PCTS tests for sigprocmask() have uncovered a defect in the way signal
masks are managed.  The test-case below specifically provides an "invalid"
*how* argument and expects the resulting signal mask to NOT change.  But
upon execution of the code below, the signal mask *is* modified.

Cristian Gafton has analyzed this and concurs about it being a kernel bug.

[snip]
#include <stdio.h>
#include <signal.h>
#include <errno.h>

#define INVALID_HOW (SIG_SETMASK + SIG_BLOCK + SIG_UNBLOCK)

main()
{
        int pid, x;
        sigset_t set, oset;
        void dumpset();

printf("**** Start *****\n");
        sigemptyset(&set);
        sigemptyset(&oset);
        dumpset(set, " set");
        dumpset(oset, "oset");

        printf("\nCalling sigprocmask(SIG_SETMASK, &set, &oset)\n");
        x = sigprocmask(SIG_SETMASK, &set, &oset);
        printf("sigprocmask(SIG_SETMASK) returns %d, errno %d\n", x,
		errno);
        if (errno) perror("sigprocmask");
	dumpset(set, " set");
        dumpset(oset, "oset");

printf("\t... adding SIGHUP to signal set\n");
        sigaddset(&set, SIGHUP);
        dumpset(set, " set");

        printf("\nCalling sigprocmask(INVALID_HOW, &set, &oset)\n");
        x = sigprocmask(INVALID_HOW, &set, &oset);
        printf("sigprocmask returns %d, errno %d\n", x, errno);
        if (errno) perror("sigprocmask");
        dumpset(set, " set");
        dumpset(oset, "oset");

        sigprocmask(SIG_SETMASK, (sigset_t *)0, &oset);
        if (sigismember(&oset, SIGHUP))
                printf("\n\t\tOOPS: signal mask altered!\n\n");
        exit(0);
}
char *n;
{
        int i = 0, *ip = (int *) &s;

        printf("%s = ", n);
        while (i < sizeof(s) / sizeof(int)) {
                /*printf("[0x%08x]", *ip++);*/
                printf("%1d", *ip++);
                if (++i % 105 == 0)
                        printf("\n");
        }
        printf("\n");
        return;
}
void
dumpset(s, n)
sigset_t s;
     dumpset(set, " set");