[PATCH] module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT_RT

[PATCH] module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT_RT

[Aaron Tomlin]

Looking at add_kallsyms(), there is absolutely no need to explicitly
disable preemption. 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 use the common RCU primitive
as this section of code can be pre-empted. In other words, it is safe to be
re-scheduled, in this context.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Fixes: 08126db5ff73 ("module: kallsyms: Fix suspicious rcu usage")
Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
---
 kernel/module/kallsyms.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
index 3e11523bc6f6..b16ff36b4b71 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],
+				     &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

Re: [PATCH] module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT_RT

[Aaron Tomlin]

On Mon 2022-07-04 07:55 -0700, Luis Chamberlain wrote:
> Aaron, thanks for the patch! Your commit log however reads like an
> optimization but it's a fix that needs to go to Linus, as such, can you
> update it to explain clearly *why* it's a fix.

Hi Luis,

Sure - will do.


Kind regards,

-- 
Aaron Tomlin