[Bug network/18237] New: dn_expand of empty name should succeed with "" when output size is 1

["nszabolcs at gmail dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=18237

            Bug ID: 18237
           Summary: dn_expand of empty name should succeed with "" when
                    output size is 1
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: network
          Assignee: unassigned at sourceware dot org
          Reporter: nszabolcs at gmail dot com

dn_expand behaves inconsistently on empty name message.

if the length of the output is at least 2 then glibc writes a single \0 byte to
name, but if it is only 1 then dn_expand fails (eventhough the result would
fit).

the following code prints

dn_expand("", name, 1) returned -1, wanted 1
dn_expand("", name, 1) failed: got ".XX" name, wanted ""
dn_expand("\xc0\2", name, 1) returned -1, wanted 2
dn_expand("\xc0\2", name, 1) failed: got ".XX" name, wanted ""



#define _DEFAULT_SOURCE
#define _BSD_SOURCE
#include <resolv.h>
#include <string.h>
#include <stdio.h>

int main(void)
{
unsigned char packet[4];
char name[4];
int r, N;

for (N=1; N<4; N++) {

memset(packet, 0, 4);
memcpy(name, "XXX", 4);
r = dn_expand(packet, packet+1, packet, name, N);
if (r!=1)
        printf("dn_expand(\"\", name, %d) returned %d, wanted 1\n", N, r);
if (name[0])
        printf("dn_expand(\"\", name, %d) failed: got \"%s\" name, wanted
\"\"\n", N, name);

/* empty name with offset pointer to 0 */
memcpy(packet, "\xc0\2\0", 4);
memcpy(name, "XXX", 4);
r = dn_expand(packet, packet+3, packet, name, N);
if (r!=2)
        printf("dn_expand(\"\\xc0\\2\", name, %d) returned %d, wanted 2\n", N,
r);
if (name[0])
        printf("dn_expand(\"\\xc0\\2\", name, %d) failed: got \"%s\" name,
wanted \"\"\n", N, name);

}
}

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