linux-kernel - [PATCH v9 10/14] module: kallsyms: Fix suspicious rcu usage

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Date:   Mon, 28 Feb 2022 23:43:18 +0000
From:   Aaron Tomlin <atomlin@...hat.com>
To:     mcgrof@...nel.org, christophe.leroy@...roup.eu, pmladek@...e.com
Cc:     cl@...ux.com, mbenes@...e.cz, akpm@...ux-foundation.org,
        jeyu@...nel.org, linux-kernel@...r.kernel.org,
        linux-modules@...r.kernel.org, void@...ifault.com,
        atomlin@...mlin.com, allen.lkml@...il.com, joe@...ches.com,
        msuchanek@...e.de, oleksandr@...alenko.name,
        jason.wessel@...driver.com, daniel.thompson@...aro.org
Subject: [PATCH v9 10/14] module: kallsyms: Fix suspicious rcu usage

No functional change.

The purpose of this patch is to address the various Sparse warnings
due to the incorrect dereference/or access of an __rcu pointer.

Signed-off-by: Aaron Tomlin <atomlin@...hat.com>
---
 kernel/module/kallsyms.c | 34 ++++++++++++++++++++++------------
 1 file changed, 22 insertions(+), 12 deletions(-)

diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
index 1b0780e20aab..a3da0686a2a6 100644
--- a/kernel/module/kallsyms.c
+++ b/kernel/module/kallsyms.c
@@ -171,14 +171,17 @@ void add_kallsyms(struct module *mod, const struct load_info *info)
 	Elf_Shdr *symsec = &info->sechdrs[info->index.sym];
 
 	/* Set up to point into init section. */
-	mod->kallsyms = mod->init_layout.base + info->mod_kallsyms_init_off;
+	mod->kallsyms = (void __rcu *)mod->init_layout.base +
+		info->mod_kallsyms_init_off;
 
+	preempt_disable();
 	/* The following is safe since this pointer cannot change */
-	mod->kallsyms->symtab = (void *)symsec->sh_addr;
-	mod->kallsyms->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
+	rcu_dereference_sched(mod->kallsyms)->symtab = (void *)symsec->sh_addr;
+	rcu_dereference_sched(mod->kallsyms)->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
 	/* Make sure we get permanent strtab: don't use info->strtab. */
-	mod->kallsyms->strtab = (void *)info->sechdrs[info->index.str].sh_addr;
-	mod->kallsyms->typetab = mod->init_layout.base + info->init_typeoffs;
+	rcu_dereference_sched(mod->kallsyms)->strtab =
+		(void *)info->sechdrs[info->index.str].sh_addr;
+	rcu_dereference_sched(mod->kallsyms)->typetab = mod->init_layout.base + info->init_typeoffs;
 
 	/*
 	 * Now populate the cut down core kallsyms for after init
@@ -187,20 +190,22 @@ void add_kallsyms(struct module *mod, const struct load_info *info)
 	mod->core_kallsyms.symtab = dst = mod->core_layout.base + info->symoffs;
 	mod->core_kallsyms.strtab = s = mod->core_layout.base + info->stroffs;
 	mod->core_kallsyms.typetab = mod->core_layout.base + info->core_typeoffs;
-	src = mod->kallsyms->symtab;
-	for (ndst = i = 0; i < mod->kallsyms->num_symtab; i++) {
-		mod->kallsyms->typetab[i] = elf_type(src + i, info);
+	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);
 		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] =
-			    mod->kallsyms->typetab[i];
+			    rcu_dereference_sched(mod->kallsyms)->typetab[i];
 			dst[ndst] = src[i];
 			dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
-			s += strscpy(s, &mod->kallsyms->strtab[src[i].st_name],
+			s += strscpy(s,
+				     &rcu_dereference_sched(mod->kallsyms)->strtab[src[i].st_name],
 				     KSYM_NAME_LEN) + 1;
 		}
 	}
+	preempt_enable();
 	mod->core_kallsyms.num_symtab = ndst;
 }
 
@@ -478,11 +483,16 @@ int module_kallsyms_on_each_symbol(int (*fn)(void *, const char *,
 
 	mutex_lock(&module_mutex);
 	list_for_each_entry(mod, &modules, list) {
-		/* We hold module_mutex: no need for rcu_dereference_sched */
-		struct mod_kallsyms *kallsyms = mod->kallsyms;
+		struct mod_kallsyms *kallsyms;
 
 		if (mod->state == MODULE_STATE_UNFORMED)
 			continue;
+
+		/* Use rcu_dereference_sched() to remain compliant with the sparse tool */
+		preempt_disable();
+		kallsyms = rcu_dereference_sched(mod->kallsyms);
+		preempt_enable();
+
 		for (i = 0; i < kallsyms->num_symtab; i++) {
 			const Elf_Sym *sym = &kallsyms->symtab[i];
 
-- 
2.34.1