storage: Allow read only backing storage

Add a new argument '-r' to prevent writes back to the backing storage.
This is useful for experimenting with the remote storage, without having
the files overwritten.

Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>

rmtfs.c

@@ -449,17 +449,23 @@ static int run_rmtfs(void)
 
 int main(int argc, char **argv)
 {
+	bool read_only = false;
 	int ret;
 	int option;
 	const char *storage_root = NULL;
 
-	while ((option = getopt(argc, argv, "o:v")) != -1) {
+	while ((option = getopt(argc, argv, "o:rv")) != -1) {
 		switch (option) {
 		/* -o sets the directory where EFS images are stored. */
 		case 'o':
 			storage_root = optarg;
 			break;
 
+		/* -r to avoid writing to storage */
+		case 'r':
+			read_only = true;
+			break;
+
 		/* -v is for verbose */
 		case 'v':
 			dbgprintf_enabled = 1;
@@ -475,7 +481,7 @@ int main(int argc, char **argv)
 	if (!rmem)
 		return 1;
 
-	ret = storage_init(storage_root);
+	ret = storage_init(storage_root, read_only);
 	if (ret) {
 		fprintf(stderr, "failed to initialize storage system\n");
 		goto close_rmtfs_mem;

rmtfs.h

@@ -25,7 +25,7 @@ ssize_t rmtfs_mem_write(struct rmtfs_mem *rmem, unsigned long phys_address, cons
 
 struct rmtfd;
 
-int storage_init(const char *storage_root);
+int storage_init(const char *storage_root, bool read_only);
 struct rmtfd *storage_open(unsigned node, const char *path);
 struct rmtfd *storage_get(unsigned node, int caller_id);
 void storage_close(struct rmtfd *rmtfd);
@@ -33,6 +33,6 @@ int storage_get_caller_id(const struct rmtfd *rmtfd);
 int storage_get_error(const struct rmtfd *rmtfd);
 void storage_exit(void);
 ssize_t storage_pread(const struct rmtfd *rmtfd, void *buf, size_t nbyte, off_t offset);
-ssize_t storage_pwrite(const struct rmtfd *rmtfd, const void *buf, size_t nbyte, off_t offset);
+ssize_t storage_pwrite(struct rmtfd *rmtfd, const void *buf, size_t nbyte, off_t offset);
 
 #endif

storage.c

@@ -1,3 +1,4 @@
+#include <sys/stat.h>
 #include <sys/types.h>
 #include <errno.h>
 #include <fcntl.h>
@@ -8,6 +9,9 @@
 #include "rmtfs.h"
 
 #define MAX_CALLERS 10
+#define STORAGE_MAX_SIZE (16 * 1024 * 1024)
+
+#define MIN(x, y) ((x) < (y) ? (x) : (y))
 
 struct partition {
 	const char *path;
@@ -20,9 +24,13 @@ struct rmtfd {
 	int fd;
 	unsigned dev_error;
 	const struct partition *partition;
+
+	void *shadow_buf;
+	size_t shadow_len;
 };
 
 static const char *storage_dir = "/boot";
+static int storage_read_only;
 
 static const struct partition partition_table[] = {
 	{ "/boot/modem_fs1", "modem_fs1" },
@@ -34,13 +42,17 @@ static const struct partition partition_table[] = {
 
 static struct rmtfd rmtfds[MAX_CALLERS];
 
-int storage_init(const char *storage_root)
+static int storage_populate_shadow_buf(struct rmtfd *rmtfd, const char *file);
+
+int storage_init(const char *storage_root, bool read_only)
 {
 	int i;
 
 	if (storage_root)
 		storage_dir = storage_root;
 
+	storage_read_only = read_only;
+
 	for (i = 0; i < MAX_CALLERS; i++) {
 		rmtfds[i].id = i;
 		rmtfds[i].fd = -1;
@@ -56,6 +68,7 @@ struct rmtfd *storage_open(unsigned node, const char *path)
 	struct rmtfd *rmtfd = NULL;
 	size_t pathlen;
 	int saved_errno;
+	int ret;
 	int fd;
 	int i;
 
@@ -90,17 +103,29 @@ found:
 	pathlen = strlen(storage_dir) + strlen(part->actual) + 2;
 	fspath = alloca(pathlen);
 	snprintf(fspath, pathlen, "%s/%s", storage_dir, part->actual);
-	fd = open(fspath, O_RDWR);
-	if (fd < 0) {
-		saved_errno = errno;
-		fprintf(stderr, "[storage] failed to open '%s' (requested '%s'): %s\n",
-				fspath, part->path, strerror(saved_errno));
-		errno = saved_errno;
-		return NULL;
+	if (!storage_read_only) {
+		fd = open(fspath, O_RDWR);
+		if (fd < 0) {
+			saved_errno = errno;
+			fprintf(stderr, "[storage] failed to open '%s' (requested '%s'): %s\n",
+					fspath, part->path, strerror(saved_errno));
+			errno = saved_errno;
+			return NULL;
+		}
+		rmtfd->fd = fd;
+		rmtfd->shadow_len = 0;
+	} else {
+		ret = storage_populate_shadow_buf(rmtfd, fspath);
+		if (ret < 0) {
+			saved_errno = errno;
+			fprintf(stderr, "[storage] failed to open '%s' (requested '%s'): %s\n",
+					fspath, part->path, strerror(saved_errno));
+			errno = saved_errno;
+			return NULL;
+		}
 	}
 
 	rmtfd->node = node;
-	rmtfd->fd = fd;
 	rmtfd->partition = part;
 
 	return rmtfd;
@@ -110,6 +135,11 @@ void storage_close(struct rmtfd *rmtfd)
 {
 	close(rmtfd->fd);
 	rmtfd->fd = -1;
+
+	free(rmtfd->shadow_buf);
+	rmtfd->shadow_buf = NULL;
+	rmtfd->shadow_len = 0;
+
 	rmtfd->partition = NULL;
 }
 
@@ -149,11 +179,93 @@ void storage_exit(void)
 
 ssize_t storage_pread(const struct rmtfd *rmtfd, void *buf, size_t nbyte, off_t offset)
 {
-	return pread(rmtfd->fd, buf, nbyte, offset);
+	ssize_t n;
+
+	if (!storage_read_only) {
+		n = pread(rmtfd->fd, buf, nbyte, offset);
+	} else {
+		n = MIN(nbyte, rmtfd->shadow_len - offset);
+		if (n > 0)
+			memcpy(buf, rmtfd->shadow_buf + offset, n);
+		else
+			n = 0;
+	}
+
+	if (n < nbyte)
+		memset(buf + n, 0, nbyte - n);
+
+	return nbyte;
 }
 
-ssize_t storage_pwrite(const struct rmtfd *rmtfd, const void *buf, size_t nbyte, off_t offset)
+ssize_t storage_pwrite(struct rmtfd *rmtfd, const void *buf, size_t nbyte, off_t offset)
 {
-	return pwrite(rmtfd->fd, buf, nbyte, offset);
+	size_t new_len = offset + nbyte;
+	void *new_buf;
+
+	if (!storage_read_only)
+		return pwrite(rmtfd->fd, buf, nbyte, offset);
+
+	if (new_len >= STORAGE_MAX_SIZE) {
+		fprintf(stderr, "write to %zd bytes exceededs max size\n", new_len);
+		errno = -EINVAL;
+		return -1;
+	}
+
+	if (new_len > rmtfd->shadow_len) {
+		new_buf = realloc(rmtfd->shadow_buf, new_len);
+		if (!new_buf) {
+			errno = -ENOMEM;
+			return -1;
+		}
+
+		rmtfd->shadow_buf = new_buf;
+		rmtfd->shadow_len = new_len;
+	}
+
+	memcpy(rmtfd->shadow_buf + offset, buf, nbyte);
+
+	return nbyte;
 }
 
+static int storage_populate_shadow_buf(struct rmtfd *rmtfd, const char *file)
+{
+	ssize_t len;
+	ssize_t n;
+	void *buf;
+	int ret;
+	int fd;
+
+	fd = open(file, O_RDONLY);
+	if (fd < 0)
+		return -1;
+
+	len = lseek(fd, 0, SEEK_END);
+	if (len < 0) {
+		ret = -1;
+		goto err_close_fd;
+	}
+
+	lseek(fd, 0, SEEK_SET);
+
+	buf = calloc(1, len);
+	if (!buf) {
+		ret = -1;
+		goto err_close_fd;
+	}
+
+	n = read(fd, buf, len);
+	if (n < 0) {
+		ret = -1;
+		goto err_close_fd;
+	}
+
+	rmtfd->shadow_buf = buf;
+	rmtfd->shadow_len = n;
+
+	ret = 0;
+
+err_close_fd:
+	close(fd);
+
+	return ret;
+}