Subject: [PATCH 03/28] module: Use proper RCU assignment in add_kallsyms().
From: Sebastian Andrzej Siewior <[email-masked]>
Date: Wed, 8 Jan 2025 10:04:32 +0100

add_kallsyms() assigns the RCU pointer module::kallsyms and setups the
structures behind it which point to init-data. The module was not
published yet, nothing can see the kallsyms pointer and the data behind
it. Also module's init function was not yet invoked.
There is no need to use rcu_dereference() here, it is just to keep
checkers quiet. The whole RCU read section is also not needed.

Use a local kallsyms pointer and setup the data structures. Assign that
pointer to the data structure at the end via rcu_assign_pointer().

Acked-by: Peter Zijlstra (Intel) <[email-masked]>
Link: https://lore.kernel.org/r/[email-masked]
Signed-off-by: Sebastian Andrzej Siewior <[email-masked]>
---
 kernel/module/kallsyms.c |   31 ++++++++++++++-----------------
 1 file changed, 14 insertions(+), 17 deletions(-)

--- a/kernel/module/kallsyms.c
+++ b/kernel/module/kallsyms.c
@@ -177,19 +177,15 @@ void add_kallsyms(struct module *mod, co
 	unsigned long strtab_size;
 	void *data_base = mod->mem[MOD_DATA].base;
 	void *init_data_base = mod->mem[MOD_INIT_DATA].base;
+	struct mod_kallsyms *kallsyms;
 
-	/* Set up to point into init section. */
-	mod->kallsyms = (void __rcu *)init_data_base +
-		info->mod_kallsyms_init_off;
+	kallsyms = init_data_base + info->mod_kallsyms_init_off;
 
-	rcu_read_lock();
-	/* The following is safe since this pointer cannot change */
-	rcu_dereference(mod->kallsyms)->symtab = (void *)symsec->sh_addr;
-	rcu_dereference(mod->kallsyms)->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
+	kallsyms->symtab = (void *)symsec->sh_addr;
+	kallsyms->num_symtab = symsec->sh_size / sizeof(Elf_Sym);
 	/* Make sure we get permanent strtab: don't use info->strtab. */
-	rcu_dereference(mod->kallsyms)->strtab =
-		(void *)info->sechdrs[info->index.str].sh_addr;
-	rcu_dereference(mod->kallsyms)->typetab = init_data_base + info->init_typeoffs;
+	kallsyms->strtab = (void *)info->sechdrs[info->index.str].sh_addr;
+	kallsyms->typetab = init_data_base + info->init_typeoffs;
 
 	/*
 	 * Now populate the cut down core kallsyms for after init
@@ -199,20 +195,19 @@ void add_kallsyms(struct module *mod, co
 	mod->core_kallsyms.strtab = s = data_base + info->stroffs;
 	mod->core_kallsyms.typetab = data_base + info->core_typeoffs;
 	strtab_size = info->core_typeoffs - info->stroffs;
-	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);
+	src = kallsyms->symtab;
+	for (ndst = i = 0; i < kallsyms->num_symtab; i++) {
+		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(mod->kallsyms)->typetab[i];
+				kallsyms->typetab[i];
 			dst[ndst] = src[i];
 			dst[ndst++].st_name = s - mod->core_kallsyms.strtab;
-			ret = strscpy(s,
-				      &rcu_dereference(mod->kallsyms)->strtab[src[i].st_name],
+			ret = strscpy(s, &kallsyms->strtab[src[i].st_name],
 				      strtab_size);
 			if (ret < 0)
 				break;
@@ -220,7 +215,9 @@ void add_kallsyms(struct module *mod, co
 			strtab_size -= ret + 1;
 		}
 	}
-	rcu_read_unlock();
+
+	/* Set up to point into init section. */
+	rcu_assign_pointer(mod->kallsyms, kallsyms);
 	mod->core_kallsyms.num_symtab = ndst;
 }