fw_cfg: insert fw_cfg file blobs via qemu cmdline

Allow user supplied files to be inserted into the fw_cfg
device before starting the guest. Since fw_cfg_add_file()
already disallows duplicate fw_cfg file names, qemu will
exit with an error message if the user supplies multiple
blobs with the same fw_cfg file name, or if a blob name
collides with a fw_cfg name programmatically added from
within the QEMU source code. A warning message will be
printed if the fw_cfg item name does not begin with the
prefix "opt/", which is recommended for external, user
provided blobs.

Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

docs/specs/fw_cfg.txt

@@ -203,3 +203,24 @@ completes fully overwriting the item's data.
 
 NOTE: This function is deprecated, and will be completely removed
 starting with QEMU v2.4.
+
+== Externally Provided Items ==
+
+As of v2.4, "file" fw_cfg items (i.e., items with selector keys above
+FW_CFG_FILE_FIRST, and with a corresponding entry in the fw_cfg file
+directory structure) may be inserted via the QEMU command line, using
+the following syntax:
+
+    -fw_cfg [name=]<item_name>,file=<path>
+
+where <item_name> is the fw_cfg item name, and <path> is the location
+on the host file system of a file containing the data to be inserted.
+
+NOTE: Users *SHOULD* choose item names beginning with the prefix "opt/"
+when using the "-fw_cfg" command line option, to avoid conflicting with
+item names used internally by QEMU. For instance:
+
+    -fw_cfg name=opt/my_item_name,file=./my_blob.bin
+
+Similarly, QEMU developers *SHOULD NOT* use item names prefixed with
+"opt/" when inserting items programmatically, e.g. via fw_cfg_add_file().

qemu-options.hx

@@ -2686,6 +2686,17 @@ STEXI
 @table @option
 ETEXI
 
+DEF("fw_cfg", HAS_ARG, QEMU_OPTION_fwcfg,
+    "-fw_cfg [name=]<name>,file=<file>\n"
+    "                add named fw_cfg entry from file\n",
+    QEMU_ARCH_ALL)
+STEXI
+@item -fw_cfg [name=]@var{name},file=@var{file}
+@findex -fw_cfg
+Add named fw_cfg entry from file. @var{name} determines the name of
+the entry in the fw_cfg file directory exposed to the guest.
+ETEXI
+
 DEF("serial", HAS_ARG, QEMU_OPTION_serial, \
     "-serial dev     redirect the serial port to char device 'dev'\n",
     QEMU_ARCH_ALL)

vl.c

@@ -492,6 +492,25 @@ static QemuOptsList qemu_semihosting_config_opts = {
     },
 };
 
+static QemuOptsList qemu_fw_cfg_opts = {
+    .name = "fw_cfg",
+    .implied_opt_name = "name",
+    .head = QTAILQ_HEAD_INITIALIZER(qemu_fw_cfg_opts.head),
+    .desc = {
+        {
+            .name = "name",
+            .type = QEMU_OPT_STRING,
+            .help = "Sets the fw_cfg name of the blob to be inserted",
+        }, {
+            .name = "file",
+            .type = QEMU_OPT_STRING,
+            .help = "Sets the name of the file from which\n"
+                    "the fw_cfg blob will be loaded",
+        },
+        { /* end of list */ }
+    },
+};
+
 /**
  * Get machine options
  *
@@ -2127,6 +2146,38 @@ char *qemu_find_file(int type, const char *name)
     return NULL;
 }
 
+static int parse_fw_cfg(void *opaque, QemuOpts *opts, Error **errp)
+{
+    gchar *buf;
+    size_t size;
+    const char *name, *file;
+
+    if (opaque == NULL) {
+        error_report("fw_cfg device not available");
+        return -1;
+    }
+    name = qemu_opt_get(opts, "name");
+    file = qemu_opt_get(opts, "file");
+    if (name == NULL || *name == '\0' || file == NULL || *file == '\0') {
+        error_report("invalid argument value");
+        return -1;
+    }
+    if (strlen(name) > FW_CFG_MAX_FILE_PATH - 1) {
+        error_report("name too long (max. %d char)", FW_CFG_MAX_FILE_PATH - 1);
+        return -1;
+    }
+    if (strncmp(name, "opt/", 4) != 0) {
+        error_report("WARNING: externally provided fw_cfg item names "
+                     "should be prefixed with \"opt/\"!");
+    }
+    if (!g_file_get_contents(file, &buf, &size, NULL)) {
+        error_report("can't load %s", file);
+        return -1;
+    }
+    fw_cfg_add_file((FWCfgState *)opaque, name, buf, size);
+    return 0;
+}
+
 static int device_help_func(void *opaque, QemuOpts *opts, Error **errp)
 {
     return qdev_device_help(opts);
@@ -2822,6 +2873,7 @@ int main(int argc, char **argv, char **envp)
     qemu_add_opts(&qemu_numa_opts);
     qemu_add_opts(&qemu_icount_opts);
     qemu_add_opts(&qemu_semihosting_config_opts);
+    qemu_add_opts(&qemu_fw_cfg_opts);
 
     runstate_init();
 
@@ -3438,6 +3490,12 @@ int main(int argc, char **argv, char **envp)
                 }
                 do_smbios_option(opts);
                 break;
+            case QEMU_OPTION_fwcfg:
+                opts = qemu_opts_parse(qemu_find_opts("fw_cfg"), optarg, 1);
+                if (opts == NULL) {
+                    exit(1);
+                }
+                break;
             case QEMU_OPTION_enable_kvm:
                 olist = qemu_find_opts("machine");
                 qemu_opts_parse(olist, "accel=kvm", 0);
@@ -4274,6 +4332,11 @@ int main(int argc, char **argv, char **envp)
 
     numa_post_machine_init();
 
+    if (qemu_opts_foreach(qemu_find_opts("fw_cfg"),
+                          parse_fw_cfg, fw_cfg_find(), NULL) != 0) {
+        exit(1);
+    }
+
     /* init USB devices */
     if (usb_enabled()) {
         if (foreach_device_config(DEV_USB, usb_parse) < 0)