Re: [PATCH] printk: avoid -Wsometimes-uninitialized warning

From: Chris Down
Date: Mon Sep 27 2021 - 09:19:23 EST

Hi Arnd,

Arnd Bergmann writes:
From: Arnd Bergmann <arnd@xxxxxxxx>

clang notices that the pi_get_entry() function would use
uninitialized data if it was called with a non-NULL module
pointer on a kernel that does not support modules:

On a !CONFIG_MODULES kernel, we _never_ pass a non-NULL module pointer. This isn't just convention: we don't even have `struct module` fully fleshed out, so it technically cannot be so.

kernel/printk/index.c:32:6: error: variable 'nr_entries' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
if (!mod) {
kernel/printk/index.c:38:13: note: uninitialized use occurs here
if (pos >= nr_entries)
kernel/printk/index.c:32:2: note: remove the 'if' if its condition is always true
if (!mod) {

Rework the condition to make it clear to the compiler that we are always
in the second case. Unfortunately the #ifdef is still required as the
definition of 'struct module' is hidden when modules are disabled.

Having IS_ENABLED and then an #ifdef seems to hurt code readability to me.

Fixes: 337015573718 ("printk: Userspace format indexing support")

Does this really fix anything, or just clang's ignorance? If the latter, clang needs to be smarter here: as far as I can see there are no occasions where there's even any opportunity for a non-NULL pointer to come in on a !CONFIG_MODULES kernel, since `struct module` isn't even complete.

Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
kernel/printk/index.c | 6 ++----
1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/index.c b/kernel/printk/index.c
index d3709408debe..b4d90bab6d4d 100644
--- a/kernel/printk/index.c
+++ b/kernel/printk/index.c
@@ -22,14 +22,12 @@ static struct pi_entry *pi_get_entry(const struct module *mod, loff_t pos)
struct pi_entry **entries;
unsigned int nr_entries;

- if (mod) {
entries = mod->printk_index_start;
nr_entries = mod->printk_index_size;
- }
- if (!mod) {
+ } else {
/* vmlinux, comes from linker symbols */
entries = __start_printk_index;
nr_entries = __stop_printk_index - __start_printk_index;