cgi-io: add download operation

Add a new `cgi-download` applet which allows to retrieve the contents
of regular files or block devices.

In order to initiate a transfer, a POST request in x-www-form-urlencoded
format must be sent to the applet, with one field "sessionid" holding
the login session and another field "path" containing the file path to
download.

Further optional fields are "filename" which - if present - will cause
the download applet to set a Content-Dispostition header and "mimetype"
which allows to let the applet respond with a specific type instead of
the default "application/octet-stream".

Below is an example for the required acl rules to grant download access
to files or block devices:

    ubus call session grant '{
        "ubus_rpc_session": "...",
        "scope": "cgi-io",
        "objects": [
            [ "download", "read" ]
        ]
    }'

    ubus call session grant '{
        "ubus_rpc_session": "...",
        "scope": "file",
        "objects": [
            [ "/etc/config/*", "read" ],
            [ "/dev/mtdblock*", "read" ]
        ]
    }'

Signed-off-by: Jo-Philipp Wich <jo@mein.io>
(cherry picked from commit ab2a2b080d)

net/cgi-io/Makefile

@@ -8,7 +8,7 @@
 include $(TOPDIR)/rules.mk
 
 PKG_NAME:=cgi-io
-PKG_RELEASE:=9
+PKG_RELEASE:=10
 
 PKG_LICENSE:=GPL-2.0-or-later
 
@@ -38,6 +38,7 @@ define Package/cgi-io/install
 	$(INSTALL_DIR) $(1)/usr/libexec $(1)/www/cgi-bin/
 	$(INSTALL_BIN) $(PKG_BUILD_DIR)/cgi-io $(1)/usr/libexec
 	$(LN) ../../usr/libexec/cgi-io $(1)/www/cgi-bin/cgi-upload
+	$(LN) ../../usr/libexec/cgi-io $(1)/www/cgi-bin/cgi-download
 	$(LN) ../../usr/libexec/cgi-io $(1)/www/cgi-bin/cgi-backup
 endef
 

net/cgi-io/src/main.c

@@ -26,6 +26,9 @@
 #include <ctype.h>
 #include <sys/stat.h>
 #include <sys/wait.h>
+#include <sys/sendfile.h>
+#include <sys/ioctl.h>
+#include <linux/fs.h>
 
 #include <libubus.h>
 #include <libubox/blobmsg.h>
@@ -645,6 +648,84 @@ main_upload(int argc, char *argv[])
 	return 0;
 }
 
+static int
+main_download(int argc, char **argv)
+{
+	char *fields[] = { "sessionid", NULL, "path", NULL, "filename", NULL, "mimetype", NULL };
+	unsigned long long size = 0;
+	char *p, buf[4096];
+	ssize_t len = 0;
+	struct stat s;
+	int rfd;
+
+	postdecode(fields, 4);
+
+	if (!fields[1] || !session_access(fields[1], "cgi-io", "download", "read"))
+		return failure(0, "Download permission denied");
+
+	if (!fields[3] || !session_access(fields[1], "file", fields[3], "read"))
+		return failure(0, "Access to path denied by ACL");
+
+	if (stat(fields[3], &s))
+		return failure(errno, "Failed to stat requested path");
+
+	if (!S_ISREG(s.st_mode) && !S_ISBLK(s.st_mode))
+		return failure(0, "Requested path is not a regular file or block device");
+
+	for (p = fields[5]; p && *p; p++)
+		if (!isalnum(*p) && !strchr(" ()<>@,;:[]?.=%", *p))
+			return failure(0, "Invalid characters in filename");
+
+	for (p = fields[7]; p && *p; p++)
+		if (!isalnum(*p) && !strchr(" .;=/-", *p))
+			return failure(0, "Invalid characters in mimetype");
+
+	rfd = open(fields[3], O_RDONLY);
+
+	if (rfd < 0)
+		return failure(errno, "Failed to open requested path");
+
+	if (S_ISBLK(s.st_mode))
+		ioctl(rfd, BLKGETSIZE64, &size);
+	else
+		size = (unsigned long long)s.st_size;
+
+	printf("Status: 200 OK\r\n");
+	printf("Content-Type: %s\r\n", fields[7] ? fields[7] : "application/octet-stream");
+
+	if (fields[5])
+		printf("Content-Disposition: attachment; filename=\"%s\"\r\n", fields[5]);
+
+	printf("Content-Length: %llu\r\n\r\n", size);
+	fflush(stdout);
+
+	while (size > 0) {
+		len = sendfile(1, rfd, NULL, size);
+
+		if (len == -1) {
+			if (errno == ENOSYS || errno == EINVAL) {
+				while ((len = read(rfd, buf, sizeof(buf))) > 0)
+					fwrite(buf, len, 1, stdout);
+
+				fflush(stdout);
+				break;
+			}
+
+			if (errno == EINTR || errno == EAGAIN)
+				continue;
+		}
+
+		if (len <= 0)
+			break;
+
+		size -= len;
+	}
+
+	close(rfd);
+
+	return 0;
+}
+
 static int
 main_backup(int argc, char **argv)
 {
@@ -718,6 +799,8 @@ int main(int argc, char **argv)
 {
 	if (strstr(argv[0], "cgi-upload"))
 		return main_upload(argc, argv);
+	else if (strstr(argv[0], "cgi-download"))
+		return main_download(argc, argv);
 	else if (strstr(argv[0], "cgi-backup"))
 		return main_backup(argc, argv);