Implement support for SOURCE_DATE_EPOCH environment variable

reproducible-builds.org suggests the use of an environment variable
as a source for time stamps:

https://reproducible-builds.org/specs/source-date-epoch/

This commit adds support for setting the default mtime from the variable,
if it is set and only defaulting to 0 if not. The timestamp given by the
command line switch takes precedence.

Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>

6 files changed, 71 insertions, 2 deletions

diff --git a/doc/gensquashfs.1 b/doc/gensquashfs.1
index d7b78a1..ab729d7 100644
--- a/doc/gensquashfs.1
+++ b/doc/gensquashfs.1
@@ -58,7 +58,7 @@ rd.
 uid=<value>;0
 gid=<value>;0
 mode=<value>;0755
-mtime=<value>;0
+mtime=<value>;\fB$SOURCE\_DATE\_EPOCH\fR if set, 0 otherwise
 .TE
 .TP
 .TP
@@ -148,6 +148,13 @@ file /bin/bash 0755 0 0
 file "/opt/my app/\\"special\\"/data" 0600 0 0
 .fi
 .in
+.SH ENVIRONEMNT VARIABLES
+If the command line switch \fB\-\-defaults\fR is not used or no default mtime
+is specified, the value of the environment variable \fBSOURCE\_DATE\_EPOCH\fR
+is used for all file and filesystem timestamps.
+
+If \fBSOURCE\_DATE\_EPOCH\fR is not set, not a parsable number or it is out of
+range, the timestamps default to 0.
 .SH AUTHOR
 Written by David Oberhollenzer.
 .SH COPYRIGHT
diff --git a/doc/tar2sqfs.1 b/doc/tar2sqfs.1
index 76178bf..7965a26 100644
--- a/doc/tar2sqfs.1
+++ b/doc/tar2sqfs.1
@@ -52,7 +52,7 @@ rd.
 uid=<value>;0
 gid=<value>;0
 mode=<value>;0755
-mtime=<value>;0
+mtime=<value>;\fB$SOURCE\_DATE\_EPOCH\fR if set, 0 otherwise
 .TE
 .TP
 .TP
@@ -95,6 +95,13 @@ If any unsupported section or extended attribute key is encountered in an
 archive a warning message is written to stderr. If the \fB\-\-no\-skip\fR
 option is set, processing aborts. By default, unknown sections and unsupported
 extended attributes are simply skipped after issuing a warning.
+.SH ENVIRONEMNT VARIABLES
+If the command line switch \fB\-\-defaults\fR is not used or no default mtime
+is specified, the value of the environment variable \fBSOURCE\_DATE\_EPOCH\fR
+is used for all file and filesystem timestamps.
+
+If \fBSOURCE\_DATE\_EPOCH\fR is not set, not a parsable number or it is out of
+range, the timestamps default to 0.
 .SH EXAMPLES
 .TP
 Turn an uncompressed tar archive into a squashfs image:
diff --git a/include/util.h b/include/util.h
index 9d5cc94..82e3177 100644
--- a/include/util.h
+++ b/include/util.h
@@ -83,4 +83,11 @@ int padd_file(int outfd, uint64_t size, size_t blocksize);
 
 uint32_t update_crc32(uint32_t crc, const void *data, size_t size);
 
+/*
+  If the environment variable SOURCE_DATE_EPOCH is set to a parsable number
+  that fits into an unsigned 32 bit value, return its value. Otherwise,
+  default to 0.
+ */
+uint32_t get_source_date_epoch(void);
+
 #endif /* UTIL_H */
diff --git a/lib/Makemodule.am b/lib/Makemodule.am
index 5a8ffaa..f49ff91 100644
--- a/lib/Makemodule.am
+++ b/lib/Makemodule.am
@@ -45,6 +45,7 @@ libutil_a_SOURCES += lib/util/print_version.c lib/util/mkdir_p.c
 libutil_a_SOURCES += lib/util/str_table.c include/str_table.h
 libutil_a_SOURCES += lib/util/dirstack.c lib/util/padd_file.c
 libutil_a_SOURCES += lib/util/read_data_at.c lib/util/crc32.c
+libutil_a_SOURCES += lib/util/source_date_epoch.c
 
 if WITH_GZIP
 libcompress_a_SOURCES += lib/comp/gzip.c
diff --git a/lib/fstree/fstree.c b/lib/fstree/fstree.c
index 3c240e7..6955c17 100644
--- a/lib/fstree/fstree.c
+++ b/lib/fstree/fstree.c
@@ -7,6 +7,7 @@
 #include "config.h"
 
 #include "fstree.h"
+#include "util.h"
 
 #include <string.h>
 #include <stdlib.h>
@@ -110,6 +111,7 @@ int fstree_init(fstree_t *fs, size_t block_size, char *defaults)
 	memset(fs, 0, sizeof(*fs));
 	fs->defaults.st_mode = S_IFDIR | 0755;
 	fs->defaults.st_blksize = block_size;
+	fs->defaults.st_mtime = get_source_date_epoch();
 	fs->block_size = block_size;
 
 	if (defaults != NULL && process_defaults(&fs->defaults, defaults) != 0)
diff --git a/lib/util/source_date_epoch.c b/lib/util/source_date_epoch.c
new file mode 100644
index 0000000..1397e52
--- /dev/null
+++ b/lib/util/source_date_epoch.c
@@ -0,0 +1,45 @@
+/* SPDX-License-Identifier: GPL-3.0-or-later */
+/*
+ * source_date_epoch.h
+ *
+ * Copyright (C) 2019 David Oberhollenzer <goliath@infraroot.at>
+ */
+#include "config.h"
+#include "util.h"
+
+#include <stdlib.h>
+#include <stdio.h>
+#include <ctype.h>
+
+uint32_t get_source_date_epoch(void)
+{
+	const char *str, *ptr;
+	uint32_t x, tval = 0;
+
+	str = getenv("SOURCE_DATE_EPOCH");
+
+	if (str == NULL || *str == '\0')
+		return 0;
+
+	for (ptr = str; *ptr != '\0'; ++ptr) {
+		if (!isdigit(*ptr))
+			goto fail_nan;
+
+		x = (*ptr) - '0';
+
+		if (tval > (UINT32_MAX - x) / 10)
+			goto fail_ov;
+
+		tval = tval * 10 + x;
+	}
+
+	return tval;
+fail_ov:
+	fprintf(stderr, "WARNING: SOURCE_DATE_EPOCH=%s does not fit into "
+		"32 bit integer\n", str);
+	return 0;
+fail_nan:
+	fprintf(stderr, "WARNING: SOURCE_DATE_EPOCH=%s is not a positive "
+		"number\n", str);
+	return 0;
+}