Hey Aaron, thanks again!
On Mon, Jul 04, 2022 at 05:17:53PM +0100, Aaron Tomlin wrote:
> To disable preemption in the context of add_kallsyms() is incorrect.
Why, what broke? Did this used to work? Was the commit in question a
regression then? Clarifying all this will help a lot.
The commit log is better but yet doesn't make it easy for me to tell
if I should send this to Linus as a fix in the rc series.
Luis
> 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 RCU
> primitive as this section of code can be safely preempted.
>
> Reported-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Fixes: 08126db5ff73 ("module: kallsyms: Fix suspicious rcu usage")
> Signed-off-by: Aaron Tomlin <atomlin@xxxxxxxxxx>
> ---
> kernel/module/kallsyms.c | 24 ++++++++++++------------
> 1 file changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
> index 3e11523bc6f6..0b6fd82d5898 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],
> + s += strlcpy(s,
> + &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
>
