linux-kernel - [PATCH] ppdev: Distribute switch variables for initialization

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Message-Id: <20200220062311.69121-1-keescook@chromium.org>
Date:   Wed, 19 Feb 2020 22:23:11 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sudip Mukherjee <sudipm.mukherjee@...il.com>,
        Sudip Mukherjee <sudip.mukherjee@...ethink.co.uk>
Cc:     Alexander Potapenko <glider@...gle.com>,
        Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org
Subject: [PATCH] ppdev: Distribute switch variables for initialization

Variables declared in a switch statement before any case statements
cannot be automatically initialized with compiler instrumentation (as
they are not part of any execution flow). With GCC's proposed automatic
stack variable initialization feature, this triggers a warning (and they
don't get initialized). Clang's automatic stack variable initialization
(via CONFIG_INIT_STACK_ALL=y) doesn't throw a warning, but it also
doesn't initialize such variables[1]. Note that these warnings (or silent
skipping) happen before the dead-store elimination optimization phase,
so even when the automatic initializations are later elided in favor of
direct initializations, the warnings remain.

To avoid these problems, move such variables into the "case" where
they're used or lift them up into the main function body.

drivers/char/ppdev.c: In function ‘pp_do_ioctl’:
drivers/char/ppdev.c:516:25: warning: statement will never be executed [-Wswitch-unreachable]
  516 |   struct ieee1284_info *info;
      |                         ^~~~

[1] https://bugs.llvm.org/show_bug.cgi?id=44916

Signed-off-by: Kees Cook <keescook@...omium.org>
---
 drivers/char/ppdev.c |   20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/drivers/char/ppdev.c b/drivers/char/ppdev.c
index 2c2381a806ae..38b46c7d1737 100644
--- a/drivers/char/ppdev.c
+++ b/drivers/char/ppdev.c
@@ -355,14 +355,19 @@ static int pp_do_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 	struct pp_struct *pp = file->private_data;
 	struct parport *port;
 	void __user *argp = (void __user *)arg;
+	struct ieee1284_info *info;
+	unsigned char reg;
+	unsigned char mask;
+	int mode;
+	s32 time32[2];
+	s64 time64[2];
+	struct timespec64 ts;
+	int ret;
 
 	/* First handle the cases that don't take arguments. */
 	switch (cmd) {
 	case PPCLAIM:
 	    {
-		struct ieee1284_info *info;
-		int ret;
-
 		if (pp->flags & PP_CLAIMED) {
 			dev_dbg(&pp->pdev->dev, "you've already got it!\n");
 			return -EINVAL;
@@ -513,15 +518,6 @@ static int pp_do_ioctl(struct file *file, unsigned int cmd, unsigned long arg)
 
 	port = pp->pdev->port;
 	switch (cmd) {
-		struct ieee1284_info *info;
-		unsigned char reg;
-		unsigned char mask;
-		int mode;
-		s32 time32[2];
-		s64 time64[2];
-		struct timespec64 ts;
-		int ret;
-
 	case PPRSTATUS:
 		reg = parport_read_status(port);
 		if (copy_to_user(argp, &reg, sizeof(reg)))