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

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Message-ID: <YVHE1qclD6ZyjvvD@chrisdown.name>
Date:   Mon, 27 Sep 2021 14:19:18 +0100
From:   Chris Down <chris@...isdown.name>
To:     Arnd Bergmann <arnd@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Jessica Yu <jeyu@...nel.org>, Arnd Bergmann <arnd@...db.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        John Ogness <john.ogness@...utronix.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        YueHaibing <yuehaibing@...wei.com>, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev
Subject: Re: [PATCH] printk: avoid -Wsometimes-uninitialized warning

Hi Arnd,

Arnd Bergmann writes:
>From: Arnd Bergmann <arnd@...db.de>
>
>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@...db.de>
>---
> 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
>