Commit e69a6614 authored by Aaron Tomlin's avatar Aaron Tomlin Committed by Luis Chamberlain
Browse files

module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT_RT 



The commit 08126db5 ("module: kallsyms: Fix suspicious rcu usage")
under PREEMPT_RT=y, disabling preemption introduced an unbounded
latency since the loop is not fixed. This change caused a regression
since previously preemption was not disabled and we would dereference
RCU-protected pointers explicitly. That being said, these pointers
cannot change.

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 and
more appropriate RCU flavour as this section of code can be safely
preempted.

Reported-by: default avatarSteven Rostedt <rostedt@goodmis.org>
Fixes: 08126db5 ("module: kallsyms: Fix suspicious rcu usage")
Signed-off-by: default avatarAaron Tomlin <atomlin@redhat.com>
Signed-off-by: default avatarLuis Chamberlain <mcgrof@kernel.org>
parent 2cc39179

kernel/module/kallsyms.c

+11 −11
Original line number Diff line number Diff line
@@ -176,14 +176,14 @@ 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
@@ -193,20 +193,20 @@ void add_kallsyms(struct module *mod, const struct load_info *info) 
	mod->core_kallsyms.strtab = s = mod->data_layout.base + info->stroffs;

	mod->core_kallsyms.typetab = mod->data_layout.base + info->core_typeoffs;

	strtab_size = info->core_typeoffs - info->stroffs;

	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)) {

			ssize_t ret;



			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;

			ret = strscpy(s,

				      &rcu_dereference_sched(mod->kallsyms)->strtab[src[i].st_name],

				      &rcu_dereference(mod->kallsyms)->strtab[src[i].st_name],

				      strtab_size);

			if (ret < 0)

				break;
@@ -214,7 +214,7 @@ void add_kallsyms(struct module *mod, const struct load_info *info) 
			strtab_size -= ret + 1;

		}

	}

	preempt_enable();

	rcu_read_unlock();

	mod->core_kallsyms.num_symtab = ndst;

}