On Thu, 10 Mar 2022 10:24:09 +0000
Aaron Tomlin <atomlin@xxxxxxxxxx> wrote:
> No functional change.
There most definitely is a functional change here.
>
> 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@xxxxxxxxxx>
> ---
> 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++) {
How big is this loop?
> + 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();
^^^^^^^^^^^^^^^^^
This just introduced an unbounded latency and will fail PREEMPT_RT.
-- Steve
> 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];
>
