.com, pmladek@...e.com,
        mbenes@...e.cz, christophe.leroy@...roup.eu,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-modules@...r.kernel.org, atomlin@...mlin.com,
        ghalat@...hat.com, oleksandr@...alenko.name, neelx@...hat.com,
        daniel.thompson@...aro.org, hch@...radead.org, tglx@...utronix.de,
        linux-rt-users@...r.kernel.org
Subject: Re: [PATCH v2] module: kallsyms: Ensure preemption in add_kallsyms()
 with PREEMPT_RT

Hey Aaron, thanks again!

On Mon, Jul 04, 2022 at 05:17:53PM +0100, Aaron Tomlin wrote:
> To disable preemption in the context of add_kallsyms() is incorrect.

Why, what broke? Did this used to work? Was the commit in question a
regression then? Clarifying all this will help a lot.

The commit log is better but yet doesn't make it easy for me to tell
if I should send this to Linus as a fix in the rc series.

  Luis

> Before kallsyms-specific data is prepared/or set-up, we ensure that
> the unformed module is known to be unique i.e. does not already exist
> (see load_module()). Therefore, we can fix this by using the common RCU
> primitive as this section of code can be safely preempted.
> 
> Reported-by: Steven Rostedt <rostedt@...dmis.org>
> Fixes: 08126db5ff73 ("module: kallsyms: Fix suspicious rcu usage")
> Signed-off-by: Aaron Tomlin <atomlin@...hat.com>
> ---
>  kernel/module/kallsyms.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
> index 3e11523bc6f6..0b6fd82d5898 100644
> --- a/kernel/module/kallsyms.c
> +++ b/kernel/module/kallsyms.c
> @@ -174,14 +174,14 @@ void add_kallsyms(struct module *mod, const struct load_info *info)
>  	mod->kallsyms = (void __rcu *)mod->init_layout.base +
>  		info->mod_kallsyms_init_off;
>  
> -	preempt_disable();
> +	rcu_read_lock();
>  	/* The following is safe since this pointer cannot change */
> -	rcu_dereference_sched(mod->kallsyms)->symtab = (void *)symsec->sh_addr;
> -	rcu_dereference_sched(mod->kallsyms)->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
> +	rcu_dereference(mod->kallsyms)->symtab = (void *)symsec->sh_addr;
> +	rcu_dereference(mod->kallsyms)->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
>  	/* Make sure we get permanent strtab: don't use info->strtab. */
> -	rcu_dereference_sched(mod->kallsyms)->strtab =
> +	rcu_dereference(mod->kallsyms)->strtab =
>  		(void *)info->sechdrs[info->index.str].sh_addr;
> -	rcu_dereference_sched(mod->kallsyms)->typetab = mod->init_layout.base + info->init_typeoffs;
> +	rcu_dereference(mod->kallsyms)->typetab = mod->init_layout.base + info->init_typeoffs;
>  
>  	/*
>  	 * Now populate the cut down core kallsyms for after init
> @@ -190,22 +190,22 @@ void add_kallsyms(struct module *mod, const struct load_info *info)
>  	mod->core_kallsyms.symtab = dst = mod->data_layout.base + info->symoffs;
>  	mod->core_kallsyms.strtab = s = mod->data_layout.base + info->stroffs;
>  	mod->core_kallsyms.typetab = mod->data_layout.base + info->core_typeoffs;
> -	src = rcu_dereference_sched(mod->kallsyms)->symtab;
> -	for (ndst = i = 0; i < rcu_dereference_sched(mod->kallsyms)->num_symtab; i++) {
> -		rcu_dereference_sched(mod->kallsyms)->typetab[i] = elf_type(src + i, info);
> +	src = rcu_dereference(mod->kallsyms)->symtab;
> +	for (ndst = i = 0; i < rcu_dereference(mod->kallsyms)->num_symtab; i++) {
> +		rcu_dereference(mod->kallsyms)->typetab[i] = elf_type(src + i, info);
>  		if (i == 0 || is_livepatch_module(mod) ||
>  		    is_core_symbol(src + i, info->sechdrs, info->hdr->e_shnum,
>  				   info->index.pcpu)) {
>  			mod->core_kallsyms.typetab[ndst] =
> -			    rcu_dereference_sched(mod->kallsyms)->typetab[i];
> +			    rcu_dereference(mod->kallsyms)->typetab[i];
>  			dst[ndst] = src[i];
>  			dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
> -			s += strscpy(s,
> -				     &rcu_dereference_sched(mod->kallsyms)->strtab[src[i].st_name],
> +			s += strlcpy(s,
> +				     &rcu_dereference(mod->kallsyms)->strtab[src[i].st_name],
>  				     KSYM_NAME_LEN) + 1;
>  		}
>  	}
> -	preempt_enable();
> +	rcu_read_unlock();
>  	mod->core_kallsyms.num_symtab = ndst;
>  }
>  
> -- 
> 2.34.3
>