security hardening: ensure suid programs have valid stdin/out/err

this behavior (opening fds 0-2 for a suid program) is explicitly
allowed (but not required) by POSIX to protect badly-written suid
programs from clobbering files they later open.

this commit does add some cost in startup code, but the availability
of auxv and the security flag will be useful elsewhere in the future.
in particular auxv is needed for static-linked vdso support, which is
still waiting to be committed (sorry nik!)

src/env/__init_security.c

@@ -0,0 +1,26 @@
+#include <stddef.h>
+#include <elf.h>
+#include <poll.h>
+#include <fcntl.h>
+#include "syscall.h"
+#include "libc.h"
+#include "atomic.h"
+
+#define AUX_CNT 24
+
+void __init_security(size_t *auxv)
+{
+	size_t i, aux[AUX_CNT] = { 0 };
+	struct pollfd pfd[3] = { {.fd=0}, {.fd=1}, {.fd=2} };
+
+	for (; auxv[0]; auxv+=2) if (auxv[0]<AUX_CNT) aux[auxv[0]] = auxv[1];
+	if (aux[AT_UID]==aux[AT_EUID] && aux[AT_GID]==aux[AT_EGID]
+		&& !aux[AT_SECURE]) return;
+
+	__syscall(SYS_poll, pfd, 3, 0);
+	for (i=0; i<3; i++)
+		if (pfd[i].revents&POLLNVAL)
+			if (__syscall(SYS_open, "/dev/null", O_RDWR)<0)
+				a_crash();
+	libc.secure = 1;
+}