[Drbd-dev] [PATCH] Correctly skip interface indexes larger than 256

[Drbd-dev] [PATCH] Correctly skip interface indexes larger than 256

[Dirk Tilger]

fscanf line probably copied from kernel printf, but printf specifies the
minimal field widwth and scanf the maximal field width.

printf (3) manual:

> Each conversion specification is introduced by the character %, and ends with
> a conversion specifier. In between there may be (in this order) zero or more
> flags, an optional minimum field width, an optional precision and an optional
> length modifier.

fscanf manual:

> Each conversion specification in format begins with either the character '%'
> or the character sequence "%n$" (see below for the distinction) followed by:
>
> * An optional '*' assignment-suppression character: scanf() reads input as
>   directed by the conversion specification, but discards the input. No
>   corresponding pointer argument is required, and this specification is not
>   included in the count of successful assignments returned by scanf().

[...]

> * An optional decimal integer which specifies the maximum field width.
>   Reading of characters stops either when this maximum is reached or when a
>   nonmatching character is found, whichever happens first. Most conversions
>   discard initial white space characters (the exceptions are noted below),
>   and these discarded characters don't count toward the maximum field width.
>   String input conversions store a terminating null byte ('\0') to mark the
>   end of the input; the maximum field width does not include this terminator.
---
 user/shared/shared_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/user/shared/shared_main.c b/user/shared/shared_main.c
index 8dec945..44a3d54 100644
--- a/user/shared/shared_main.c
+++ b/user/shared/shared_main.c
@@ -186,7 +186,7 @@ int have_ip_ipv6(const char *ip)
 
 	while (fscanf
 	       (if_inet6,
-		X32(08) X32(08) X32(08) X32(08) " %*02x %*02x %*02x %*02x %s",
+		X32(08) X32(08) X32(08) X32(08) " %*x %*x %*x %*x %s",
 		b, b + 1, b + 2, b + 3, name) > 0) {
 		for (i = 0; i < 4; i++)
 			addr6.s6_addr32[i] = cpu_to_be32(b[i]);
-- 
1.9.1

Re: [Drbd-dev] [PATCH] Correctly skip interface indexes larger than 256

[Roland Kammerer]

On Sun, Nov 16, 2014 at 06:35:43PM +0400, Dirk Tilger wrote:
> -		X32(08) X32(08) X32(08) X32(08) " %*02x %*02x %*02x %*02x %s",
> +		X32(08) X32(08) X32(08) X32(08) " %*x %*x %*x %*x %s",

Are numbers larger than 0xff even possible?

We read the string from "/proc/net/if_inet6", and to me it looks like
the interfaces limits the input range to values <= 255 [1].

Would be great if you could provide an example where it really makes a
difference, i.e. how to get lager values than 255 in the ignored format
string positions.

Regards, rck

[1] http://www.tldp.org/HOWTO/Linux+IPv6-HOWTO/proc-net.html

Re: [Drbd-dev] [PATCH] Correctly skip interface indexes larger than 256

[Roland Kammerer]

On Mon, Nov 17, 2014 at 01:28:36PM +0100, Roland Kammerer wrote:
> On Sun, Nov 16, 2014 at 06:35:43PM +0400, Dirk Tilger wrote:
> > -		X32(08) X32(08) X32(08) X32(08) " %*02x %*02x %*02x %*02x %s",
> > +		X32(08) X32(08) X32(08) X32(08) " %*x %*x %*x %*x %s",
> 
> Are numbers larger than 0xff even possible?
> 

Yes, it is possible...

Thank you Dirk for the hint and the patch. Applied to our internal repo.

Regards, rck