2022-07-11 17:31:42

by Aaron Tomlin

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

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 <[email protected]>
Fixes: 08126db5ff73 ("module: kallsyms: Fix suspicious rcu usage")
Signed-off-by: Aaron Tomlin <[email protected]>
---
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

2022-07-11 17:48:55

by Luis Chamberlain

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

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 <[email protected]>
> Fixes: 08126db5ff73 ("module: kallsyms: Fix suspicious rcu usage")
> Signed-off-by: Aaron Tomlin <[email protected]>

Thanks! Queued up!

Luis