Re: [Qemu-devel] [PATCH] READCONFIG: Allow reading the configuration from a pre-existing filedescriptor

[Eric Blake]

On 01/25/2012 03:23 PM, Ronnie Sahlberg wrote:
> Update the readconfig filename parsing to allow specifying an existing, 
> inherited, filedescriptor as 'fd:<n>'
> This is useful when you want to pass potentially sensitive onfiguration data 
> to qemu without having it hit the filesystem/stable-storage
> 
> Signed-off-by: Ronnie Sahlberg <address@hidden>
> ---
>  qemu-config.c   |   15 +++++++++++++--
>  qemu-options.hx |    3 ++-
>  2 files changed, 15 insertions(+), 3 deletions(-)

Reviewed-by: Eric Blake <address@hidden>

> 
> diff --git a/qemu-config.c b/qemu-config.c
> index b030205..c12c5eb 100644
> --- a/qemu-config.c
> +++ b/qemu-config.c
> @@ -770,8 +770,19 @@ out:
>  
>  int qemu_read_config_file(const char *filename)
>  {
> -    FILE *f = fopen(filename, "r");
> -    int ret;
> +    FILE *f;
> +    int ret, fd;
> +
> +    if (strncmp(filename, "fd:", 3)) {
> +        f = fopen(filename, "r");
> +    } else {
> +        errno = 0;
> +        fd = strtol(filename + 3, NULL, 10);

This means

-readconfig fd:4junk

will read fd 4.  I don't know if there is a policy on ignoring vs.
rejecting ill-formed command line, which is why I'm okay with the patch
as-is if it meets project policy; but you might want to consider passing
a non-NULL pointer for the second argument and rejecting an empty string
or trailing junk.

> +        if (errno != 0) {
> +            return -errno;
> +        }

POSIX says that strtol("", NULL, 10) may return 0 without setting errno
(that is, you can't rely on EINVAL in that case).  That's another
argument for _always_ passing a non-NULL pointer and to see if you
accidentally parsed an empty string, since you don't want to have
another FILE* competing with stdin.  [Libvirt forbids the direct use of
strtol and friends, and instead provides wrapper functions that take
care of the sanity checking that is not mandated by POSIX; it may be
worth introducing a qemu_strtol that does likewise, but that's a
different cleanup project with wider scope.]

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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