Hello,
I'm using busybox on a AT91SAM9261 (arm920t) processor with gcc 4.3.4.
Yesterday I ran into some problems using dnsd. DNS queries from a
windows host were not properly decoded and hence not answered. Digging
into the code the problem turned out to be two different problems, both
related to halfword alignment.
The first one (in dnsd_main()) is related to the alignment of buffer
where the incoming network packets are stored. It was declared and
allocated as uint8_t buf[] in dnsd.c:462. This tells the compiler that
the buffer is containing byte data and gives it the freedom to allocate
the buffer at any address. In my setup it starts at an odd address. When
processing an incoming packet later on in process_packet() the buffer is
accessed as containing 2 byte data, which leads to alignment problems.
Allocating the buffer as uint16_t buf[] solves the problem for me.
The second one (in process_packet()) was particularly hard to track
down. It boils down to move_from_unaligned16(), which is #defined as 2
byte memcpy() in platform.h not working correctly for 16 bit source
pointer types. For these the compiler assumes the pointer to already be
aligned and replaces the 2 byte memcpy() by a half word load and a half
word store assembler instruction. The solution is to ensure the source
pointer is pointing to a single byte data type.
Please have a look at the attached patch which solves both problems. I'm
not sure whether the solution for the second problem should be done in
platform.h.
Best regards
Lars Reemts
--
Lars Reemts, Lange Wieke 1, 26817 Rhauderfehn, Germany
Tel: +49 4952 942290, Fax: +49 4952 942291
mailto: [email protected]
diff -urpN busybox-1.16.0.orig//networking/dnsd.c busybox-1.16.0/networking/dnsd.c
--- busybox-1.16.0.orig//networking/dnsd.c 2010-04-12 16:03:20.000000000 +0200
+++ busybox-1.16.0/networking/dnsd.c 2010-04-12 16:09:39.000000000 +0200
@@ -353,13 +353,11 @@ static int process_packet(struct dns_ent
{
char *answstr;
struct dns_head *head;
- struct dns_prop *unaligned_qprop;
+ struct dns_prop qprop;
char *query_string;
uint8_t *answb;
uint16_t outr_rlen;
uint16_t outr_flags;
- uint16_t type;
- uint16_t class;
int querystr_len;
head = (struct dns_head *)buf;
@@ -377,22 +375,18 @@ static int process_packet(struct dns_ent
query_string = (void *)(head + 1);
/* caller guarantees strlen is <= MAX_PACK_LEN */
querystr_len = strlen(query_string) + 1;
- /* may be unaligned! */
- unaligned_qprop = (void *)(query_string + querystr_len);
- querystr_len += sizeof(unaligned_qprop);
+ memcpy( &qprop, query_string + querystr_len, sizeof(qprop) );
+ querystr_len += sizeof(qprop);
/* where to append answer block */
- answb = (void *)(unaligned_qprop + 1);
-
+ answb = (uint8_t*)query_string + querystr_len;
/* QR = 1 "response", RCODE = 4 "Not Implemented" */
outr_flags = htons(0x8000 | 4);
- move_from_unaligned16(type, &unaligned_qprop->type);
- if (type != htons(REQ_A) && type != htons(REQ_PTR)) {
+ if (qprop.type != htons(REQ_A) && qprop.type != htons(REQ_PTR)) {
/* we can't handle the query type */
goto empty_packet;
}
- move_from_unaligned16(class, &unaligned_qprop->class);
- if (class != htons(1)) { /* not class INET? */
+ if (qprop.class != htons(1)) { /* not class INET? */
goto empty_packet;
}
/* OPCODE != 0 "standard query" ? */
@@ -405,9 +399,9 @@ static int process_packet(struct dns_ent
/* need to convert lengths to dots before we can use it in non-debug */
bb_info_msg("%s", query_string);
#endif
- answstr = table_lookup(conf_data, type, query_string);
+ answstr = table_lookup(conf_data, qprop.type, query_string);
outr_rlen = 4;
- if (answstr && type == htons(REQ_PTR)) {
+ if (answstr && qprop.type == htons(REQ_PTR)) {
/* return a host name */
outr_rlen = strlen(answstr) + 1;
}
@@ -459,7 +453,9 @@ int dnsd_main(int argc UNUSED_PARAM, cha
unsigned lsa_size;
int udps, opts;
uint16_t port = 53;
- uint8_t buf[MAX_PACK_LEN + 1];
+ /* allocate the buffer as uint16_t[] to make sure it is aligned at a uint16 boundary */
+ uint16_t _buf[MAX_PACK_LEN/2 + 1];
+ uint8_t *buf = (uint8_t*)_buf;
opts = getopt32(argv, "vi:c:t:p:d", &listen_interface, &fileconf, &sttl, &sport);
//if (opts & 0x1) // -v
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