Re: [PATCH v1 2/3] x86/xen/msr: Fix uninitialized symbol 'err'

[Ingo Molnar <mingo@xxxxxxxxxx>]

* Xin Li (Intel) <xin@xxxxxxxxx> wrote:

> xen_read_msr_safe() currently passes an uninitialized argument err to
> xen_do_read_msr().  But as xen_do_read_msr() may not set the argument,
> xen_read_msr_safe() could return err with an unpredictable value.
> 
> To ensure correctness, initialize err to 0 (representing success)
> in xen_read_msr_safe().
> 
> Because xen_read_msr_safe() is essentially a wrapper of xen_do_read_msr(),
> the latter should be responsible for initializing the value of *err to 0.
> Thus initialize *err to 0 in xen_do_read_msr().
> 
> Fixes: 502ad6e5a619 ("x86/msr: Change the function type of 
> native_read_msr_safe()")
> Reported-by: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> Closes: https://lore.kernel.org/xen-devel/aBxNI_Q0-MhtBSZG@stanley.mountain/
> Signed-off-by: Xin Li (Intel) <xin@xxxxxxxxx>
> ---
>  arch/x86/xen/enlighten_pv.c | 5 ++++-
>  1 file changed, 4 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index 3be38350f044..01f1d441347e 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -1091,6 +1091,9 @@ static u64 xen_do_read_msr(u32 msr, int *err)
>  {
>       u64 val = 0;    /* Avoid uninitialized value for safe variant. */
>  
> +     if (err)
> +             *err = 0;
> +
>       if (pmu_msr_chk_emulated(msr, &val, true))
>               return val;
>  
> @@ -1162,7 +1165,7 @@ static void xen_do_write_msr(u32 msr, u64 val, int *err)
>  
>  static int xen_read_msr_safe(u32 msr, u64 *val)
>  {
> -     int err;
> +     int err = 0;
>  
>       *val = xen_do_read_msr(msr, &err);
>       return err;

So why not initialize 'err' with 0 in both callers, xen_read_msr_safe() 
and xen_read_msr(), and avoid all the initialization trouble in 
xen_do_read_msr()?

Thanks,

        Ingo