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 <rostedt@goodmis.org>
Fixes: 08126db5ff73 ("module: kallsyms: Fix suspicious rcu usage")
Signed-off-by: Aaron Tomlin <atomlin@redhat.com>
Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>

diff --git a/kernel/module/kallsyms.c b/kernel/module/kallsyms.c
index 18c23545b98450ca8efeeeb2428924b8b652a274..77e75bead5698c3ffe86d0a4a991543484ff2c69 100644 (file)
--- 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;
 }