Dec 6 -- The Self-Healing Road to Perdition

Article:  https://ibr.cs.tu-bs.de/advent/06-sigaction/
Workload: ~89 source-code lines

06-sigaction/Makefile

+PROG = sigaction
+
+${PROG}: ${PROG}.c
+	gcc $< -o $@ -Wall
+
+run: ${PROG}
+	./${PROG}
+
+strace: ${PROG}
+	strace -ff ./{$PROG}
+
+clean:
+	rm -f ./${PROG}

06-sigaction/sigaction.c

+#undef _GNU_SOURCE
+#define _GNU_SOURCE
+#include <unistd.h>
+#include <string.h>
+#include <signal.h>
+#include <sys/mman.h>
+#include <stdio.h>
+#include <signal.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdlib.h>
+#include <sys/ucontext.h>
+
+int PAGE_SIZE;
+
+extern int main(void);
+
+/* See Day 2: clone.
+ *
+ * Example: syscall_write("foobar = ", 23);
+ */
+int syscall_write(char *msg, int64_t number, char base) {
+    write(1, msg, strlen(msg));
+
+    char buffer[sizeof(number) * 8];
+    char *p = &buffer[sizeof(number) * 8];
+    int len = 1;
+    *(--p) = '\n';
+    if (number < 0) {
+        write(1, "-", 1);
+        number *= -1;
+    }
+    do {
+        *(--p) =  "0123456789abcdef"[number % base];
+        number /= base;
+        len ++;
+    } while (number != 0);
+    write(1, p, len);
+    return 0;
+}
+
+
+/* We have three different fault handlers to make our program
+ * nearly "immortal":
+ *
+ * 1. sa_sigint:  Is invoked on Control-C.
+ * 2. sa_sigsegv: Handle segmentation faults
+ * 3. sa_sigill:  Jump over illegal instructions
+*/
+
+volatile bool do_exit = false;
+
+int main(void) {
+    // We get the actual page-size for this system. On x86, this
+    // always return 4096, as this is the size of regular pages on
+    // this architecture. We need this in the SIGSEGV handler.
+    PAGE_SIZE = sysconf(_SC_PAGESIZE);
+
+
+    // We generate an invalid pointer that points _somewhere_! This is
+    // undefined behavior, and we only hope for the best here. Perhaps
+    // we should install a signal handler for SIGSEGV beforehand....
+    uint32_t * addr = (uint32_t*)0xdeadbeef;
+
+    // This will provoke a SIGSEGV
+    *addr = 23;
+
+    // Two ud2 instructions are exactly 4 bytes long
+#define INVALID_OPCODE_32_BIT() asm("ud2; ud2;")
+
+    // This will provoke a SIGILL
+    INVALID_OPCODE_32_BIT();
+
+    // Happy faulting, until someone sets the do_exit variable.
+    // Perhaps the SIGINT handler?
+    while(!do_exit) {
+        sleep(1);
+        addr += 22559;
+        *addr = 42;
+        INVALID_OPCODE_32_BIT();
+    }
+
+    { // Like in the mmap exercise, we use pmap to show our own memory
+      // map, before exiting.
+        char cmd[256];
+        snprintf(cmd, 256, "pmap %d", getpid());
+        printf("---- system(\"%s\"):\n", cmd);
+        system(cmd);
+    }
+
+    return 0;
+}