RE: [PATCH 5/5] Tools: hv: kvp: Fix a warning of buffer overflow with gcc 8.0.1

Dexuan Cui
2018-10-17T19:57:10Z

> From: devel <driverdev-devel-bounces@linuxdriverproject.org> On Behalf Of
> Greg KH
> Sent: Tuesday, October 16, 2018 22:07
>
> On Wed, Oct 17, 2018 at 03:14:06AM +0000, kys@linuxonhyperv.com wrote:
> > From: Dexuan Cui <decui@microsoft.com>
> >
> > The patch fixes:
> >
> > hv_kvp_daemon.c: In function 'kvp_set_ip_info':
> > hv_kvp_daemon.c:1305:2: note: 'snprintf' output between 41 and 4136 bytes
> > into a destination of size 4096
> > ...
> > --- a/tools/hv/hv_kvp_daemon.c
> > +++ b/tools/hv/hv_kvp_daemon.c
> > @@ -1176,7 +1176,7 @@ static int kvp_set_ip_info(char *if_name, struct
> hv_kvp_ipaddr_value *new_val)
> > int error = 0;
> > char if_file[PATH_MAX];
> > FILE *file;
> > - char cmd[PATH_MAX];
> > + char cmd[PATH_MAX * 2];
>
> Overkill?

Well, I found the -Wformat-truncation warning can be suppressed by checking
the return value of snprintf, so I'm going to do a new patch for KY to resend like this:

@@ -1178,6 +1178,7 @@ static int kvp_set_ip_info(char *if_name, struct hv_kvp_ipaddr_value *new_val)
FILE *file;
char cmd[PATH_MAX];
char *mac_addr;
+ int str_len;

/*
* Set the configuration for the specified interface with
@@ -1301,8 +1302,19 @@ static int kvp_set_ip_info(char *if_name, struct hv_kvp_ipaddr_value *new_val)
* invoke the external script to do its magic.
*/

- snprintf(cmd, sizeof(cmd), KVP_SCRIPTS_PATH "%s %s",
- "hv_set_ifconfig", if_file);
+ str_len = snprintf(cmd, sizeof(cmd), KVP_SCRIPTS_PATH "%s %s",
+ "hv_set_ifconfig", if_file);
+
+ /*
+ * This is a little overcautious, but it's necessary to suppress some
+ * false warnings from gcc 8.0.1.
+ */
+ if (str_len <= 0 || (unsigned int)str_len >= sizeof(cmd)) {
+ syslog(LOG_ERR, "Cmd '%s' (len=%d) may be too long",
+ cmd, str_len);
+ return HV_E_FAIL;
+ }

Thanks,
-- Dexuan