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

[Chris Down]

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 (IS_ENABLED(CONFIG_MODULES) && mod) {
> #ifdef CONFIG_MODULES
> -	if (mod) {
> 		entries = mod->printk_index_start;
> 		nr_entries = mod->printk_index_size;
> -	}
> #endif
> -
> -	if (!mod) {
> +	} else {
> 		/* vmlinux, comes from linker symbols */
> 		entries = __start_printk_index;
> 		nr_entries = __stop_printk_index - __start_printk_index;
> --
> 2.29.2