misc

Signed-off-by: Mattias Andrée <maandree@kth.se>

11 files changed, 841 insertions, 157 deletions

diff --git a/Makefile b/Makefile
index 0e2dff4..07072f1 100644
--- a/Makefile
+++ b/Makefile
@@ -10,7 +10,9 @@ OBJ =\
 	text.o\
 	avg.o\
 	rnd.o\
-	sig.o
+	sig.o\
+	ask.o\
+	map.o
 
 HDR =\
 	common.h
diff --git a/README b/README
index 54ace7b..c3706b1 100644
--- a/README
+++ b/README
@@ -3,7 +3,7 @@ NAME
 
 SYNOPSIS
 	deadshred [-b blocksize] [-o offset] [-l length | -e postend]
-	          [-r] device [< random-source]
+	          [-rY] device [map-file] [< random-source]
 
 DESCRIPTION
 	The deadshred utility fills a file or block devices with
@@ -36,13 +36,24 @@ OPTIONS
 		Start writing from the end instead of from the
 		beginning on the first pass over the device.
 
+	-Y
+		Do not ask for confirmation.
+
 OPERANDS
-	The following operand is supported:
+	The following operands are supported:
 
-	file
+	device
 		The file to override. Must be either a regular file or
 		a block device.
 
+	map-file
+		If the file map-file exists and is non-empty, it
+		specifies what sections in the file to overwrite;
+		this file will be periodically, and upon exit,
+		updated to remove parts that has been successfully
+		overwritten. This file will be unlinked when it
+		becomes empty.
+
 STDIN
 	Unless the standard input is a terminal device, it shall be an
 	unless source of either random data or a particular byte to
diff --git a/TODO b/TODO
index 6d59925..a852aa3 100644
--- a/TODO
+++ b/TODO
@@ -1,3 +1,3 @@
-Add shred map operand
+Test shred map support
 Test direction alternation
 Verify that aborting does not cause sections to be lost in the shred map
diff --git a/ask.c b/ask.c
new file mode 100644
index 0000000..bb8fa50
--- /dev/null
+++ b/ask.c
@@ -0,0 +1,115 @@
+/* See LICENSE file for copyright and license details. */
+#include "common.h"
+#if defined(__linux__)
+# include <sys/sysmacros.h>
+#endif
+
+
+int
+confirm(const char *device, off_t off, off_t len, off_t end)
+{
+	char buf[1024];
+	int ttyfd = -1;
+	int need_close;
+	int ret = 1;
+	int custom_section = (off >= 0 || len >= 0 || end >= 0);
+	const char *type;
+	ssize_t i, r;
+	int confirm_state = 0;
+	struct stat st;
+
+	if (isatty(STDIN_FILENO)) {
+		ttyfd = STDIN_FILENO;
+		need_close = 0;
+	} else {
+		ttyfd = open("/dev/tty", O_RDWR);
+		if (ttyfd < 0)
+			return 1;
+		need_close = 1;
+	}
+
+	if (getpgrp() != tcgetpgrp(ttyfd))
+		goto out;
+
+	if (custom_section) {
+		if (off < 0)
+			off = 0;
+		if (end >= 0)
+			len = end - off;
+		else if (len >= 0)
+			end = off + len;
+		else if (off == 0)
+			custom_section = 0;
+		if (end >= 0 && end < off)
+			eprintf("the value of -e option is lower than value of -o option");
+	}
+
+	if (stat(device, &st))
+		eprintf("stat %s:", device);
+	switch (st.st_mode & S_IFMT) {
+	case S_IFREG:
+		type = "file";
+		break;
+	case S_IFBLK:
+		type = "device";
+		break;
+	default:
+		eprintf("%s: not a regular file or block device", device);
+	}
+
+	if (dprintf(ttyfd, "Are you sure you want to erase the contents of %s?\n", device) < 0)
+		goto printerror;
+
+	if (S_ISREG(st.st_mode)) {
+		if (dprintf(ttyfd, "\nThe file is %s large.\n", exact_and_human_size(st.st_size, buf, 1)) < 0)
+			goto printerror;
+#if defined(__linux__)
+	} else if (S_ISBLK(st.st_mode)) {
+		char *devname = get_device_name(major(st.st_rdev), minor(st.st_rdev));
+		if (devname) {
+			/* TODO display device identity and size (look in /sys/class/block/<devname>/) */
+		}
+#endif
+	}
+
+	if (custom_section) {
+		if (dprintf(ttyfd, "\nYou have requested to only erase part of the %s:\n", type) < 0 ||
+		    (off >= 0 && dprintf(ttyfd, "\tFirst byte: %s\n", exact_and_human_size(off, buf, 0)) < 0) ||
+		    (len >= 0 && dprintf(ttyfd, "\tNumber of bytes: %s\n", exact_and_human_size(len, buf, 0)) < 0) ||
+		    (end >= 0 && dprintf(ttyfd, "\tEnd of overwrite: %s\n", exact_and_human_size(end, buf, 1)) < 0))
+			goto printerror;
+	}
+
+	if (dprintf(ttyfd, "\nAffirm by entering 'yes' in majuscule without quotes.\n") < 0)
+		goto printerror;
+
+	ret = 0;
+	for (;;) {
+		r = read(ttyfd, buf, sizeof(buf));
+		if (!r) {
+			goto out;
+		} else if (r < 0) {
+			if (errno == EINTR)
+				continue;
+			eprintf("read %s:", need_close ? "/dev/tty" : "<stdin>");
+		}
+		for (i = 0; i < r; i++) {
+			if (buf[i] != "YES\n"[confirm_state++])
+				goto out;
+			if (buf[i] == '\n') {
+				ret = 1;
+				goto out;
+			}
+		}
+	}
+
+out:
+	if (need_close)
+		close(ttyfd);
+	if (dprintf(ttyfd, "%s\n", ret ? "" : "User did not affirm action. Aborting...") < 0)
+		goto printerror;
+	return ret;
+
+printerror:
+	eprintf("dprintf %s:", need_close ? "/dev/tty" : "<stdin>");
+}
diff --git a/common.h b/common.h
index 241f280..ad4b8a8 100644
--- a/common.h
+++ b/common.h
@@ -40,17 +40,23 @@ struct status {
 	int write_average_i;
 	struct timespec write_average_begin_times[WRAVG_STEPS];
 	off_t write_average_amounts[WRAVG_STEPS];
+
+	size_t nspans;
+	size_t span_off;
+	size_t spans_size;
+	struct span *spans;
 };
 #define STATUS_INIT {.pass_nr = 1, .last_success = {-1, 0}, .direction = FORWARDS}
-
-
-/* deadshred.c */
-extern struct status status;
-extern _Atomic volatile sig_atomic_t exiting;
+#define STATUS_TRANSFER_SIZE (offsetof(struct status, nspans) + sizeof(size_t))
 
 
 /* io.c */
+void writeall(int fd, const void *data, size_t n, const char *fname);
+void filecpy(int fd, off_t src, off_t len, const char *fname);
 off_t filesize(int fd, const char *fname);
+#if defined(__linux__)
+char *get_device_name(unsigned int devmajor, unsigned int devminor);
+#endif
 
 
 /* fmt.c */
@@ -62,6 +68,7 @@ __attribute__((__pure__))
 off_t unhumansize(const char *s, char flag);
 const char *durationstr(const struct timespec *dur, char *buf, int second_decimals);
 const char *humanbytespersecond(double bytes_per_second, char *buf);
+const char *exact_and_human_size(off_t bytes, char *buf, int with_unit);
 
 
 /* text.c */
@@ -87,6 +94,19 @@ void used_random(ssize_t amount);
 
 
 /* sig.c */
+extern _Atomic volatile sig_atomic_t exiting;
 void setup_sighandler(void);
 void block_sigs(void);
 void unblock_sigs(void);
+
+
+/* ask.c */
+int confirm(const char *device, off_t off, off_t len, off_t end);
+
+
+/* map.c */
+void add_span(struct status *status, off_t off, off_t amount, size_t blocksize, int try_join);
+void dump_map(int fd, const struct status *status, const char *fname);
+off_t measure_map_dump(const struct status *status);
+off_t load_map(int fd, struct status *status, enum direction direction, const char *fname);
+void update_map(int fd, const struct status *status, const char *fname);
diff --git a/deadshred.1 b/deadshred.1
index e445c05..8d36103 100644
--- a/deadshred.1
+++ b/deadshred.1
@@ -13,8 +13,9 @@ deadshred \- override the contents of a device that may be broken
 |
 -e
 .IR postend ]
-[-r]
+[-rY]
 .I device
+.RI [ map-file ]
 [<
 .IR random-source ]
 
@@ -67,6 +68,9 @@ to overwrite.
 .B -r
 Start writing from the end instead of from the
 beginning on the first pass over the device.
+.TP
+.B -Y
+Do not ask for confirmation.
 .PP
 The value of the
 .B -belo
@@ -95,11 +99,21 @@ is recognised as a synonym, but the value is otherwise
 case sensitive).
 
 .SH OPERANDS
-The following operand is supported:
+The following operands are supported:
 .TP
-.I file
+.I device
 The file to override. Must be either a regular file or
 a block device.
+.TP
+.I map-file
+If the file
+.I map-file exists
+and is non-empty, it specifies what sections in the
+.I device
+to overwrite; this file will be periodically, and
+upon exit, updated to remove parts that has been
+successfully overwritten. This file will be unlinked
+when it becomes empty.
 
 .SH STDIN
 Unless the standard input is a terminal device, it shall be an
@@ -108,10 +122,12 @@ fill the device with.
 
 .SH STDOUT
 If the process is terminated using either of the signals
-SIGTERM or SIGINT, the process will write to standard output
+SIGTERM or SIGINT, and a
+.I map-file
+was not specified, the process will write to standard output
 a map of sections that has not been overwritten yet. The
 output will be the concatenation of one string per section,
-each on the format
+followed by a <newline>, each on the format
 .PP
 .nf
 \fB\(dq%s%x-%x/%x\(dq,\fP <\fB\(dq0x\(dq\fP for the first section, \fB\(dq,\(dq\fP otherwise>\fB,\fP
@@ -120,6 +136,24 @@ each on the format
               <\fIthe block size that should be used when trying to overwrite\fP>
 .fi
 
+.SH INPUT FILES
+If a
+.I map-file
+is specified, its content shall either be empty or
+conform to the format specified for the standard
+output. See the
+.B STDOUT
+section for more information.
+
+.SH OUTPUT FILES
+If a
+.I map-file
+is specified, the data written to it will conform
+to the format specified for the standard
+output. See the
+.B STDOUT
+section for more information.
+
 .SH NOTES
 While the
 .B deadshred
diff --git a/deadshred.c b/deadshred.c
index d7f2f5f..24d02fa 100644
--- a/deadshred.c
+++ b/deadshred.c
@@ -3,15 +3,17 @@
 #include <libsimple-arg.h>
 
 
-USAGE("[-b blocksize] [-o offset] [-l length | -e postend] [-r] device [< random-source]");
+USAGE("[-b blocksize] [-o offset] [-l length | -e postend] [-rY] device [map-file] [< random-source]");
 
 
-struct status status = STATUS_INIT;
-_Atomic volatile sig_atomic_t exiting = 0;
+struct auxthread_input {
+	struct status initial_status;
+	int map_fd;
+	const char *map_fname;
+};
+
 
-static struct span *spans = NULL;
-static size_t nspans = 0;
-static size_t spans_size = 0;
+static struct status status = STATUS_INIT;
 
 static off_t total_size = 0;
 static char total_size_1000[256];
@@ -24,38 +26,7 @@ static struct timespec start_time;
 
 static const struct timespec status_print_interval = {0, MILLISECONDS(500)};
 static const struct timespec poll_timeout          = {1, 0};
-static int status_print_pipe[2];
-
-
-static void
-add_span(off_t off, off_t amount, size_t blocksize, int try_join)
-{
-	off_t end = off + amount;
-
-	while ((off_t)(blocksize >> 1) >= amount)
-		blocksize >>= 1;
-
-	if (try_join) {
-		if (off == spans[nspans - 1U].end || end == spans[nspans - 1U].start) {
-			spans[nspans - 1U].start = MIN(off, spans[nspans - 1U].start);
-			spans[nspans - 1U].end = MAX(end, spans[nspans - 1U].end);
-			spans[nspans - 1U].bad += amount;
-			spans[nspans - 1U].blocksize = MAX(blocksize, spans[nspans - 1U].blocksize);
-			return;
-		}
-	}
-
-	if (nspans == spans_size) {
-		spans_size += 1024;
-		spans = ereallocarray(spans, spans_size, sizeof(*spans));
-	}
-
-	spans[nspans].start = off;
-	spans[nspans].end = end;
-	spans[nspans].bad = amount;
-	spans[nspans].blocksize = blocksize;
-	nspans++;
-}
+static int auxthread_pipe[2];
 
 
 static void
@@ -127,14 +98,27 @@ print_status(int done, struct status *s)
 }
 
 
+static void
+preprint(void)
+{
+	fprintf(stderr, "\n\n\n\n\n\n\033[K");
+}
+
+
 static void *
-status_print_loop(void *initial_status)
+auxthread_loop(void *input)
 {
-	struct pollfd pfd = {.fd = status_print_pipe[0], .events = POLLIN};
+	struct pollfd pfd = {.fd = auxthread_pipe[0], .events = POLLIN};
 	ssize_t r;
 	int terminate = 0;
-	struct status s = *(struct status *)initial_status;
-	size_t status_off;
+	int map_fd = ((struct auxthread_input *)input)->map_fd;
+	const char *map_fname = ((struct auxthread_input *)input)->map_fname;
+	struct status s = ((struct auxthread_input *)input)->initial_status;
+	size_t offset, spans_transfer_size;
+
+	s.spans = NULL;
+	s.spans_size = 0;
+	s.span_off = 0;
 
 	unblock_sigs();
 
@@ -149,22 +133,46 @@ status_print_loop(void *initial_status)
 		case 0:
 			break;
 		default:
-			status_off = 0;
+			offset = 0;
 		read_status_again:
-			r = read(status_print_pipe[0], &((char *)&s)[status_off], sizeof(s) - status_off);
-			if (r == (ssize_t)(sizeof(s) - status_off)) {
-				goto have_time;
-			} else if (!r) {
+			r = read(auxthread_pipe[0], &((char *)&s)[offset], STATUS_TRANSFER_SIZE - offset);
+			if (!r) {
 				terminate = 1;
 				break;
 			} else if (r < 0) {
 				if (errno == EINTR)
 					goto read_status_again;
 				eprintf("read <internal pipe>:");
-			} else {
-				status_off += (size_t)r;
+			} else if (r != (ssize_t)(STATUS_TRANSFER_SIZE - offset)) {
+				offset += (size_t)r;
 				goto read_status_again;
 			}
+
+			if (!s.nspans)
+				goto have_time;
+			if (s.spans_size != s.nspans) {
+				s.spans_size = s.nspans;
+				s.spans = ereallocarray(s.spans, s.spans_size, sizeof(*s.spans));
+			}
+
+			offset = 0;
+			spans_transfer_size = s.nspans * sizeof(*s.spans);
+		read_spans_again:
+			r = read(auxthread_pipe[0], &((char *)s.spans)[offset], spans_transfer_size - offset);
+			if (!r) {
+				terminate = 1;
+				break;
+			} else if (r < 0) {
+				if (errno == EINTR)
+					goto read_spans_again;
+				eprintf("read <internal pipe>:");
+			} else if (r != (ssize_t)(spans_transfer_size - offset)) {
+				offset += (size_t)r;
+				goto read_spans_again;
+			}
+
+			update_map(map_fd, &s, map_fname);
+			goto have_time;
 		}
 		if (clock_gettime(clck, &s.now))
 			eprintf("clock_gettime %s:", clkcstr);
@@ -177,16 +185,16 @@ status_print_loop(void *initial_status)
 		wravg_update_time(&s);
 	} while (!terminate);
 
+	free(s.spans);
 	return NULL;
 }
 
 
 static void
-shredspan(int fd, struct span *span, const char *fname)
+shredspan(int fd, struct span *span, const char *fname, int have_map)
 {
 	off_t off, n;
 	ssize_t r;
-	size_t status_off;
 	struct timespec when = {0, 0};
 	int bad = span->bad > 0;
 	int first_fail = 1;
@@ -201,8 +209,8 @@ shredspan(int fd, struct span *span, const char *fname)
 				span->start = off;
 			else
 				span->end = off;
-			close(status_print_pipe[1]);
-			status_print_pipe[1] = -1;
+			close(auxthread_pipe[1]);
+			auxthread_pipe[1] = -1;
 			return;
 		}
 
@@ -213,22 +221,51 @@ shredspan(int fd, struct span *span, const char *fname)
 			status.last_success = status.now;
 		}
 		if (libsimple_cmptimespec(&status.now, &when) >= 0) {
+			size_t saved_nspans = status.nspans;
+			size_t spans_transfer_size;
+			size_t spans_offset = 0;
+			size_t status_off = 0;
 			libsimple_sumtimespec(&when, &status.now, &status_print_interval);
-			status_off = 0;
+			if (have_map && 0) /* TODO periodically 1 instead 0 */
+				status.nspans -= status.span_off;
+			else
+				status.nspans = 0;
+			spans_transfer_size = status.nspans * sizeof(*status.spans);
+			spans_transfer_size += spans_offset = status.span_off * sizeof(*status.spans);
 		write_status_again:
-			r = write(status_print_pipe[1], &((const char *)&status)[status_off], sizeof(status) - status_off);
-			if (r != (ssize_t)(sizeof(status) - status_off)) {
+			r = write(auxthread_pipe[1], &((const char *)&status)[status_off], STATUS_TRANSFER_SIZE - status_off);
+			if (r != (ssize_t)(STATUS_TRANSFER_SIZE - status_off)) {
 				if (r >= 0) {
 					status_off += (size_t)r;
 					goto write_status_again;
 				} else if (r == EINTR) {
-					if (exiting)
+					if (exiting) {
+						status.nspans = saved_nspans;
 						goto userexit;
+					}
 					goto write_status_again;
 				} else {
 					eprintf("write <internal pipe>:");
 				}
 			}
+			status.nspans = saved_nspans;
+			if (spans_offset < spans_transfer_size) {
+			write_spans_again:
+				r = write(auxthread_pipe[1], &((const char *)status.spans)[spans_offset],
+				          spans_transfer_size - spans_offset);
+				if (r != (ssize_t)(spans_transfer_size - spans_offset)) {
+					if (r >= 0) {
+						spans_offset += (size_t)r;
+						goto write_spans_again;
+					} else if (r == EINTR) {
+						if (exiting)
+							goto userexit;
+						goto write_spans_again;
+					} else {
+						eprintf("write <internal pipe>:");
+					}
+				}
+			}
 		}
 		if (status.direction == FORWARDS) {
 			n = MIN((off_t)span->blocksize, span->end - off);
@@ -252,7 +289,7 @@ shredspan(int fd, struct span *span, const char *fname)
 			}
 			if (errno != EIO)
 				eprintf("pwrite %s <buffer> %zu %ji:", fname, (size_t)n, (intmax_t)off);
-			add_span(off, n, span->blocksize == 1U ? 1U : span->blocksize >> 1, !first_fail);
+			add_span(&status, off, n, span->blocksize == 1U ? 1U : span->blocksize >> 1, !first_fail);
 			first_fail = 0;
 			if (status.direction == FORWARDS)
 				off += n;
@@ -271,7 +308,7 @@ shredspan(int fd, struct span *span, const char *fname)
 			off += (off_t)r;
 		} else if ((off_t)r < n) {
 			n -= (off_t)r;
-			add_span(off + (off_t)r, n, span->blocksize == 1U ? 1U : span->blocksize >> 1, !first_fail);
+			add_span(&status, off + (off_t)r, n, span->blocksize == 1U ? 1U : span->blocksize >> 1, !first_fail);
 			first_fail = 0;
 			if (status.direction == FORWARDS)
 				off += n;
@@ -300,43 +337,16 @@ shredspan(int fd, struct span *span, const char *fname)
 }
 
 
-static void
-dump_map(int fd, const char *fname)
-{
-	size_t i;
-	int r;
-
-	if (!nspans)
-		return;
-
-	for (i = 0; i < nspans; i++) {
-		r = dprintf(fd, "%s%jx-%jx/%zx",
-		            i ? "," : "0x",
-		            (uintmax_t)spans[i].start,
-		            (uintmax_t)spans[i].end,
-		            spans[i].blocksize);
-		if (r < 0)
-			goto fail;
-	}
-	r = dprintf(fd, "\n");
-	if (r < 0)
-		goto fail;
-
-	return;
-
-fail:
-	eprintf("dprintf %s:", fname);
-}
-
-
 int
 main(int argc, char *argv[])
 {
 	off_t off = -1, len = -1, end = -1, blk = -1;
 	size_t i, j;
 	int fd, reverse_direction = 0;
-	pthread_t status_print_thread;
-	struct status initial_status;
+	int ask_for_confirmation = 1;
+	int map_fd = -1;
+	pthread_t auxthread;
+	struct auxthread_input auxthread_input;
 
 	ARGBEGIN {
 	case 'b':
@@ -362,21 +372,18 @@ main(int argc, char *argv[])
 	case 'r':
 		reverse_direction = 1;
 		break;
+	case 'Y':
+		ask_for_confirmation = 0;
+		break;
 	default:
 		usage();
 	} ARGEND;
 
-	if (argc != 1)
+	if (argc < 1 || argc > 2)
 		usage();
 
-	/* TODO If in foreground, and /dev/tty can be opened,
-	 *      use /dev/tty — however if stdin is a tty, use
-	 *      stdin instead — to request configuration unless
-	 *      bypassed with new option (-Y). The prompt shall
-	 *      be identify the device and display it's size,
-	 *      also the section the wipe shall be displayed
-	 *      if -elo was used. The use shall be required
-	 *      to enter 'yes' in upper case. */
+	if (ask_for_confirmation && !confirm(argv[0], off, len, end))
+		return 1;
 
 	fd = open(argv[0], O_WRONLY | O_DSYNC);
 	if (fd < 0)
@@ -384,52 +391,62 @@ main(int argc, char *argv[])
 
 	init_random(STDIN_FILENO, "<stdin>", blk);
 
-	spans = emalloc(sizeof(*spans));
-	spans[0].start = 0;
-	spans[0].end = filesize(fd, argv[0]);
-	spans[0].bad = 0;
-	spans[0].blocksize = max_blksize();
-	nspans = 1U;
-	spans_size = 1U;
+	status.spans = emalloc(sizeof(*status.spans));
+	status.spans[0].start = 0;
+	status.spans[0].end = filesize(fd, argv[0]);
+	status.spans[0].bad = 0;
+	status.spans[0].blocksize = max_blksize();
+	status.nspans = 1U;
+	status.spans_size = 1U;
 
 	if (off >= 0) {
-		if (off > spans[0].end)
-			eprintf("value of -o flag is beyond the end of the file");
-		spans[0].start = off;
+		if (off > status.spans[0].end)
+			eprintf("value of -o option is beyond the end of the file");
+		status.spans[0].start = off;
 	}
 	if (len >= 0) {
-		if (len > OFF_MAX - spans[0].start)
-			eprintf("the sum of the values of -o and -l flag is too large");
-		end = spans[0].start + len;
-		if (end > spans[0].end)
-			eprintf("the sum of the values of -o and -l flag is beyond the end of the file");
-		spans[0].end = end;
+		if (len > OFF_MAX - status.spans[0].start)
+			eprintf("the sum of the values of -o and -l option is too large");
+		end = status.spans[0].start + len;
+		if (end > status.spans[0].end)
+			eprintf("the sum of the values of -o and -l option is beyond the end of the file");
+		status.spans[0].end = end;
 	} else if (end >= 0) {
-		if (end > spans[0].end)
-			eprintf("the value of -e flag is beyond the end of the file");
-		spans[0].end = end;
+		if (end > status.spans[0].end)
+			eprintf("the value of -e option is beyond the end of the file");
+		status.spans[0].end = end;
 	}
 
-	total_size = spans[0].end - spans[0].start;
+	total_size = status.spans[0].end - status.spans[0].start;
 	if (!total_size) {
 		weprintf("attempting to shred 0 bytes");
 		close(fd);
 		return 0;
 	}
-	while ((off_t)spans[0].blocksize >> 1 > total_size)
-		spans[0].blocksize >>= 1;
+	while ((off_t)status.spans[0].blocksize >> 1 > total_size)
+		status.spans[0].blocksize >>= 1;
 
 	humansize1000(total_size, total_size_1000);
 	humansize1024(total_size, total_size_1024);
 
-	if (pipe(status_print_pipe))
+	if (reverse_direction)
+		status.direction ^= 1;
+
+	if (argv[1]) {
+		map_fd = open(argv[1], O_RDWR | O_CREAT, 0666);
+		if (map_fd < 0)
+			eprintf("open %s O_RDWR|O_CREAT 0666:", argv[1]);
+		if (lseek(map_fd, 0, SEEK_SET))
+			eprintf("lseek %s 0 SEEK_SET:", argv[1]);
+		status.shredded = load_map(fd, &status, status.direction, argv[1]);
+	}
+
+	if (pipe(auxthread_pipe))
 		eprintf("pipe:");
 
 	setup_sighandler();
 	block_sigs();
 
-	if (reverse_direction)
-		status.direction ^= 1;
 	if (clock_gettime(clck, &start_time)) {
 		clck = CLOCK_MONOTONIC;
 		clkcstr = "CLOCK_MONOTONIC";
@@ -439,33 +456,38 @@ main(int argc, char *argv[])
 	wravg_init(&start_time, &status);
 	status.last_success = start_time;
 
-	initial_status = status;
-	errno = pthread_create(&status_print_thread, NULL, &status_print_loop, &initial_status);
+	auxthread_input.initial_status = status;
+	auxthread_input.map_fd = map_fd;
+	auxthread_input.map_fname = argv[1];
+	errno = pthread_create(&auxthread, NULL, &auxthread_loop, &auxthread_input);
 	if (errno)
 		eprintf("pthread_create NULL:");
 
+	libsimple_eprintf_preprint = &preprint;
+
 	for (;;) {
-		size_t old_nspans = nspans;
-		for (i = 0; i < old_nspans && !exiting; i++)
-			shredspan(fd, &spans[i], argv[0]);
-		if (exiting) {
-			memmove(&spans[0], &spans[i], (nspans -= i) * sizeof(*spans));
-			break;
+		size_t old_nspans = status.nspans;
+		for (i = 0; i < old_nspans; i++) {
+			status.span_off = i;
+			shredspan(fd, &status.spans[i], argv[0], map_fd >= 0);
+			if (exiting)
+				goto out;
 		}
-		for (i = 0, j = nspans, nspans -= old_nspans; i < nspans;)
-			spans[i++] = spans[--j];
-		if (!nspans)
+		for (i = 0, j = status.nspans, status.nspans -= old_nspans; i < status.nspans;)
+			status.spans[i++] = status.spans[--j];
+		if (!status.nspans)
 			break;
 		status.direction ^= 1;
 		status.pass_nr += 1U;
 	}
 
+out:
 	close(fd);
-	if (status_print_pipe[1] >= 0)
-		close(status_print_pipe[1]);
+	if (auxthread_pipe[1] >= 0)
+		close(auxthread_pipe[1]);
 	destroy_random();
 
-	errno = pthread_join(status_print_thread, NULL);
+	errno = pthread_join(auxthread, NULL);
 	if (errno)
 		weprintf("pthread_join:");
 
@@ -473,10 +495,19 @@ main(int argc, char *argv[])
 		eprintf("clock_gettime %s:", clkcstr);
 	print_status(1, &status);
 
-	if (nspans) {
-		dump_map(STDOUT_FILENO, "<stdout>");
-		if (close(STDOUT_FILENO))
-			eprintf("write <stdout>");
+	if (status.nspans > status.span_off) {
+		if (map_fd >= 0) {
+			update_map(map_fd, &status, argv[0]);
+			if (close(map_fd))
+				eprintf("write %s:", argv[0]);
+		} else {
+			dump_map(STDOUT_FILENO, &status, "<stdout>");
+			if (close(STDOUT_FILENO))
+				eprintf("write <stdout>:");
+		}
+	} else if (argv[1]) {
+		if (unlink(argv[1]))
+			weprintf("unlink %s:", argv[1]);
 	}
 	return 0;
 }
diff --git a/fmt.c b/fmt.c
index 4c931dd..7b73ed7 100644
--- a/fmt.c
+++ b/fmt.c
@@ -187,3 +187,19 @@ humanbytespersecond(double bytes_per_second, char *buf)
 
 	return buf;
 }
+
+
+const char *
+exact_and_human_size(off_t bytes, char *buf, int with_unit)
+{
+	char *p = buf;
+	p += sprintf(p, "%ji%s", (intmax_t)bytes, !with_unit ? "" : bytes == 1 ? " byte" : " bytes");
+	if (bytes >= 1024) {
+		p = stpcpy(p, " (");
+		humansize1000(bytes, p);
+		p = stpcpy(strchr(p, '\0'), ", ");
+		humansize1024(bytes, p);
+		p = stpcpy(strchr(p, '\0'), ")");
+	}
+	return buf;
+}
diff --git a/io.c b/io.c
index 6ee9e29..98afe53 100644
--- a/io.c
+++ b/io.c
@@ -2,6 +2,61 @@
 #include "common.h"
 
 
+void
+writeall(int fd, const void *data, size_t n, const char *fname)
+{
+	const char *text = data;
+	ssize_t r;
+
+	while (n) {
+		r = write(fd, text, n);
+		if (r < 0) {
+			if (errno == EINTR)
+				continue;
+			eprintf("write %s:", fname);
+		}
+		text = &text[r];
+		n -= (size_t)r;
+	}
+}
+
+
+void
+filecpy(int fd, off_t src, off_t len, const char *fname)
+{
+	char buf[8UL << 10];
+	ssize_t r;
+	size_t n;
+	size_t off;
+
+	while (len) {
+	read_again:
+		r = pread(fd, buf, sizeof(buf), src);
+		if (r <= 0) {
+			if (!r)
+				eprintf("%s: unexpected end of file", fname);
+			if (errno == EINTR)
+				goto read_again;
+			eprintf("pread %s:", fname);
+		}
+		src += (off_t)r;
+		len -= (off_t)r;
+		n = (size_t)r;
+
+		for (off = 0; off < n;) {
+		write_again:
+			r = write(fd, &buf[off], n - off);
+			if (r < 0) {
+				if (errno == EINTR)
+					goto write_again;
+				eprintf("write %s:", fname);
+			}
+			off += (size_t)r;
+		}
+	}
+}
+
+
 off_t
 filesize(int fd, const char *fname)
 {
@@ -23,3 +78,75 @@ filesize(int fd, const char *fname)
 
 	return st.st_size;
 }
+
+
+#if defined(__linux__)
+
+char *
+get_device_name(unsigned int devmajor, unsigned int devminor)
+{
+	char dev[2 * 3 * sizeof(int) + 3];
+	size_t devlen;
+	char buf[PATH_MAX];
+	struct dirent *f;
+	DIR *dir;
+	int dfd, fd;
+	ssize_t r;
+	size_t off;
+
+	devlen = (size_t)sprintf(dev, "%u:%u\n", devmajor, devminor);
+
+	dir = opendir("/sys/class/block");
+	if (!dir) {
+		weprintf("opendir /sys/class/block:");
+		return NULL;
+	}
+	dfd = dirfd(dir);
+	if (dfd < 0) {
+		weprintf("dirfd /sys/class/block:");
+		goto fail;
+	}
+
+	while ((errno = 0, f = readdir(dir))) {
+		if (f->d_name[0] == '.')
+			continue;
+		if (strlen(f->d_name) > sizeof(buf) - sizeof("/dev"))
+			continue;
+		stpcpy(stpcpy(buf, f->d_name), "/dev");
+		fd = openat(dfd, buf, O_RDONLY);
+		if (fd < 0)
+			continue;
+
+		off = 0;
+		while (off < sizeof(buf)) {
+			r = read(fd, &buf[off], sizeof(buf) - off);
+			if (!r)
+				goto complete;
+			if (r < 0) {
+				if (errno == EINTR)
+					continue;
+				goto next;
+			}
+			off += (size_t)r;
+		}
+		goto next;
+
+	complete:
+		if (off == devlen && !memcmp(dev, buf, devlen)) {
+			close(fd);
+			closedir(dir);
+			return strdup(f->d_name);
+		}
+
+	next:
+		close(fd);
+	}
+
+	if (errno)
+		weprintf("readdir /sys/class/block:");
+fail:
+	closedir(dir);
+	return NULL;
+}
+
+#endif
diff --git a/map.c b/map.c
new file mode 100644
index 0000000..e72b02a
--- /dev/null
+++ b/map.c
@@ -0,0 +1,325 @@
+/* See LICENSE file for copyright and license details. */
+#include "common.h"
+
+
+void
+add_span(struct status *status, off_t off, off_t amount, size_t blocksize, int try_join)
+{
+	off_t end = off + amount;
+
+	while ((off_t)(blocksize >> 1) >= amount)
+		blocksize >>= 1;
+
+	if (try_join) {
+		if (off == status->spans[status->nspans - 1U].end || end == status->spans[status->nspans - 1U].start) {
+			status->spans[status->nspans - 1U].start = MIN(off, status->spans[status->nspans - 1U].start);
+			status->spans[status->nspans - 1U].end = MAX(end, status->spans[status->nspans - 1U].end);
+			status->spans[status->nspans - 1U].bad += amount;
+			status->spans[status->nspans - 1U].blocksize = MAX(blocksize, status->spans[status->nspans - 1U].blocksize);
+			return;
+		}
+	}
+
+	if (status->nspans == status->spans_size) {
+		status->spans_size += 1024;
+		status->spans = ereallocarray(status->spans, status->spans_size, sizeof(*status->spans));
+	}
+
+	status->spans[status->nspans].start = off;
+	status->spans[status->nspans].end = end;
+	status->spans[status->nspans].bad = amount;
+	status->spans[status->nspans].blocksize = blocksize;
+	status->nspans++;
+}
+
+
+void
+dump_map(int fd, const struct status *status, const char *fname)
+{
+	size_t i;
+	int r;
+
+	if (status->nspans == status->span_off)
+		return;
+
+	for (i = status->span_off; i < status->nspans; i++) {
+		r = dprintf(fd, "%s%jx-%jx/%zx",
+		            i ? "," : "0x",
+		            (uintmax_t)status->spans[i].start,
+		            (uintmax_t)status->spans[i].end,
+		            status->spans[i].blocksize);
+		if (r < 0)
+			goto fail;
+	}
+	r = dprintf(fd, "\n");
+	if (r < 0)
+		goto fail;
+
+	return;
+
+fail:
+	eprintf("dprintf %s:", fname);
+}
+
+
+off_t
+measure_map_dump(const struct status *status)
+{
+	size_t i;
+	int r;
+	off_t ret = 1;
+
+	if (status->nspans == status->span_off)
+		return 0;
+
+	for (i = status->span_off; i < status->nspans; i++) {
+		r = snprintf(NULL, 0, "%s%jx-%jx/%zx",
+		             i ? "," : "0x",
+		             (uintmax_t)status->spans[i].start,
+		             (uintmax_t)status->spans[i].end,
+		             status->spans[i].blocksize);
+		if (r < 0)
+			eprintf("snprintf:");
+		ret += r;
+	}
+
+	return ret;
+}
+
+
+static int
+spancmp(const void *av, const void *bv)
+{
+	const struct span *a = av, *b = bv;
+	return a->start < b->start ? -1 : a->start > b->start;
+}
+
+
+off_t
+load_map(int fd, struct status *status, enum direction direction, const char *fname)
+{
+	char buf[8UL << 10], *end;
+	size_t off = 0;
+	size_t p = 0, start = 0, i, j;
+	ssize_t r;
+	int empty = 1, state = 0, ended = 0;
+	uintmax_t v1 = 0, v2 = 0, v3 = 0, *vp;
+	struct span refspan = status->spans[0], t;
+	off_t preshredded, min_start;
+
+	for (;;) {
+		r = read(fd, &buf[off], sizeof(buf) - off);
+		if (r <= 0) {
+			if (!r)
+				break;
+			if (errno == EINTR)
+				continue;
+			eprintf("read %s:", fname);
+		}
+		off += (size_t)r;
+		if (empty) {
+			if (off < 2)
+				continue;
+			status->nspans = 0;
+			empty = 0;
+			if (buf[0] != '0' || buf[1] != 'x')
+				goto invalid;
+			p = 2U;
+		}
+		if (ended)
+			goto invalid;
+
+		while (p < off) {
+			switch (state) {
+			case 0:
+				errno = 0;
+				/* fall through */
+			case 2:
+			case 4:
+				start = p;
+				if (!isxdigit(buf[p]))
+					goto invalid;
+				p++;
+				state += 1;
+				break;
+			case 1:
+				vp = &v1;
+				goto value;
+			case 3:
+				vp = &v2;
+				goto value;
+			case 5:
+				vp = &v3;
+			value:
+				if (isxdigit(buf[p])) {
+					p++;
+					break;
+				}
+				if (state == 1) {
+					if (buf[p] != '-')
+						goto invalid;
+					state += 1;
+				} else {
+					if (state == 3 && buf[p] == '/') {
+						state += 1;
+					} else if (buf[p] == ',') {
+						v3 = refspan.blocksize;
+						state += 3;
+					} else if (buf[p] == '\n') {
+						v3 = refspan.blocksize;
+						state += 3;
+						ended = 1;
+					} else {
+						goto invalid;
+					}
+					
+				}
+				buf[p++] = '\0';
+				*vp = strtoumax(&buf[start], &end, 16);
+				if (errno || *end)
+					goto invalid;
+				if (state == 6) {
+					state = 0;
+					if (v1 >= v2 || v2 > (uintmax_t)OFF_MAX)
+						goto invalid;
+					if (v3 > v2 - v1)
+						v3 = v2 - v1;
+					if (v3 > (uintmax_t)SIZE_MAX)
+						v3 = ((uintmax_t)SIZE_MAX >> 1) + 1U;
+					if (v3 > refspan.blocksize)
+						v3 = refspan.blocksize;
+
+					if ((off_t)v1 < refspan.start || (off_t)v2 > refspan.end)
+						eprintf("%s: contains out of bounds span", fname);
+
+					if (status->nspans == status->spans_size) {
+						status->spans_size += 1024;
+						status->spans = ereallocarray(status->spans, status->spans_size,
+						                              sizeof(*status->spans));
+					}
+
+					status->spans[status->nspans].start = (off_t)v1;
+					status->spans[status->nspans].end = (off_t)v2;
+					status->spans[status->nspans].bad = 0;
+					status->spans[status->nspans].blocksize = (size_t)v3;
+					status->nspans++;
+				}
+				break;
+			}
+		}
+
+		off = 0;
+	}
+
+	if (!ended)
+		eprintf("%s: truncated file content", fname);
+
+	if (empty)
+		return 0;
+
+	qsort(status->spans, status->nspans, sizeof(*status->spans), &spancmp);
+	min_start = -1;
+	preshredded = refspan.end - refspan.start;
+	for (i = 0; i < status->nspans; i++) {
+		if (status->spans[i].start < min_start)
+			goto invalid;
+		if (status->spans[i].start == min_start &&
+		    status->spans[i].blocksize == status->spans[i - 1U].blocksize) {
+			status->spans[i - 1U].end = status->spans[i].end;
+			memmove(&status->spans[i], &status->spans[i + 1U],
+			        (--status->nspans - i) * sizeof(*status->spans));
+			i--;
+		}
+		min_start = status->spans[i].end;
+		preshredded -= status->spans[i].end - status->spans[i].start;
+	}
+	if (direction == BACKWARDS) {
+		for (i = 0, j = status->nspans; --j > i; i++) {
+			t = status->spans[i];
+			status->spans[i] = status->spans[j];
+			status->spans[j] = t;
+		}
+	}
+		
+	return preshredded;
+
+invalid:
+	eprintf("%s: invalid file content", fname);
+}
+
+
+void
+update_map(int fd, const struct status *status, const char *fname)
+{
+#define STR(S) (S), sizeof(S) - 1U
+
+	static char dots[4096] = {0};
+
+	off_t old_len, new_len, pos;
+	ssize_t r;
+
+	if (!dots[0])
+		memset(dots, '.', sizeof(dots));
+
+	old_len = lseek(fd, 0, SEEK_CUR);
+	if (old_len < 0)
+		eprintf("lseek %s 0 SEEK_CUR:", fname);
+
+	new_len = measure_map_dump(status);
+
+	pos = old_len;
+	if (pos < new_len) {
+		while (pos < new_len - 1) {
+			r = write(fd, dots, (size_t)MIN((off_t)sizeof(dots), new_len - 1 - pos));
+			if (r < 0) {
+				if (errno == EINTR)
+					continue;
+				eprintf("write %s:", fname);
+			}
+			pos += (off_t)r;
+		}
+		do {
+			r = write(fd, "\n", 1U);
+		} while (r < 0 && errno == EINTR);
+		if (r < 0)
+			eprintf("write %s:", fname);
+	}
+
+	writeall(fd, STR(">> FIRST LINE IS CORRECT UNLESS THE AS ANOTHER NOTICE BELOW THIS ONE >>\n"), fname);
+
+	filecpy(fd, 0, old_len, fname);
+
+	writeall(fd, STR(">> LINE ABOVE IS THE CORRECT ONE >>\n"), fname);
+
+	for (;;) {
+		if (!fdatasync(fd))
+			break;
+		if (errno != EINTR)
+			eprintf("fdatasync %s:", fname);
+	}
+
+	if (lseek(fd, 0, SEEK_SET))
+		eprintf("lseek %s 0 SEEK_SET:", fname);
+
+	dump_map(fd, status, fname);
+
+	pos = lseek(fd, 0, SEEK_CUR);
+	if (pos < 0)
+		eprintf("lseek %s 0 SEEK_CUR:", fname);
+
+	for (;;) {
+		if (!ftruncate(fd, pos))
+			break;
+		if (errno != EINTR)
+			eprintf("ftruncate %s %ji:", fname, (intmax_t)pos);
+	}
+
+	for (;;) {
+		if (!fsync(fd))
+			break;
+		if (errno != EINTR)
+			eprintf("fsync %s:", fname);
+	}
+
+#undef STR
+}
diff --git a/sig.c b/sig.c
index c7900fd..d77a46f 100644
--- a/sig.c
+++ b/sig.c
@@ -2,6 +2,9 @@
 #include "common.h"
 
 
+_Atomic volatile sig_atomic_t exiting = 0;
+
+
 static void
 sighandler(int signo)
 {