Re: [Qemu-devel] [PATCH] qemu-config: Accept empty option values

[Eric Blake <eblake@redhat.com>]

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On 04/08/2015 12:16 PM, Eduardo Habkost wrote:
> Currently it is impossible to set an option in a config file to an empty
> string, because the parser matches only lines containing non-empty
> strings between double-quotes.
> 
> As sscanf() "[" conversion specifier only matches non-empty strings, add
> a special case for empty strings.

I avoid sscanf() as a rule (as it's behavior on %d is undefined in the
face of malicious input), so I had to read the man page; but you are
right.  Libvirt is trying to completely ban use of *scanf for that
reason; but obviously qemu is not quite so opposed to it.

> 
> Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> ---
>  util/qemu-config.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/util/qemu-config.c b/util/qemu-config.c
> index 2d32ce7..9f9577d 100644
> --- a/util/qemu-config.c
> +++ b/util/qemu-config.c
> @@ -413,7 +413,8 @@ int qemu_config_parse(FILE *fp, QemuOptsList **lists, const char *fname)
>              opts = qemu_opts_create(list, NULL, 0, &error_abort);
>              continue;
>          }
> -        if (sscanf(line, " %63s = \"%1023[^\"]\"", arg, value) == 2) {
> +        if (sscanf(line, " %63s = \"%1023[^\"]\"", arg, value) == 2 ||
> +            (value[0] = '\0', sscanf(line, " %63s = \"\"", arg) == 1)) {

This is one of the few times I've seen sscanf used in a well-defined
manner (albeit still arbitrarily limiting, in that we have fixed-size
buffers) - but having to rely on a comma operator to get there makes
this look quite arcane.  I still wonder if hand-rolling a real scanner
would beat the compactness of sscanf by making the code intentions a
little more discernible, and have the benefits of avoiding my sscanf
red-flag checker.  But my wonder is not enough to stop me from accepting
this hack as-is.

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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