diff mbox

[2/2] drivers/rtc/rtc-ds1685.c: don't try to micromanage sysfs output size

Message ID 1448373085-6353-2-git-send-email-linux@rasmusvillemoes.dk
State Accepted
Headers show

Commit Message

Rasmus Villemoes Nov. 24, 2015, 1:51 p.m. UTC 
...and don't do it wrong.

"not ok or N/A" has length 13. Add the trailing newline, and the
snprintf return value will be 14. However, we lied to snprintf and
told it that only 13 bytes were available. Hence snprintf has only
written "not ok or N/" and a trailing '\0' to the buffer. Next we
continue lying, this time to the upper sysfs layer, claiming that we
wrote 14 meaningful bytes to the buffer. That'll make the upper layer
copy "not ok or N/" plus two nul bytes to user space (one nul byte
from snprintf, the other since sysfs takes care to clear the buffer
before giving it to the ->show method).

In the other cases, the claimed buffer size is closer to sufficient,
but we'll still get a nul byte instead of a newline written to user
space.  There's absolutely no reason to try to predict the output
size, and there's plenty of room in the buffer, so just use sprintf.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 drivers/rtc/rtc-ds1685.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

Comments

Alexandre Belloni Nov. 30, 2015, 7:06 p.m. UTC | #1 
On 24/11/2015 at 14:51:24 +0100, Rasmus Villemoes wrote :
> ...and don't do it wrong.
> 
> "not ok or N/A" has length 13. Add the trailing newline, and the
> snprintf return value will be 14. However, we lied to snprintf and
> told it that only 13 bytes were available. Hence snprintf has only
> written "not ok or N/" and a trailing '\0' to the buffer. Next we
> continue lying, this time to the upper sysfs layer, claiming that we
> wrote 14 meaningful bytes to the buffer. That'll make the upper layer
> copy "not ok or N/" plus two nul bytes to user space (one nul byte
> from snprintf, the other since sysfs takes care to clear the buffer
> before giving it to the ->show method).
> 
> In the other cases, the claimed buffer size is closer to sufficient,
> but we'll still get a nul byte instead of a newline written to user
> space.  There's absolutely no reason to try to predict the output
> size, and there's plenty of room in the buffer, so just use sprintf.
> 
> Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
> ---
>  drivers/rtc/rtc-ds1685.c | 12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
Applied, thanks.
diff mbox

Patch

diff --git a/drivers/rtc/rtc-ds1685.c b/drivers/rtc/rtc-ds1685.c
index 5038122aa8de..535050fc5e9f 100644
--- a/drivers/rtc/rtc-ds1685.c
+++ b/drivers/rtc/rtc-ds1685.c
@@ -1114,7 +1114,7 @@  ds1685_rtc_sysfs_battery_show(struct device *dev,
 
 	ctrld = rtc->read(rtc, RTC_CTRL_D);
 
-	return snprintf(buf, 13, "%s\n",
+	return sprintf(buf, "%s\n",
 			(ctrld & RTC_CTRL_D_VRT) ? "ok" : "not ok or N/A");
 }
 static DEVICE_ATTR(battery, S_IRUGO, ds1685_rtc_sysfs_battery_show, NULL);
@@ -1137,7 +1137,7 @@  ds1685_rtc_sysfs_auxbatt_show(struct device *dev,
 	ctrl4a = rtc->read(rtc, RTC_EXT_CTRL_4A);
 	ds1685_rtc_switch_to_bank0(rtc);
 
-	return snprintf(buf, 13, "%s\n",
+	return sprintf(buf, "%s\n",
 			(ctrl4a & RTC_CTRL_4A_VRT2) ? "ok" : "not ok or N/A");
 }
 static DEVICE_ATTR(auxbatt, S_IRUGO, ds1685_rtc_sysfs_auxbatt_show, NULL);
@@ -1160,9 +1160,7 @@  ds1685_rtc_sysfs_serial_show(struct device *dev,
 	ds1685_rtc_get_ssn(rtc, ssn);
 	ds1685_rtc_switch_to_bank0(rtc);
 
-	return snprintf(buf, 24, "%8phC\n", ssn);
-
-	return 0;
+	return sprintf(buf, "%8phC\n", ssn);
 }
 static DEVICE_ATTR(serial, S_IRUGO, ds1685_rtc_sysfs_serial_show, NULL);
 
@@ -1285,7 +1283,7 @@  ds1685_rtc_sysfs_ctrl_regs_show(struct device *dev,
 	tmp = rtc->read(rtc, reg_info->reg) & reg_info->bit;
 	ds1685_rtc_switch_to_bank0(rtc);
 
-	return snprintf(buf, 2, "%d\n", (tmp ? 1 : 0));
+	return sprintf(buf, "%d\n", (tmp ? 1 : 0));
 }
 
 /**
@@ -1621,7 +1619,7 @@  ds1685_rtc_sysfs_time_regs_show(struct device *dev,
 	tmp = ds1685_rtc_bcd2bin(rtc, tmp, bcd_reg_info->mask,
 				 bin_reg_info->mask);
 
-	return snprintf(buf, 4, "%d\n", tmp);
+	return sprintf(buf, "%d\n", tmp);
 }
 
 /**