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Put on my asbestos underwear and commit the patch that I posted to -arch

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some time ago that changes kern.randompid from a boolean to a randomness
range for the next pid assigment.  Too high causes a lot of extra work
to scan for free pids, and too low merely wastes randomness entropy.  It's
still possible to select a completely random range by using PID_MAX (100k)
or -1 as a shortcut to mean "the whole range".
Also, don't waste randomness when doing a wraparound.
This commit is contained in:
Peter Wemm 1999-12-06 11:13:50 +00:00
parent 547f2153bb
commit bb6a234e47
Notes: svn2git 2020-12-20 02:59:44 +00:00 
svn path=/head/; revision=54203
1 changed files with 36 additions and 3 deletions
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39
sys/kern/kern_fork.c
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@ -142,8 +142,37 @@ rfork(p, uap)
int nprocs = 1; /* process 0 */
static int nextpid = 0;
/*
* Random component to nextpid generation. We mix in a random factor to make
* it a little harder to predict. We sanity check the modulus value to avoid
* doing it in critical paths. Don't let it be too small or we pointlessly
* waste randomness entropy, and don't let it be impossibly large. Using a
* modulus that is too big causes a LOT more process table scans and slows
* down fork processing as the pidchecked caching is defeated.
*/
static int randompid = 0;
SYSCTL_INT(_kern, OID_AUTO, randompid, CTLFLAG_RW, &randompid, 0, "");
static int
sysctl_kern_randompid SYSCTL_HANDLER_ARGS
{
int error, pid;
pid = randompid;
error = sysctl_handle_int(oidp, &pid, 0, req);
if (error || !req->newptr)
return (error);
if (pid < 0 || pid > PID_MAX - 100) /* out of range */
pid = PID_MAX - 100;
else if (pid < 2) /* NOP */
pid = 0;
else if (pid < 100) /* Make it reasonable */
pid = 100;
randompid = pid;
return (error);
}
SYSCTL_PROC(_kern, OID_AUTO, randompid, CTLTYPE_INT|CTLFLAG_RW,
0, 0, sysctl_kern_randompid, "I", "Random PID modulus");
int
fork1(p1, flags, procp)
@ -250,14 +279,18 @@ fork1(p1, flags, procp)
* ready to use (from nextpid+1 through pidchecked-1).
*/
nextpid++;
if (randompid)
nextpid += arc4random() % randompid;
retry:
/*
* If the process ID prototype has wrapped around,
* restart somewhat above 0, as the low-numbered procs
* tend to include daemons that don't exit.
*/
if (nextpid >= PID_MAX || randompid) {
nextpid = (randompid) ? arc4random() % PID_MAX : 100;
if (nextpid >= PID_MAX) {
nextpid = nextpid % PID_MAX;
if (nextpid < 100)
nextpid += 100;
pidchecked = 0;
}
if (nextpid >= pidchecked) {