[PATCH v2, variant A] tty: nozomi: avoid sprintf buffer overflow

[PATCH v2, variant A] tty: nozomi: avoid sprintf buffer overflow

[Arnd Bergmann]

Testing with a gcc-7 snapshot produced an internal compiler error
for this file:

drivers/tty/nozomi.c: In function 'receive_flow_control':
drivers/tty/nozomi.c:919:12: internal compiler error: in get_substring_ranges_for_loc, at input.c:1388
 static int receive_flow_control(struct nozomi *dc)

I've reported this at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78569
but also noticed that the code line contains a stack overflow, as it prints
a string into a slightly shorter fixed-length 'tmp' variable.

I don't see any point in the temporary variable, we can simply use
pr_debug() to print the output directly. This change should not change
any of the output but avoids both the stack overflow and the gcc
crash.

The stack overflow will not happen unless a module load parameter is
also set to enable the debug messages.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
I'm posting two versions of the same fix, please choose one or the other
as you see fit.
---
 drivers/tty/nozomi.c | 12 +++---------
 1 file changed, 3 insertions(+), 9 deletions(-)

diff --git a/drivers/tty/nozomi.c b/drivers/tty/nozomi.c
index e2020a691058..b17dbfd4f313 100644
--- a/drivers/tty/nozomi.c
+++ b/drivers/tty/nozomi.c
@@ -68,14 +68,8 @@
 /* Default debug printout level */
 #define NOZOMI_DEBUG_LEVEL 0x00
 
-#define P_BUF_SIZE 128
-#define NFO(_err_flag_, args...)				\
-do {								\
-	char tmp[P_BUF_SIZE];					\
-	snprintf(tmp, sizeof(tmp), ##args);			\
-	printk(_err_flag_ "[%d] %s(): %s\n", __LINE__,		\
-		__func__, tmp);				\
-} while (0)
+#define NFO(fmt, args...)					\
+	pr_debug("[%d] %s(): " fmt "\n", __LINE__, __func__,  ##args);
 
 #define DBG1(args...) D_(0x01, ##args)
 #define DBG2(args...) D_(0x02, ##args)
@@ -91,7 +85,7 @@ do {								\
 static int debug = NOZOMI_DEBUG_LEVEL;
 
 #define D(lvl, args...)  do \
-			{if (lvl & debug) NFO(KERN_DEBUG, ##args); } \
+			{if (lvl & debug) NFO(args); } \
 			while (0)
 #define D_(lvl, args...) D(lvl, ##args)
 
-- 
2.9.0

[PATCH v2, variant B] tty: nozomi: avoid sprintf buffer overflow

[Arnd Bergmann]

Testing with a gcc-7 snapshot produced an internal compiler error
for this file:

drivers/tty/nozomi.c: In function 'receive_flow_control':
drivers/tty/nozomi.c:919:12: internal compiler error: in get_substring_ranges_for_loc, at input.c:1388
 static int receive_flow_control(struct nozomi *dc)

I've reported this at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78569
but also noticed that the code line contains a stack overflow, as it prints
a string into a slightly shorter fixed-length 'tmp' variable.

A lot of the code here is unnecessary and can be expressed in a simpler
way, relying on the fact that removing the 'DEBUG' macro will also get
rid of all pr_debug() calls. This change should not change any of the
output but avoids both the stack overflow and the gcc crash.

The stack overflow will not happen unless a module load parameter is
also set to enable the debug messages.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
---
 drivers/tty/nozomi.c | 47 ++++++++++++-----------------------------------
 1 file changed, 12 insertions(+), 35 deletions(-)

diff --git a/drivers/tty/nozomi.c b/drivers/tty/nozomi.c
index e2020a691058..39b3723a32a6 100644
--- a/drivers/tty/nozomi.c
+++ b/drivers/tty/nozomi.c
@@ -63,44 +63,23 @@
 
 #define VERSION_STRING DRIVER_DESC " 2.1d"
 
-/*    Macros definitions */
-
 /* Default debug printout level */
 #define NOZOMI_DEBUG_LEVEL 0x00
-
-#define P_BUF_SIZE 128
-#define NFO(_err_flag_, args...)				\
-do {								\
-	char tmp[P_BUF_SIZE];					\
-	snprintf(tmp, sizeof(tmp), ##args);			\
-	printk(_err_flag_ "[%d] %s(): %s\n", __LINE__,		\
-		__func__, tmp);				\
-} while (0)
-
-#define DBG1(args...) D_(0x01, ##args)
-#define DBG2(args...) D_(0x02, ##args)
-#define DBG3(args...) D_(0x04, ##args)
-#define DBG4(args...) D_(0x08, ##args)
-#define DBG5(args...) D_(0x10, ##args)
-#define DBG6(args...) D_(0x20, ##args)
-#define DBG7(args...) D_(0x40, ##args)
-#define DBG8(args...) D_(0x80, ##args)
-
-#ifdef DEBUG
-/* Do we need this settable at runtime? */
 static int debug = NOZOMI_DEBUG_LEVEL;
+module_param(debug, int, S_IRUGO | S_IWUSR);
 
-#define D(lvl, args...)  do \
-			{if (lvl & debug) NFO(KERN_DEBUG, ##args); } \
-			while (0)
-#define D_(lvl, args...) D(lvl, ##args)
-
-/* These printouts are always printed */
+/*    Macros definitions */
+#define DBG_(lvl, fmt, args...)				\
+do {							\
+	if (lvl & debug)				\
+		pr_debug("[%d] %s(): " fmt "\n",	\
+			 __LINE__, __func__,  ##args);	\
+} while (0)
 
-#else
-static int debug;
-#define D_(lvl, args...)
-#endif
+#define DBG1(args...) DBG_(0x01, ##args)
+#define DBG2(args...) DBG_(0x02, ##args)
+#define DBG3(args...) DBG_(0x04, ##args)
+#define DBG4(args...) DBG_(0x08, ##args)
 
 /* TODO: rewrite to optimize macros... */
 
@@ -1943,7 +1922,5 @@ static __exit void nozomi_exit(void)
 module_init(nozomi_init);
 module_exit(nozomi_exit);
 
-module_param(debug, int, S_IRUGO | S_IWUSR);
-
 MODULE_LICENSE("Dual BSD/GPL");
 MODULE_DESCRIPTION(DRIVER_DESC);
-- 
2.9.0

Re: [PATCH v2, variant B] tty: nozomi: avoid sprintf buffer overflow

[Greg Kroah-Hartman]

On Tue, Nov 29, 2016 at 12:51:04PM +0100, Arnd Bergmann wrote:
> Testing with a gcc-7 snapshot produced an internal compiler error
> for this file:
> 
> drivers/tty/nozomi.c: In function 'receive_flow_control':
> drivers/tty/nozomi.c:919:12: internal compiler error: in get_substring_ranges_for_loc, at input.c:1388
>  static int receive_flow_control(struct nozomi *dc)
> 
> I've reported this at https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78569
> but also noticed that the code line contains a stack overflow, as it prints
> a string into a slightly shorter fixed-length 'tmp' variable.
> 
> A lot of the code here is unnecessary and can be expressed in a simpler
> way, relying on the fact that removing the 'DEBUG' macro will also get
> rid of all pr_debug() calls. This change should not change any of the
> output but avoids both the stack overflow and the gcc crash.

I like this, but I don't think this patch went far enough, we should get
rid of the "special" macro entirely and just use dev_dbg() everywhere as
the dynamic debugging interface is exactly for this type of thing (you
can turn them on or off per-line, no need for odd "levels".)

So I'll take this patch now to remove the obvious error, but follow-on
patches to fix it up "correctly" would be great.

thanks,

greg k-h