Re: [Xen-devel] [PATCH 1/7] libxc: fix uninitialized variable in xc_cpuid_pv_policy()

[Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>]

Jennifer Herbert writes ("[Xen-devel] [PATCH 1/7] libxc: fix uninitialized 
variable in xc_cpuid_pv_policy()"):
> If xc_domain_get_guest_width were to fail, guest_width is not set, and
> hence guest_64bit becomes undefined.
> Fix is to initialise to 0, and report error if call fails.
...
> diff --git a/tools/libxc/xc_cpuid_x86.c b/tools/libxc/xc_cpuid_x86.c
> index c97f91a..847b701 100644
> --- a/tools/libxc/xc_cpuid_x86.c
> +++ b/tools/libxc/xc_cpuid_x86.c
> @@ -437,14 +437,16 @@ static void xc_cpuid_pv_policy(
>  {
>      DECLARE_DOMCTL;
>      unsigned int guest_width;
> -    int guest_64bit;
> +    int guest_64bit = 0;

I'm not a huge fan of this style, which some people might describe as
`defensive initialisations'.  They turn failures to initialise a
variable (which can be detected by tools like Coverity and some
compilers), into uses of the wrong value.

> -    xc_domain_get_guest_width(xch, domid, &guest_width);
> -    guest_64bit = (guest_width == 8);
> +    if (xc_domain_get_guest_width(xch, domid, &guest_width) == 0)
> +        guest_64bit = (guest_width == 8);
> +    else
> +        ERROR("Could not read guest word width.");

Surely after failure of xc_domain_get_guest_width we should not
blunder on, making unwarranted assumptions about the guest bit width.

Unfortunately xc_cpuid_pv_policy doesn't return an error code.  I
think it needs to.  So that's rather a yak.

Sorry,
Ian.

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