linux-kernel - [PATCH] [v2] printk: avoid -Wsometimes-uninitialized warning

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Date:   Tue, 28 Sep 2021 11:34:33 +0200
From:   Arnd Bergmann <arnd@...nel.org>
To:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Chris Down <chris@...isdown.name>,
        John Ogness <john.ogness@...utronix.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        YueHaibing <yuehaibing@...wei.com>, Jessica Yu <jeyu@...nel.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        linux-kernel@...r.kernel.org, llvm@...ts.linux.dev
Subject: [PATCH] [v2] printk: avoid -Wsometimes-uninitialized warning

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:

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.

Fixes: 337015573718 ("printk: Userspace format indexing support")
Suggested-by: Steven Rostedt <rostedt@...dmis.org>
Signed-off-by: Arnd Bergmann <arnd@...db.de>
--
v2: use a simpler trick of having an 'else' in the #ifdef
    block, as Steven suggested.
---
 kernel/printk/index.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/kernel/printk/index.c b/kernel/printk/index.c
index d3709408debe..43b45a916ff6 100644
--- a/kernel/printk/index.c
+++ b/kernel/printk/index.c
@@ -26,10 +26,9 @@ static struct pi_entry *pi_get_entry(const struct module *mod, loff_t pos)
 	if (mod) {
 		entries = mod->printk_index_start;
 		nr_entries = mod->printk_index_size;
-	}
+	} else 
 #endif
-
-	if (!mod) {
+	{
 		/* vmlinux, comes from linker symbols */
 		entries = __start_printk_index;
 		nr_entries = __stop_printk_index - __start_printk_index;
-- 
2.29.2