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: Steven Rostedt <rostedt@goodmis.org>
Fixes: 08126db5 ("module: kallsyms: Fix suspicious rcu usage")
Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

kernel/module/kallsyms.c

@@ -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;
 }