netsnmp_asn1_packet_parse(3) - Linux man page

Name

asn1 parsing and datatype manipulation routines. -

Defines

#define NULL 0

Functions

int asn_check_packet (u_char *pkt, size_t len)

u_char * asn_parse_int (u_char *data, size_t *datalength, u_char *type, long *intp, size_t intsize)

u_char * asn_parse_unsigned_int (u_char *data, size_t *datalength, u_char *type, u_long *intp, size_t intsize)

u_char * asn_build_int (u_char *data, size_t *datalength, u_char type, long *intp, size_t intsize)

u_char * asn_build_unsigned_int (u_char *data, size_t *datalength, u_char type, u_long *intp, size_t intsize)

u_char * asn_parse_string (u_char *data, size_t *datalength, u_char *type, u_char *string, size_t *strlength)

u_char * asn_build_string (u_char *data, size_t *datalength, u_char type, const u_char *string, size_t strlength)

u_char * asn_parse_header (u_char *data, size_t *datalength, u_char *type)

u_char * asn_parse_sequence (u_char *data, size_t *datalength, u_char *type, u_char expected_type, const char *estr)

u_char * asn_build_header (u_char *data, size_t *datalength, u_char type, size_t length)

u_char * asn_build_sequence (u_char *data, size_t *datalength, u_char type, size_t length)

u_char * asn_parse_length (u_char *data, u_long *length)

u_char * asn_build_length (u_char *data, size_t *datalength, size_t length)

u_char * asn_parse_objid (u_char *data, size_t *datalength, u_char *type, oid *objid, size_t *objidlength)

u_char * asn_build_objid (u_char *data, size_t *datalength, u_char type, oid *objid, size_t objidlength)

u_char * asn_parse_null (u_char *data, size_t *datalength, u_char *type)

u_char * asn_build_null (u_char *data, size_t *datalength, u_char type)

u_char * asn_parse_bitstring (u_char *data, size_t *datalength, u_char *type, u_char *string, size_t *strlength)

u_char * asn_build_bitstring (u_char *data, size_t *datalength, u_char type, u_char *string, size_t strlength)

u_char * asn_parse_unsigned_int64 (u_char *data, size_t *datalength, u_char *type, struct counter64 *cp, size_t countersize)

u_char * asn_build_unsigned_int64 (u_char *data, size_t *datalength, u_char type, struct counter64 *cp, size_t countersize)

int asn_realloc (u_char **pkt, size_t *pkt_len)

Detailed Description

Note on

Re-allocating reverse ASN.1 encoder functions. Synopsis:

 u_char *buf = (u_char*)malloc(100);
 u_char type = (ASN_UNIVERSAL | ASN_PRIMITIVE | ASN_INTEGER);
 size_t buf_len = 100, offset = 0;
 long data = 12345;
 int allow_realloc = 1;

if (asn_realloc_rbuild_int(&buf, &buf_len, &offset, allow_realloc,
                            type, &data, sizeof(long)) == 0) {
     error;
 }

NOTE WELL: after calling one of these functions with allow_realloc non-zero, buf might have moved, buf_len might have grown and offset will have increased by the size of the encoded data. You should **NEVER** do something like this:

 u_char *buf = (u_char *)malloc(100), *ptr;
 u_char type = (ASN_UNIVERSAL | ASN_PRIMITIVE | ASN_INTEGER);
 size_t buf_len = 100, offset = 0;
 long data1 = 1234, data2 = 5678;
 int rc = 0, allow_realloc = 1;

rc  = asn_realloc_rbuild_int(&buf, &buf_len, &offset, allow_realloc,
                                type, &data1, sizeof(long));
 ptr = buf[buf_len - offset];   / * points at encoding of data1 * /
 if (rc == 0) {
      error;
 }
 rc  = asn_realloc_rbuild_int(&buf, &buf_len, &offset, allow_realloc,
                              type, &data2, sizeof(long));
 make use of ptr here;

ptr is **INVALID** at this point. In general, you should store the offset value and compute pointers when you need them:

 u_char *buf = (u_char *)malloc(100), *ptr;
 u_char type = (ASN_UNIVERSAL | ASN_PRIMITIVE | ASN_INTEGER);
 size_t buf_len = 100, offset = 0, ptr_offset;
 long data1 = 1234, data2 = 5678;
 int rc = 0, allow_realloc = 1;

rc  = asn_realloc_rbuild_int(&buf, &buf_len, &offset, allow_realloc,
                              type, &data1, sizeof(long));
 ptr_offset = offset;
 if (rc == 0) {
      error;
 }
 rc  = asn_realloc_rbuild_int(&buf, &buf_len, &offset, allow_realloc,
                              type, &data2, sizeof(long));
 ptr = buf + buf_len - ptr_offset
 make use of ptr here;

Here, you can see that ptr will be a valid pointer even if the block of memory has been moved, as it may well have been. Plenty of examples of usage all over asn1.c, snmp_api.c, snmpusm.c.

The other thing you should **NEVER** do is to pass a pointer to a buffer on the stack as the first argument when allow_realloc is non-zero, unless you really know what you are doing and your machine/compiler allows you to free non-heap memory. There are rumours that such things exist, but many consider them no more than the wild tales of a fool.

Of course, you can pass allow_realloc as zero, to indicate that you do not wish the packet buffer to be reallocated for some reason; perhaps because it is on the stack. This may be useful to emulate the functionality of the old API:

 u_char my_static_buffer[100], *cp = NULL;
 size_t my_static_buffer_len = 100;
 float my_pi = (float)22/(float)7;

cp = asn_rbuild_float(my_static_buffer, &my_static_buffer_len,
                       ASN_OPAQUE_FLOAT, &my_pi, sizeof(float));
 if (cp == NULL) {
 error;
 }

IS EQUIVALENT TO:

 u_char my_static_buffer[100];
 size_t my_static_buffer_len = 100, my_offset = 0;
 float my_pi = (float)22/(float)7;
 int rc = 0;

rc = asn_realloc_rbuild_float(&my_static_buffer, &my_static_buffer_len,
                               &my_offset, 0,
                               ASN_OPAQUE_FLOAT, &my_pi, sizeof(float));
 if (rc == 0) {
   error;
 }