data/lab/arena: Add access-counter challenge

A challenge to exercise mprotect() and an intro in SIGSEGV handling.

Signed-off-by: Razvan Deaconescu <[email protected]>

content/chapters/data/lab/content/arena.md

@@ -62,6 +62,46 @@ test_brk                         ........................ passed ...  25
 Total:                                                           100/100
 ```
 
+## Access Counter
+
+Navigate to the `support/access-counter/` directory.
+
+Your goal is to update the `src/access_counter.c` source code file to capture memory access exceptions (i.e. the `SIGSEGV` signal) and to update page permissions in order for the acces to eventually succeed.
+Use `mprotect` to update the protection of the pages in stages: read, write and then exec.
+Each time an update is made, the `counter` variable is increased;
+this is used for testing.
+
+The `pages` array stores information about accessed pages.
+Assume the `MAX_PAGES` size of the array is enough to store information.
+When an existing page is accessed and causes a memory exception, the permission is update, in the stages mentioned above: read, write, and then exec.
+When a new page is accessed, a new entry is filled in the `pages` array, initialized with read protection.
+Use `mmap()` to reserve virtual pages.
+Use anonymous mapping (i.e. the `MAP_ANONYMOUS`) flag.
+Use any permissions required.
+
+To test it, enter the `tests/` directory and run:
+
+```console
+make check
+```
+
+In case of a correct solution, you will get an output such as:
+
+```text
+./run_all_tests.sh
+test_acess_read                  ........................ passed ...   9
+test_acess_write                 ........................ passed ...   9
+test_acess_exec                  ........................ passed ...  10
+test_acess_read_write            ........................ passed ...  12
+test_acess_read_exec             ........................ passed ...  12
+test_acess_write_exec            ........................ passed ...  12
+test_acess_exec_read             ........................ passed ...  12
+test_acess_exec_write            ........................ passed ...  12
+test_acess_write_read            ........................ passed ...  12
+
+Total:                                                           100/100
+```
+
 ## Memory Support
 
 **Manual memory management** (MMM) is one of the most difficult tasks.

content/chapters/data/lab/solution/access-counter/src/Makefile

@@ -0,0 +1,14 @@
+CFLAGS ?= -Wall -Wextra
+CPPFLAGS ?= -I../utils
+
+.PHONY: all clean
+
+all: access_counter.o ../utils/log/log.o
+
+access_counter.o: access_counter.c ../utils/utils.h ../utils/log/log.h
+
+../utils/log/log.o: ../utils/log/log.c ../utils/log/log.h
+
+clean:
+	-rm -f access_counter.o ../utils/log/log.o
+	-rm -f *~

content/chapters/data/lab/solution/access-counter/src/access_counter.c

@@ -0,0 +1,96 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <signal.h>
+#include <sys/mman.h>
+
+#include "utils.h"
+
+struct page_info {
+	int prot;
+	void *start;
+};
+
+unsigned long counter;
+
+#define MAX_PAGES	16
+static struct page_info pages[MAX_PAGES];
+static size_t num_pages = 0;
+
+/* This is useful for filling the contents when execution rights are provided. */
+static void do_nothing(void)
+{
+}
+
+/*
+ * The actual SIGSEVG handler.
+ */
+static void access_handler(int signum, siginfo_t *si, void *arg)
+{
+	long page_size = sysconf(_SC_PAGESIZE);
+	void *start;
+	int rc;
+	unsigned int i;
+
+	log_debug("Enter handler");
+
+	counter++;
+
+	if (signum != SIGSEGV) {
+		fprintf(stderr, "Unable to handle signal %d (%s)\n", signum, strsignal(signum));
+		return;
+	}
+
+	/* TODO 2: Obtain page strart address in start variable. */
+	start = (void *) ((unsigned long) si->si_addr & ~0xFFFUL);
+	log_debug("start: %p", start);
+
+	for (i = 0; i < num_pages; i++)
+		if (pages[i].start == start)
+			break;
+
+	if (i >= num_pages && i < MAX_PAGES) {
+		pages[i].start = start;
+		pages[i].prot = PROT_NONE;
+		num_pages += 1;
+	}
+
+	log_debug("i = %u", i);
+
+	/* TODO 21: Update page proctections with mprotect(). */
+	switch (pages[i].prot) {
+	case PROT_NONE:
+		rc = mprotect(start, page_size, PROT_READ);
+		DIE(rc < 0, "mprotect");
+		pages[i].prot = PROT_READ;
+		break;
+	case PROT_READ:
+		rc = mprotect(start, page_size, PROT_READ | PROT_WRITE);
+		DIE(rc < 0, "mprotect");
+		pages[i].prot = PROT_WRITE;
+		break;
+	case PROT_WRITE:
+		rc = mprotect(start, page_size, PROT_READ | PROT_WRITE | PROT_EXEC);
+		DIE(rc < 0, "mprotect");
+		/* Optimistically copy 64 bytes of code. Should be more than enough. */
+		memcpy(start, do_nothing, 64);
+		pages[i].prot = PROT_EXEC;
+		break;
+	default:
+		break;
+	}
+}
+
+void register_sigsegv_handler(void)
+{
+	struct sigaction sa;
+	int rc;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_sigaction = access_handler;
+	sa.sa_flags = SA_SIGINFO;
+
+	rc = sigaction (SIGSEGV, &sa, NULL);
+	DIE(rc < 0, "sigaction");
+}

content/chapters/data/lab/solution/access-counter/src/access_counter.h

@@ -0,0 +1,7 @@
+#ifndef ACCESS_PRINTER_H_
+#define ACCESS_PRINTER_H_	1
+
+extern unsigned long counter;
+void register_sigsegv_handler(void);
+
+#endif

content/chapters/data/lab/support/access-counter/src/Makefile

@@ -0,0 +1,14 @@
+CFLAGS ?= -Wall -Wextra
+CPPFLAGS ?= -I../utils
+
+.PHONY: all clean
+
+all: access_counter.o ../utils/log/log.o
+
+access_counter.o: access_counter.c ../utils/utils.h ../utils/log/log.h
+
+../utils/log/log.o: ../utils/log/log.c ../utils/log/log.h
+
+clean:
+	-rm -f access_counter.o ../utils/log/log.o
+	-rm -f *~

content/chapters/data/lab/support/access-counter/src/access_counter.c

@@ -0,0 +1,73 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <signal.h>
+#include <sys/mman.h>
+
+#include "utils.h"
+
+struct page_info {
+	int prot;
+	void *start;
+};
+
+unsigned long counter;
+
+#define MAX_PAGES	16
+static struct page_info pages[MAX_PAGES];
+static size_t num_pages = 0;
+
+/* This is useful for filling the contents when execution rights are provided. */
+static void do_nothing(void)
+{
+}
+
+/*
+ * The actual SIGSEVG handler.
+ */
+static void access_handler(int signum, siginfo_t *si, void *arg)
+{
+	long page_size = sysconf(_SC_PAGESIZE);
+	void *start;
+	int rc;
+	unsigned int i;
+
+	log_debug("Enter handler");
+
+	counter++;
+
+	if (signum != SIGSEGV) {
+		fprintf(stderr, "Unable to handle signal %d (%s)\n", signum, strsignal(signum));
+		return;
+	}
+
+	/* TODO: Obtain page strart address in start variable. */
+
+	for (i = 0; i < num_pages; i++)
+		if (pages[i].start == start)
+			break;
+
+	if (i >= num_pages && i < MAX_PAGES) {
+		pages[i].start = start;
+		pages[i].prot = PROT_NONE;
+		num_pages += 1;
+	}
+
+	log_debug("i = %u", i);
+
+	/* TODO: Update page proctections with mprotect(). */
+}
+
+void register_sigsegv_handler(void)
+{
+	struct sigaction sa;
+	int rc;
+
+	memset(&sa, 0, sizeof(sa));
+	sa.sa_sigaction = access_handler;
+	sa.sa_flags = SA_SIGINFO;
+
+	rc = sigaction (SIGSEGV, &sa, NULL);
+	DIE(rc < 0, "sigaction");
+}

content/chapters/data/lab/support/access-counter/src/access_counter.h

@@ -0,0 +1,7 @@
+#ifndef ACCESS_PRINTER_H_
+#define ACCESS_PRINTER_H_	1
+
+extern unsigned long counter;
+void register_sigsegv_handler(void);
+
+#endif

content/chapters/data/lab/support/access-counter/tests/.gitignore

@@ -0,0 +1 @@
+/test

content/chapters/data/lab/support/access-counter/tests/Makefile

@@ -0,0 +1,44 @@
+SRC_PATH ?= ../src
+FULL_SRC_PATH = $(realpath $(SRC_PATH))
+CPPFLAGS = -I. -I$(realpath $(SRC_PATH)) -I../utils
+CFLAGS = -Wall -Wextra
+# Remove the line below to disable debugging support.
+CFLAGS += -g -O0
+
+SRCS = $(wildcard test*.c)
+OBJS = $(patsubst %.c,%.o,$(SRCS))
+EXECS = $(patsubst %.c,%,$(SRCS))
+
+
+.PHONY: all src check lint clean
+
+all: src $(EXECS)
+
+$(EXECS): %:%.o graded_test.o $(SRC_PATH)/access_counter.o ../utils/log/log.o | src
+	$(CC) $(LDFLAGS) -o $@ $^ $(LDLIBS)
+
+$(OBJS): %.o:%.c graded_test.h
+
+graded_test.o: graded_test.c graded_test.h
+
+../utils/log/log.o: ../utils/log/log.c ../utils/log/log.h
+
+src:
+	make -C $(FULL_SRC_PATH)
+
+check:
+	make -C $(FULL_SRC_PATH) clean
+	make clean
+	make -i SRC_PATH=$(FULL_SRC_PATH)
+	./run_all_tests.sh
+
+lint:
+	-cd .. && checkpatch.pl -f src/*.c tests/*.c tests/*/*.c
+	-cd .. && checkpatch.pl -f checker/*.sh tests/*.sh
+	-cd .. && cpplint --recursive src/ tests/ checker/
+	-cd .. && shellcheck checker/*.sh tests/*.sh
+
+clean:
+	-rm -f *~
+	-rm -f graded_test.o $(OBJS)
+	-rm -f $(EXECS)