Subject: Re: [PATCH iproute2] utils: don't match empty strings as prefixes

From: Stephen Hemminger <[email-masked]>
Date: Tue, 9 Jul 2019 14:37:58 -0700

On Tue,  9 Jul 2019 22:40:40 +0200
Matteo Croce <[email-masked]> wrote:

> iproute has an utility function which checks if a string is a prefix for
> another one, to allow use of abbreviated commands, e.g. 'addr' or 'a'
> instead of 'address'.
> 
> This routine unfortunately considers an empty string as prefix
> of any pattern, leading to undefined behaviour when an empty
> argument is passed to ip:
> 
>     # ip ''
>     1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
>         link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
>         inet 127.0.0.1/8 scope host lo
>            valid_lft forever preferred_lft forever
>         inet6 ::1/128 scope host
>            valid_lft forever preferred_lft forever
> 
>     # tc ''
>     qdisc noqueue 0: dev lo root refcnt 2
> 
>     # ip address add 192.0.2.0/24 '' 198.51.100.1 dev dummy0
>     # ip addr show dev dummy0
>     6: dummy0: <BROADCAST,NOARP> mtu 1500 qdisc noop state DOWN group default qlen 1000
>         link/ether 02:9d:5e:e9:3f:c0 brd ff:ff:ff:ff:ff:ff
>         inet 192.0.2.0/24 brd 198.51.100.1 scope global dummy0
>            valid_lft forever preferred_lft forever
> 
> Rewrite matches() so it takes care of an empty input, and doesn't
> scan the input strings three times: the actual implementation
> does 2 strlen and a memcpy to accomplish the same task.
> 
> Signed-off-by: Matteo Croce <[email-masked]>
> ---
>  include/utils.h |  2 +-
>  lib/utils.c     | 14 +++++++++-----
>  2 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/include/utils.h b/include/utils.h
> index 927fdc17..f4d12abb 100644
> --- a/include/utils.h
> +++ b/include/utils.h
> @@ -198,7 +198,7 @@ int nodev(const char *dev);
>  int check_ifname(const char *);
>  int get_ifname(char *, const char *);
>  const char *get_ifname_rta(int ifindex, const struct rtattr *rta);
> -int matches(const char *arg, const char *pattern);
> +int matches(const char *prefix, const char *string);
>  int inet_addr_match(const inet_prefix *a, const inet_prefix *b, int bits);
>  int inet_addr_match_rta(const inet_prefix *m, const struct rtattr *rta);
>  
> diff --git a/lib/utils.c b/lib/utils.c
> index be0f11b0..73ce19bb 100644
> --- a/lib/utils.c
> +++ b/lib/utils.c
> @@ -887,13 +887,17 @@ const char *get_ifname_rta(int ifindex, const struct rtattr *rta)
>  	return name;
>  }
>  
> -int matches(const char *cmd, const char *pattern)
> +/* Check if 'prefix' is a non empty prefix of 'string' */
> +int matches(const char *prefix, const char *string)
>  {
> -	int len = strlen(cmd);
> +	if (!*prefix)
> +		return 1;
> +	while(*string && *prefix == *string) {
> +		prefix++;
> +		string++;
> +	}
>  
> -	if (len > strlen(pattern))
> -		return -1;
> -	return memcmp(pattern, cmd, len);
> +	return *prefix;
>  }
>  
>  int inet_addr_match(const inet_prefix *a, const inet_prefix *b, int bits)

ERROR: space required before the open parenthesis '('
#134: FILE: lib/utils.c:895:
+	while(*string && *prefix == *string) {

total: 1 errors, 1 warnings, 30 lines checked

The empty prefix string is a bug and should not be allowed.
Also return value should be same as old code (yours isn't).