Re: [Qemu-devel] [PATCH] fw_cfg: insert string blobs via qemu cmdline

[Laszlo Ersek]

On 09/27/15 00:55, Gabriel L. Somlo wrote:
> Allow users to provide custom fw_cfg blobs with ascii string
> payloads specified directly on the qemu command line.
> 
> Suggested-by: Jordan Justen <address@hidden>
> Suggested-by: Laszlo Ersek <address@hidden>
> Signed-off-by: Gabriel Somlo <address@hidden>
> ---
>  docs/specs/fw_cfg.txt |  5 +++++
>  qemu-options.hx       |  7 ++++++-
>  vl.c                  | 30 ++++++++++++++++++++++++------
>  3 files changed, 35 insertions(+), 7 deletions(-)
> 
> diff --git a/docs/specs/fw_cfg.txt b/docs/specs/fw_cfg.txt
> index b5f4b5d..4d85701 100644
> --- a/docs/specs/fw_cfg.txt
> +++ b/docs/specs/fw_cfg.txt
> @@ -236,6 +236,11 @@ 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>,content=<string>
> +

Please consider spelling out that these blobs will NOT be NUL-terminated
when viewed on the guest. (It kinda follows from all the other fw_cfg
things, but once we leave host-side files for qemu command line strings,
it might become non-obvious to users.)

So something like, "... the content presented to the guest will not be
NUL-terminated, same as if the string was written to a host-side file
first".

Please also stress that such content (and actually name too, but that's
more generic) should be plain ASCII; let's sidestep any tricks a shell's
locale settings could pull on us.

>  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:
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 328404c..0b6f5bd 100644
> --- a/qemu-options.hx
> +++ b/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>,content=<str>\n"
> +    "                add named fw_cfg entry from string\n",

Looks good. I got worried for a second that spelling out -fw_cfg twice
is not consistent with the rest of the documentation, but there are many
other such examples (-smbios, -netdev, -net, -chardev etc).

>      QEMU_ARCH_ALL)
>  STEXI
>  @item -fw_cfg address@hidden,address@hidden
>  @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.
> +
> address@hidden -fw_cfg address@hidden,address@hidden
> +Add named fw_cfg entry from string.
>  ETEXI
>  
>  DEF("serial", HAS_ARG, QEMU_OPTION_serial, \

Looks good. I checked with -chardev, and indeed @findex stands only
after the first occurrence of the option.

> diff --git a/vl.c b/vl.c
> index e211f6a..7f35f40 100644
> --- a/vl.c
> +++ b/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 = "content",
> +            .type = QEMU_OPT_STRING,
> +            .help = "Sets the content of the fw_cfg blob to be inserted",
>          },
>          { /* end of list */ }
>      },
> @@ -2236,7 +2240,7 @@ static int parse_fw_cfg(void *opaque, QemuOpts *opts, 
> Error **errp)
>  {
>      gchar *buf;
>      size_t size;
> -    const char *name, *file;
> +    const char *name, *file, *content;
>  
>      if (opaque == NULL) {
>          error_report("fw_cfg device not available");
> @@ -2244,8 +2248,17 @@ 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') {
> -        error_report("invalid argument value");
> +    content = qemu_opt_get(opts, "content");
> +
> +#define HAVE_OPT(arg) ((arg) && (*arg))

Not sure if this is usual in QEMU, but if it is, please also #undef the
macro after you are done using it.

Also, I recommend renaming HAVE_OPT() to NONEMPTY_STR().

> +
> +    /* we need name and either a file or the content string */
> +    if (!(HAVE_OPT(name) && (HAVE_OPT(file) || HAVE_OPT(content)))) {
> +        error_report("invalid argument(s)!");

Please drop the exclamation mark.

> +        return -1;
> +    }
> +    if (HAVE_OPT(file) && HAVE_OPT(content)) {
> +        error_report("file and content are mutually exclusive!");

Ditto.

>          return -1;
>      }
>      if (strlen(name) > FW_CFG_MAX_FILE_PATH - 1) {
> @@ -2256,9 +2269,14 @@ 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)) {
> -        error_report("can't load %s", file);
> -        return -1;
> +    if (HAVE_OPT(content)) {
> +        buf = g_strdup(content);
> +        size = strlen(buf);

I wonder if we should really duplicate the content here. I think we
could get away with just linking the option string into fw_cfg.

But, for consistency with the other branch, I don't mind. :)

Also, strlen() is okay (it will not expose the terminating NUL to the
guest), but I think we shouldn't *allocate* / duplicate that NUL either.
g_strdup() does though.

How about calling strlen() first, and then replacing g_strdup() with
g_memdup()?

Not crazy important though.

> +    } 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;
> 

Just small nits; looks okay to me otherwise. Thank you for coding this
up (and thanks Jordan for the suggestion), because this way we can
insert "-fw_cfg name=opt/ovmf/PcdXXX,content=Y" in a libvirt domain XML
directly, using <qemu:arg>, without referencing external files.

Thanks!
Laszlo