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$Id: 085_sessreg_implement_hostname_hashing.diff 689 2005-10-19 22:11:30Z dnusinow $
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The sessreg program assumes that hostnames in utmp entries are unique in
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the last four characters. When two entries are "test:0" and "fist:0", for
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example, this assumption fails.
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Rectify this problem by hashing the entire hostname field from the utmp
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entry, so that we can tell unlike hostnames apart with much greater
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reliability. There is still a possiblility of one hash collision in 2^32.
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This patch by Maximiliano Curia and Damián Viano, using a public-domain
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hash algorithm by Bob Jenkins.
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Not submitted to XFree86.
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Index: sessreg/sessreg.c
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===================================================================
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--- sessreg/sessreg.c.orig 2005-10-18 19:45:25.000000000 -0400
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+++ sessreg/sessreg.c 2005-10-18 21:36:02.000000000 -0400
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const char *host, Time_t date, int addp);
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+/* used for hashing ut_id */
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+typedef unsigned long int ub4; /* unsigned 4-byte quantities */
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+typedef unsigned char ub1; /* unsigned 1-byte quantities */
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+#define hashsize(n) ((ub4)1<<(n))
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+#define hashmask(n) (hashsize(n)-1)
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+ub4 hash(register ub1 *k, register ub4 length, register ub4 initval);
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int wflag, uflag, lflag;
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char *wtmp_file, *utmp_file, *line;
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bzero (u->ut_name, sizeof (u->ut_name));
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- * this is a bit crufty, but
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- * follows the apparent conventions in
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- * the ttys file. ut_id is only 4 bytes
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- * long, and the last 4 bytes of the line
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- * name are written into it, left justified.
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+ * The ut_id is 4 bytes long. We make a hash of the line
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+ * received, preceding it by ":" to prevent clashing with
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- if (i >= sizeof (u->ut_id))
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- i -= sizeof (u->ut_id);
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- (void) strncpy (u->ut_id, line + i, sizeof (u->ut_id));
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+ h = hash(line, strlen(line),0x9e3779b9);
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+ h = (h & hashmask((sizeof(u->ut_id)-sizeof(char))*8));
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+ (void) strncpy (u->ut_id + 1,(char *) &h, sizeof (u->ut_id)-sizeof(char));
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+ * Clearing ut_id may result in race conditions leading to corrupted
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+ * utmp entries and and potential security holes.
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+ /* TODO: CHECK this */
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bzero (u->ut_id, sizeof (u->ut_id));
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u->ut_pid = getppid ();
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+--------------------------------------------------------------------
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+mix -- mix 3 32-bit values reversibly.
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+For every delta with one or two bits set, and the deltas of all three
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+ high bits or all three low bits, whether the original value of a,b,c
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+ is almost all zero or is uniformly distributed,
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+* If mix() is run forward or backward, at least 32 bits in a,b,c
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+ have at least 1/4 probability of changing.
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+* If mix() is run forward, every bit of c will change between 1/3 and
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+ 2/3 of the time. (Well, 22/100 and 78/100 for some 2-bit deltas.)
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+mix() was built out of 36 single-cycle latency instructions in a
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+ structure that could supported 2x parallelism, like so:
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+ a -= c; x = (c>>13);
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+ c -= b; x = (b>>13);
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+ Unfortunately, superscalar Pentiums and Sparcs can't take advantage
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+ of that parallelism. They've also turned some of those single-cycle
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+ latency instructions into multi-cycle latency instructions. Still,
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+ this is the fastest good hash I could find. There were about 2^^68
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+ to choose from. I only looked at a billion or so.
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+--------------------------------------------------------------------
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+#define mix(a,b,c) \
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+ a -= b; a -= c; a ^= (c>>13); \
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+ b -= c; b -= a; b ^= (a<<8); \
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+ c -= a; c -= b; c ^= (b>>13); \
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+ a -= b; a -= c; a ^= (c>>12); \
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+ b -= c; b -= a; b ^= (a<<16); \
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+ c -= a; c -= b; c ^= (b>>5); \
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+ a -= b; a -= c; a ^= (c>>3); \
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+ b -= c; b -= a; b ^= (a<<10); \
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+ c -= a; c -= b; c ^= (b>>15); \
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+--------------------------------------------------------------------
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+hash() -- hash a variable-length key into a 32-bit value
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+ k : the key (the unaligned variable-length array of bytes)
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+ len : the length of the key, counting by bytes
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+ initval : can be any 4-byte value
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+Returns a 32-bit value. Every bit of the key affects every bit of
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+the return value. Every 1-bit and 2-bit delta achieves avalanche.
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+About 6*len+35 instructions.
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+The best hash table sizes are powers of 2. There is no need to do
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+mod a prime (mod is sooo slow!). If you need less than 32 bits,
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+use a bitmask. For example, if you need only 10 bits, do
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+ h = (h & hashmask(10));
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+In which case, the hash table should have hashsize(10) elements.
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+If you are hashing n strings (ub1 **)k, do it like this:
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+ for (i=0, h=0; i<n; ++i) h = hash( k[i], len[i], h);
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+By Bob Jenkins, 1996. bob_jenkins@burtleburtle.net. You may use this
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+code any way you wish, private, educational, or commercial. It's free.
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+[On 27 May 2004, Bob Jenkins further clarified the above statement.
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+ From: Bob Jenkins <bob_jenkins@burtleburtle.net>
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+ Date: Thu, 27 May 2004 22:33:06 -0700
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+ To: Margarita Manterola <marga@marga.com.ar>
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+ Subject: Re: Hash function
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+ The algorithm is public domain. I ask that I be referenced as the
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+ source of the algorithm, but I can't enforce that, since being public
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+ domain means I've reserved no rights at all.
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+-- Branden Robinson, 2004-06-06]
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+See http://burtleburtle.net/bob/hash/evahash.html
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+Use for hash table lookup, or anything where one collision in 2^^32 is
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+acceptable. Do NOT use for cryptographic purposes.
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+--------------------------------------------------------------------
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+hash(register ub1 *k, register ub4 length, register ub4 initval)
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+ register ub4 a,b,c,len;
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+ /* Set up the internal state */
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+ a = b = 0x9e3779b9; /* the golden ratio; an arbitrary value */
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+ c = initval; /* the previous hash value */
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+ /*---------------------------------------- handle most of the key */
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+ a += (k[0] +((ub4)k[1]<<8) +((ub4)k[2]<<16) +((ub4)k[3]<<24));
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+ b += (k[4] +((ub4)k[5]<<8) +((ub4)k[6]<<16) +((ub4)k[7]<<24));
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+ c += (k[8] +((ub4)k[9]<<8) +((ub4)k[10]<<16)+((ub4)k[11]<<24));
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+ k += 12; len -= 12;
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+ /*------------------------------------- handle the last 11 bytes */
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+ switch(len) /* all the case statements fall through */
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+ case 11: c+=((ub4)k[10]<<24);
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+ case 10: c+=((ub4)k[9]<<16);
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+ case 9 : c+=((ub4)k[8]<<8);
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+ /* the first byte of c is reserved for the length */
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+ case 8 : b+=((ub4)k[7]<<24);
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+ case 7 : b+=((ub4)k[6]<<16);
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+ case 6 : b+=((ub4)k[5]<<8);
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+ case 4 : a+=((ub4)k[3]<<24);
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+ case 3 : a+=((ub4)k[2]<<16);
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+ case 2 : a+=((ub4)k[1]<<8);
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+ /* case 0: nothing left to add */
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+ /*-------------------------------------------- report the result */
added
removed
debian/patches/01_sessreg_implement_hostname_hashing.diff
