Re: [PATCH] eeepc-laptop: remove possible use of uninitialized value

Paul Bolle
2014-09-04T07:08:13Z

On Thu, 2014-09-04 at 00:53 +0200, Frans Klaver wrote:
> In store_sys_acpi, if count equals zero, or parse_arg()s sscanf call
> fails, 'value' remains possibly uninitialized. In that case 'value'
> shouldn't be used to produce the store_sys_acpi()s return value.
>
> Only test the return value of set_acpi() if we can actually call it.
> Return rv otherwise.
>
> Signed-off-by: Frans Klaver <fransklaver@gmail.com>
> ---
> drivers/platform/x86/eeepc-laptop.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/platform/x86/eeepc-laptop.c b/drivers/platform/x86/eeepc-laptop.c
> index bd533c2..41f12ba 100644
> --- a/drivers/platform/x86/eeepc-laptop.c
> +++ b/drivers/platform/x86/eeepc-laptop.c
> @@ -279,10 +279,10 @@ static ssize_t store_sys_acpi(struct device *dev, int cm,
> int rv, value;
>
> rv = parse_arg(buf, count, &value);
> - if (rv > 0)
> - value = set_acpi(eeepc, cm, value);
> - if (value < 0)
> - return -EIO;
> + if (rv > 0) {
> + if (set_acpi(eeepc, cm, value) < 0)
> + return -EIO;
> + }
> return rv;
> }
>

The warning that this code (currently) generated triggered me to submit
https://lkml.org/lkml/2014/7/1/150 , which uses a different approach to
get rid of it. I received no reactions so far. Here's that patch again:

------------>8------------
From: Paul Bolle <pebolle@tiscali.nl>
Subject: [PATCH] eeepc-laptop: simplify parse_arg()

parse_arg() has three possible return values:
-EINVAL if sscanf(), in short, fails;
zero if "count" is zero; and
"count" in all other cases

But "count" will never be zero. See, parse_arg() is called by the
various store functions. And the callchain of these functions starts
with sysfs_kf_write(). And that function checks for a zero "count". So
we can stop checking for a zero "count", drop the "count" argument
entirely, and transform parse_arg() into a function that returns zero on
success or a negative error. That, in turn, allows to make those store
functions just return "count" on success. The net effect is that the
code becomes a bit easier to understand.

A nice side effect is that this GCC warning is silenced too:
drivers/platform/x86/eeepc-laptop.c: In function ‘store_sys_acpi’:
drivers/platform/x86/eeepc-laptop.c:279:10: warning: ‘value’ may be used uninitialized in this function [-Wmaybe-uninitialized]
int rv, value;

Which is, of course, the reason to have a look at parse_arg().

Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
---
drivers/platform/x86/eeepc-laptop.c | 34 +++++++++++++++++-----------------
1 file changed, 17 insertions(+), 17 deletions(-)

diff --git a/drivers/platform/x86/eeepc-laptop.c b/drivers/platform/x86/eeepc-laptop.c
index bd533c22be57..78515b850165 100644
--- a/drivers/platform/x86/eeepc-laptop.c
+++ b/drivers/platform/x86/eeepc-laptop.c
@@ -263,13 +263,11 @@ static int acpi_setter_handle(struct eeepc_laptop *eeepc, int cm,
/*
* Sys helpers
*/
-static int parse_arg(const char *buf, unsigned long count, int *val)
+static int parse_arg(const char *buf, int *val)
{
- if (!count)
- return 0;
if (sscanf(buf, "%i", val) != 1)
return -EINVAL;
- return count;
+ return 0;
}

static ssize_t store_sys_acpi(struct device *dev, int cm,
@@ -278,12 +276,13 @@ static ssize_t store_sys_acpi(struct device *dev, int cm,
struct eeepc_laptop *eeepc = dev_get_drvdata(dev);
int rv, value;

- rv = parse_arg(buf, count, &value);
- if (rv > 0)
- value = set_acpi(eeepc, cm, value);
+ rv = parse_arg(buf, &value);
+ if (rv < 0)
+ return rv;
+ value = set_acpi(eeepc, cm, value);
if (value < 0)
return -EIO;
- return rv;
+ return count;
}

static ssize_t show_sys_acpi(struct device *dev, int cm, char *buf)
@@ -377,13 +376,13 @@ static ssize_t store_cpufv(struct device *dev,
return -EPERM;
if (get_cpufv(eeepc, &c))
return -ENODEV;
- rv = parse_arg(buf, count, &value);
+ rv = parse_arg(buf, &value);
if (rv < 0)
return rv;
- if (!rv || value < 0 || value >= c.num)
+ if (value < 0 || value >= c.num)
return -EINVAL;
set_acpi(eeepc, CM_ASL_CPUFV, value);
- return rv;
+ return count;
}

static ssize_t show_cpufv_disabled(struct device *dev,
@@ -402,7 +401,7 @@ static ssize_t store_cpufv_disabled(struct device *dev,
struct eeepc_laptop *eeepc = dev_get_drvdata(dev);
int rv, value;

- rv = parse_arg(buf, count, &value);
+ rv = parse_arg(buf, &value);
if (rv < 0)
return rv;

@@ -412,7 +411,7 @@ static ssize_t store_cpufv_disabled(struct device *dev,
pr_warn("cpufv enabled (not officially supported "
"on this model)\n");
eeepc->cpufv_disabled = false;
- return rv;
+ return count;
case 1:
return -EPERM;
default:
@@ -1042,10 +1041,11 @@ static ssize_t store_sys_hwmon(void (*set)(int), const char *buf, size_t count)
{
int rv, value;

- rv = parse_arg(buf, count, &value);
- if (rv > 0)
- set(value);
- return rv;
+ rv = parse_arg(buf, &value);
+ if (rv < 0)
+ return rv;
+ set(value);
+ return count;
}

static ssize_t show_sys_hwmon(int (*get)(void), char *buf)
--

Paul Bolle