RE: [PATCH V2 5/5] Tools: hv: kvp: Fix a warning of buffer overflow with gcc 8.0.1

[David Laight]

From: Dan Carpenter
> Sent: 18 October 2018 07:33
> 
> On Thu, Oct 18, 2018 at 05:09:32AM +, k...@linuxonhyperv.com wrote:
> > From: Dexuan Cui 
> >
> > 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
> >
> > The "(unsigned int)str_len" is to avoid:
> >
> > hv_kvp_daemon.c:1309:30: warning: comparison of integer expressions of
> > different signedness: 'int' and 'long unsigned int' [-Wsign-compare]

I usually use 'str_len + 0u' rather than a cast.

> Ugh...  Any tool with the most basic flow analysis would realize this
> was a false positive.  We use at least three static analyzers which
> catch signedness bugs.  Can we turn off GCC's warning on this until they
> improve it a bit?

Yes, would be nice if it attempted to follow the valid domain of variables.

I recently had to change:
unsigned char a, b;
unsigned int c;
...
if (a + b < c)
To stop a 'signedness' warning.

David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, 
UK
Registration No: 1397386 (Wales)

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Re: [PATCH V2 5/5] Tools: hv: kvp: Fix a warning of buffer overflow with gcc 8.0.1

[Dan Carpenter]

On Thu, Oct 18, 2018 at 05:09:32AM +, k...@linuxonhyperv.com wrote:
> From: Dexuan Cui 
> 
> 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
> 
> The "(unsigned int)str_len" is to avoid:
> 
> hv_kvp_daemon.c:1309:30: warning: comparison of integer expressions of
> different signedness: 'int' and 'long unsigned int' [-Wsign-compare]
> 

Ugh...  Any tool with the most basic flow analysis would realize this
was a false positive.  We use at least three static analyzers which
catch signedness bugs.  Can we turn off GCC's warning on this until they
improve it a bit?

regards,
dan carpenter

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[PATCH V2 5/5] Tools: hv: kvp: Fix a warning of buffer overflow with gcc 8.0.1

[kys]

From: Dexuan Cui 

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

The "(unsigned int)str_len" is to avoid:

hv_kvp_daemon.c:1309:30: warning: comparison of integer expressions of
different signedness: 'int' and 'long unsigned int' [-Wsign-compare]

Signed-off-by: Dexuan Cui 
Cc: K. Y. Srinivasan 
Cc: Haiyang Zhang 
Cc: Stephen Hemminger 
Signed-off-by: K. Y. Srinivasan 
---
 tools/hv/hv_kvp_daemon.c | 15 +--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/tools/hv/hv_kvp_daemon.c b/tools/hv/hv_kvp_daemon.c
index bbb2a8ef367c..d7e06fe0270e 100644
--- a/tools/hv/hv_kvp_daemon.c
+++ b/tools/hv/hv_kvp_daemon.c
@@ -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 +1302,18 @@ 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;
+   }
+
if (system(cmd)) {
syslog(LOG_ERR, "Failed to execute cmd '%s'; error: %d %s",
cmd, errno, strerror(errno));
-- 
2.18.0

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