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bird/nest/a-path.c
Ondrej Zajicek c8a6b9a3d1 Rewrite of buggy AS path matching. 
Old AS path maching supposes thath AS number appears
only once in AS path, but that is not true. It also
contains some bugs related to AS path sets.

New code does not use any assumptions about semantic
structure of AS path. It is asymptotically slower than
the old code, but on real paths it is not significant.

It also allows '?' for matching one arbitrary AS number.
2009-04-17 01:48:36 +02:00

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/*
* BIRD -- Path Operations
*
* (c) 2000 Martin Mares <mj@ucw.cz>
* (c) 2000 Pavel Machek <pavel@ucw.cz>
*
* Can be freely distributed and used under the terms of the GNU GPL.
*/
#include "nest/bird.h"
#include "nest/route.h"
#include "nest/attrs.h"
#include "lib/resource.h"
#include "lib/unaligned.h"
#include "lib/string.h"
/* Global AS4 support, shared by all BGP instances.
* This specifies whether BA_AS_PATH attributes contain 2 or 4 B per ASN
*/
int bgp_as4_support = 1;
static void
put_as(byte *data, u32 as)
{
if (bgp_as4_support)
put_u32(data, as);
else if (as <= 0xFFFF)
put_u16(data, as);
else
bug("put_as: Try to put 32bit AS to 16bit AS Path");
}
static inline u32
get_as(byte *data)
{
return bgp_as4_support ? get_u32(data) : get_u16(data);
}
struct adata *
as_path_prepend(struct linpool *pool, struct adata *olda, u32 as)
{
int bs = bgp_as4_support ? 4 : 2;
struct adata *newa;
if (olda->length && olda->data[0] == AS_PATH_SEQUENCE && olda->data[1] < 255)
/* Starting with sequence => just prepend the AS number */
{
int nl = olda->length + bs;
newa = lp_alloc(pool, sizeof(struct adata) + nl);
newa->length = nl;
newa->data[0] = AS_PATH_SEQUENCE;
newa->data[1] = olda->data[1] + 1;
memcpy(newa->data + bs + 2, olda->data + 2, olda->length - 2);
}
else /* Create new path segment */
{
int nl = olda->length + bs + 2;
newa = lp_alloc(pool, sizeof(struct adata) + nl);
newa->length = nl;
newa->data[0] = AS_PATH_SEQUENCE;
newa->data[1] = 1;
memcpy(newa->data + bs + 2, olda->data, olda->length);
}
put_as(newa->data + 2, as);
return newa;
}
int
as_path_convert_to_old(struct adata *path, byte *dst, int *new_used)
{
byte *src = path->data;
byte *src_end = src + path->length;
byte *dst_start = dst;
u32 as;
int i, n;
*new_used = 0;
while (src < src_end)
{
n = src[1];
*dst++ = *src++;
*dst++ = *src++;
for(i=0; i<n; i++)
{
as = get_u32(src);
if (as > 0xFFFF)
{
as = AS_TRANS;
*new_used = 1;
}
put_u16(dst, as);
src += 4;
dst += 2;
}
}
return dst - dst_start;
}
int
as_path_convert_to_new(struct adata *path, byte *dst, int req_as)
{
byte *src = path->data;
byte *src_end = src + path->length;
byte *dst_start = dst;
u32 as;
int i, t, n;
while ((src < src_end) && (req_as > 0))
{
t = *src++;
n = *src++;
if (t == AS_PATH_SEQUENCE)
{
if (n > req_as)
n = req_as;
req_as -= n;
}
else // t == AS_PATH_SET
req_as--;
*dst++ = t;
*dst++ = n;
for(i=0; i<n; i++)
{
as = get_u16(src);
put_u32(dst, as);
src += 2;
dst += 4;
}
}
return dst - dst_start;
}
void
as_path_format(struct adata *path, byte *buf, unsigned int size)
{
int bs = bgp_as4_support ? 4 : 2;
byte *p = path->data;
byte *e = p + path->length;
byte *end = buf + size - 16;
int sp = 1;
int l, isset;
while (p < e)
{
if (buf > end)
{
strcpy(buf, " ...");
return;
}
isset = (*p++ == AS_PATH_SET);
l = *p++;
if (isset)
{
if (!sp)
*buf++ = ' ';
*buf++ = '{';
sp = 0;
}
while (l-- && buf <= end)
{
if (!sp)
*buf++ = ' ';
buf += bsprintf(buf, "%u", get_as(p));
p += bs;
sp = 0;
}
if (isset)
{
*buf++ = ' ';
*buf++ = '}';
sp = 0;
}
}
*buf = 0;
}
int
as_path_getlen(struct adata *path)
{
int bs = bgp_as4_support ? 4 : 2;
int res = 0;
u8 *p = path->data;
u8 *q = p+path->length;
int len;
while (p<q)
{
switch (*p++)
{
case AS_PATH_SET: len = *p++; res++; p += bs * len; break;
case AS_PATH_SEQUENCE: len = *p++; res += len; p += bs * len; break;
default: bug("as_path_getlen: Invalid path segment");
}
}
return res;
}
int
as_path_get_first(struct adata *path, u32 *orig_as)
{
int bs = bgp_as4_support ? 4 : 2;
int found = 0;
u32 res = 0;
u8 *p = path->data;
u8 *q = p+path->length;
int len;
while (p<q)
{
switch (*p++)
{
case AS_PATH_SET:
if (len = *p++)
{
found = 1;
res = get_as(p);
p += bs * len;
}
break;
case AS_PATH_SEQUENCE:
if (len = *p++)
{
found = 1;
res = get_as(p + bs * (len - 1));
p += bs * len;
}
break;
default: bug("as_path_get_first: Invalid path segment");
}
}
*orig_as = res;
return found;
}
int
as_path_get_last(struct adata *path, u32 *last_as)
{
u8 *p = path->data;
if ((path->length == 0) || (p[0] != AS_PATH_SEQUENCE) || (p[1] == 0))
return 0;
else
{
*last_as = get_as(p+2);
return 1;
}
}
int
as_path_is_member(struct adata *path, u32 as)
{
int bs = bgp_as4_support ? 4 : 2;
u8 *p = path->data;
u8 *q = p+path->length;
int i, n;
while (p<q)
{
n = p[1];
p += 2;
for(i=0; i<n; i++)
{
if (get_as(p) == as)
return 1;
p += bs;
}
}
return 0;
}
struct pm_pos
{
u8 set;
u8 mark;
union
{
char *sp;
u32 asn;
} val;
};
static int
parse_path(struct adata *path, struct pm_pos *pos)
{
int bs = bgp_as4_support ? 4 : 2;
u8 *p = path->data;
u8 *q = p + path->length;
struct pm_pos *opos = pos;
int i, j, len;
while (p < q)
switch (*p++)
{
case AS_PATH_SET:
pos->set = 1;
pos->mark = 0;
pos->val.sp = p;
len = *p;
p += 1 + bs * len;
pos++;
break;
case AS_PATH_SEQUENCE:
len = *p++;
for (i = 0; i < len; i++)
{
pos->set = 0;
pos->mark = 0;
pos->val.asn = get_as(p);
p += bs;
pos++;
}
break;
default:
bug("as_path_match: Invalid path component");
}
return pos - opos;
}
static int
pm_match(struct pm_pos *pos, u32 asn)
{
if (! pos->set)
return pos->val.asn == asn;
int bs = bgp_as4_support ? 4 : 2;
u8 *p = pos->val.sp;
int len = *p++;
int i;
for (i = 0; i < len; i++)
if (get_as(p + i * bs) == asn)
return 1;
return 0;
}
static void
pm_mark(struct pm_pos *pos, int i, int plen, int *nl, int *nh)
{
int j;
if (pos[i].set)
pos[i].mark = 1;
for (j = i + 1; (j < plen) && pos[j].set && (! pos[j].mark); j++)
pos[j].mark = 1;
pos[j].mark = 1;
/* We are going downwards, therefore every mark is
new low and just the first mark is new high */
*nl = i + (pos[i].set ? 0 : 1);
if (*nh < 0)
*nh = j;
}
/* AS path matching is nontrivial. Because AS path can
* contain sets, it is not a plain wildcard matching. A set
* in an AS path is interpreted as it might represent any
* sequence of AS numbers from that set (possibly with
* repetitions). So it is also a kind of a pattern,
* more complicated than a path mask.
*
* The algorithm for AS path matching is a variant
* of nondeterministic finite state machine, where
* positions in AS path are states, and items in
* path mask are input for that finite state machine.
* During execution of the algorithm we maintain a set
* of marked states - a state is marked if it can be
* reached by any walk through NFSM with regard to
* currently processed part of input. When we process
* next part of mask, we advance each marked state.
* We start with marked first position, when we
* run out of marked positions, we reject. When
* we process the whole mask, we accept iff final position
* (auxiliary position after last real position in AS path)
* is marked.
*/
int
as_path_match(struct adata *path, struct f_path_mask *mask)
{
struct pm_pos pos[2048 + 1];
int plen = parse_path(path, pos);
int l, h, i, nh, nl;
/* l and h are bound of interval of positions where
are marked states */
pos[plen].set = 0;
pos[plen].mark = 0;
l = h = 0;
pos[0].mark = 1;
while (mask)
{
/* We remove this mark to not step after pos[plen] */
pos[plen].mark = 0;
switch (mask->kind)
{
case PM_ASTERISK:
for (i = l; i <= plen; i++)
pos[i].mark = 1;
h = plen;
break;
case PM_QUESTION:
case PM_ASN:
nh = -1;
for (i = h; i >= l; i--)
if (pos[i].mark)
{
pos[i].mark = 0;
if ((mask->kind == PM_QUESTION) || pm_match(pos + i, mask->val))
pm_mark(pos, i, plen, &nl, &nh);
}
if (nh < 0)
return 0;
h = nh;
l = nl;
break;
}
mask = mask->next;
}
return pos[plen].mark;
}
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