[PATCH v3 -next 0/1] module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT_RT

[PATCH v3 -next 0/1] module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT_RT

[Aaron Tomlin]

Hi Luis,

Please note this was rebased on mcgrof/modules-linus, to date i.e. that
includes Adrian's commit 35adf9a4e55e ("modules: Fix corruption of
/proc/kallsyms").

Aaron Tomlin (1):
  module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT_RT

 kernel/module/kallsyms.c | 22 +++++++++++-----------
 1 file changed, 11 insertions(+), 11 deletions(-)


base-commit: 35adf9a4e55e0b0a9d5e313e65ad83681dc32e9a
prerequisite-patch-id: 523b361c5a0b563bf8066422e897960cb2a79412
prerequisite-patch-id: 480a05b3fd339ffdeae8790dc541a587bec71df1
prerequisite-patch-id: 25ef6425ba3c164e5afd5b9862042da3687e01ce

-- 
2.34.3

[PATCH v3 -next 1/1] module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT_RT

[Aaron Tomlin]

The commit 08126db5ff73 ("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: 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 18c23545b984..77e75bead569 100644
--- a/kernel/module/kallsyms.c
+++ b/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;
 }
 
-- 
2.34.3

Re: [PATCH v3 -next 1/1] module: kallsyms: Ensure preemption in add_kallsyms() with PREEMPT_RT

[Luis Chamberlain]

On Mon, Jul 11, 2022 at 06:17:19PM +0100, Aaron Tomlin wrote:
> The commit 08126db5ff73 ("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: 08126db5ff73 ("module: kallsyms: Fix suspicious rcu usage")
> Signed-off-by: Aaron Tomlin <atomlin@redhat.com>

Thanks! Queued up!

  Luis