servers/rs: Add sanity check on executable size

RS used to do a stat, and check that the stat was successful, but it
didn't check that the executable was not actually empty.

This patch adds a check to detect file which are smaller than the minimum
ELF size possible.

Change-Id: I22b004084f17e7d395e2c1377add7bc8b51c3d5e

minix/lib/libexec/exec_elf.c

@@ -58,6 +58,8 @@ static int elf_ph_sane(Elf_Phdr *phdr)
 static int elf_unpack(char *exec_hdr,
 	int hdr_len, Elf_Ehdr **hdr, Elf_Phdr **phdr)
 {
+  assert(hdr_len >= sizeof(Elf_Ehdr));
+
   *hdr = (Elf_Ehdr *) exec_hdr;
   if(!elf_sane(*hdr)) {
   	return ENOEXEC;

minix/servers/rs/manager.c

@@ -7,6 +7,8 @@
 
 #include <paths.h>
 
+#include <sys/exec_elf.h>
+
 #include "inc.h"
 
 #include "kernel/proc.h"
@@ -1272,6 +1274,9 @@ struct rproc *rp;
   if (r != 0) 
       return -errno;
 
+  if (sb.st_size < sizeof(Elf_Ehdr))
+      return ENOEXEC;
+
   fd= open(e_name, O_RDONLY);
   if (fd == -1)
       return -errno;