A much improved eample program.

man2/mprotect.2

@@ -121,46 +121,83 @@ has any effect different from
 .B PROT_READ
 is architecture and kernel version dependent.
 .SH EXAMPLE
+.PP
+The program below allocates 4 pages of memory, makes the third
+of these pages read-only, and then executes a loop that walks upwards
+through the allocated region modifying bytes.
+
+An example of what we might see when running the program is the
+following:
+
+.in +0.5i
 .nf
+$ ./a.out
+Start of region:        0x804c000
+Got SIGSEGV at address: 0x804e000
+.fi
+.in
+.nf
+
+#include <unistd.h>
+#include <signal.h>
 #include <stdio.h>
+#include <malloc.h>
 #include <stdlib.h>
 #include <errno.h>
 #include <sys/mman.h>
 
-#include <limits.h>    /* for PAGESIZE */
+char *buffer;
-#ifndef PAGESIZE
+
-#define PAGESIZE 4096
+static void
-#endif
+handler(int sig, siginfo_t *si, void *unused)
+{
+    printf("Got SIGSEGV at address: 0x%lx\\n", 
+            (long) si->si_addr);
+    exit(EXIT_FAILURE);
+}
 
 int
-main(void)
+main(int argc, char *argv[])
 {
     char *p;
-    char c;
+    int pagesize;
+    struct sigaction sa;
 
-    /* Allocate a buffer; it will have the default
+    sa.sa_flags = SA_SIGINFO;
-       protection of PROT_READ|PROT_WRITE. */
+    sigemptyset(&sa.sa_mask);
-    p = malloc(1024+PAGESIZE-1);
+    sa.sa_sigaction = handler;
-    if (!p) {
+    if (sigaction(SIGSEGV, &sa, NULL) == -1) {
-        perror("Couldn't malloc(1024)");
+        perror("sigaction");
-        exit(errno);
+        exit(EXIT_FAILURE);
     }
 
-    /* Align to a multiple of PAGESIZE, assumed to be a power of two */
+    pagesize = sysconf(_SC_PAGE_SIZE);
-    p = (char *)(((int) p + PAGESIZE-1) & ~(PAGESIZE-1));
+    if (pagesize == -1) {
-
+        perror("sysconf");
-    c = p[666];         /* Read; ok */
+        exit(EXIT_FAILURE);
-    p[666] = 42;        /* Write; ok */
-
-    /* Mark the buffer read-only. */
-    if (mprotect(p, 1024, PROT_READ)) {
-        perror("Couldn't mprotect");
-        exit(errno);
     }
 
-    c = p[666];         /* Read; ok */
+    /* Allocate a buffer aligned on a page boundary; 
-    p[666] = 42;        /* Write; program dies on SIGSEGV */
+       initial protection is PROT_READ | PROT_WRITE */
 
+    buffer = memalign(pagesize, 4 * pagesize);
+    if (buffer == NULL) {
+        perror("memalign");
+        exit(EXIT_FAILURE);
+    }
+
+    printf("Start of region:        0x%lx\\n", (long) buffer);
+
+    if (mprotect(buffer + pagesize * 2, pagesize, 
+                PROT_NONE) == -1) {
+        perror("mprotect");
+        exit(EXIT_FAILURE);
+    }
+
+    for (p = buffer ; ; )
+        *(p++) = 'a';
+
+    printf("Loop completed\\n");     /* Should never happen */
     exit(EXIT_SUCCESS);
 }
 .fi