fw_cfg: insert string blobs via qemu cmdline

Allow users to provide custom fw_cfg blobs with ascii string
payloads specified directly on the qemu command line.

Suggested-by: Jordan Justen <jordan.l.justen@intel.com>
Suggested-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gabriel Somlo <somlo@cmu.edu>
Message-id: 1443544141-26568-1-git-send-email-somlo@cmu.edu
Reviewd-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

docs/specs/fw_cfg.txt

@@ -216,6 +216,21 @@ the following syntax:
 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.
 
+Small enough items may be provided directly as strings on the command
+line, using the syntax:
+
+    -fw_cfg [name=]<item_name>,string=<string>
+
+The terminating NUL character of the content <string> will NOT be
+included as part of the fw_cfg item data, which is consistent with
+the absence of a NUL terminator for items inserted via the file option.
+
+Both <item_name> and, if applicable, the content <string> are passed
+through by QEMU without any interpretation, expansion, or further
+processing. Any such processing (potentially performed e.g., by the shell)
+is outside of QEMU's responsibility; as such, using plain ASCII characters
+is recommended.
+
 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:

qemu-options.hx

@@ -2724,13 +2724,18 @@ ETEXI
 
 DEF("fw_cfg", HAS_ARG, QEMU_OPTION_fwcfg,
     "-fw_cfg [name=]<name>,file=<file>\n"
-    "                add named fw_cfg entry from file\n",
+    "                add named fw_cfg entry from file\n"
+    "-fw_cfg [name=]<name>,string=<str>\n"
+    "                add named fw_cfg entry from string\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.
+
+@item -fw_cfg [name=]@var{name},string=@var{str}
+Add named fw_cfg entry from string.
 ETEXI
 
 DEF("serial", HAS_ARG, QEMU_OPTION_serial, \

vl.c

@@ -512,6 +512,10 @@ static QemuOptsList qemu_fw_cfg_opts = {
             .type = QEMU_OPT_STRING,
             .help = "Sets the name of the file from which\n"
                     "the fw_cfg blob will be loaded",
+        }, {
+            .name = "string",
+            .type = QEMU_OPT_STRING,
+            .help = "Sets content of the blob to be inserted from a string",
         },
         { /* end of list */ }
     },
@@ -2239,11 +2243,16 @@ char *qemu_find_file(int type, const char *name)
     return NULL;
 }
 
+static inline bool nonempty_str(const char *str)
+{
+    return str && *str;
+}
+
 static int parse_fw_cfg(void *opaque, QemuOpts *opts, Error **errp)
 {
     gchar *buf;
     size_t size;
-    const char *name, *file;
+    const char *name, *file, *str;
 
     if (opaque == NULL) {
         error_report("fw_cfg device not available");
@@ -2251,8 +2260,15 @@ static int parse_fw_cfg(void *opaque, QemuOpts *opts, Error **errp)
     }
     name = qemu_opt_get(opts, "name");
     file = qemu_opt_get(opts, "file");
-    if (name == NULL || *name == '\0' || file == NULL || *file == '\0') {
+    str = qemu_opt_get(opts, "string");
-        error_report("invalid argument value");
+
+    /* we need name and either a file or the content string */
+    if (!(nonempty_str(name) && (nonempty_str(file) || nonempty_str(str)))) {
+        error_report("invalid argument(s)");
+        return -1;
+    }
+    if (nonempty_str(file) && nonempty_str(str)) {
+        error_report("file and string are mutually exclusive");
         return -1;
     }
     if (strlen(name) > FW_CFG_MAX_FILE_PATH - 1) {
@@ -2263,9 +2279,14 @@ static int parse_fw_cfg(void *opaque, QemuOpts *opts, Error **errp)
         error_report("WARNING: externally provided fw_cfg item names "
                      "should be prefixed with \"opt/\"!");
     }
-    if (!g_file_get_contents(file, &buf, &size, NULL)) {
+    if (nonempty_str(str)) {
-        error_report("can't load %s", file);
+        size = strlen(str); /* NUL terminator NOT included in fw_cfg blob */
-        return -1;
+        buf = g_memdup(str, size);
+    } else {
+        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;