fix setgroups behavior in multithreaded process

this function is outside the scope of the standards, but logically
should behave like the set*id functions whose effects are
process-global.

src/linux/setgroups.c

@@ -1,8 +1,36 @@
 #define _GNU_SOURCE
 #include <unistd.h>
+#include <signal.h>
 #include "syscall.h"
+#include "libc.h"
+
+struct ctx {
+	size_t count;
+	const gid_t *list;
+	int ret;
+};
+
+static void do_setgroups(void *p)
+{
+	struct ctx *c = p;
+	if (c->ret<0) return;
+	int ret = __syscall(SYS_setgroups, c->count, c->list);
+	if (ret && !c->ret) {
+		/* If one thread fails to set groups after another has already
+		 * succeeded, forcibly killing the process is the only safe
+		 * thing to do. State is inconsistent and dangerous. Use
+		 * SIGKILL because it is uncatchable. */
+		__block_all_sigs(0);
+		__syscall(SYS_kill, __syscall(SYS_getpid), SIGKILL);
+	}
+	c->ret = ret;
+}
 
 int setgroups(size_t count, const gid_t list[])
 {
-	return syscall(SYS_setgroups, count, list);
+	/* ret is initially nonzero so that failure of the first thread does not
+	 * trigger the safety kill above. */
+	struct ctx c = { .count = count, .list = list, .ret = 1 };
+	__synccall(do_setgroups, &c);
+	return __syscall_ret(c.ret);
 }