Re: [PATCH][4.15] gnttab: work around "may be used uninitialized" warning

[Julien Grall <julien@xen.org>]

Hi Jan,

On 10/03/2021 10:13, Jan Beulich wrote:
> Sadly I was wrong to suggest dropping vaddrs' initializer during review
> of v2 of the patch introducing this code. gcc 4.3 can't cope.

What's the error? Are you sure this is not going to hiding a potential 
miscompilation of the function?

> 
> Fixes: 52531c734ea1 ("xen/gnttab: Rework resource acquisition")
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> 
> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -4026,7 +4026,7 @@ int gnttab_acquire_resource(
>       struct grant_table *gt = d->grant_table;
>       unsigned int i, final_frame;
>       mfn_t tmp;
> -    void **vaddrs;
> +    void **vaddrs = NULL;
I am a bit nervous to inialize vaddrs to NULL for a few reasons:
   1) It is not 100% obvious (e.g. it takes more than a second) that 
vaddrs will always be initialized.
   2) A compiler will not be able to help us if we are adding code 
without initialized vaddrs.

It also feels wrong to me to try to write Xen in a way that will make a 
10 years compiler happy...

If we still want to support them, then maybe a better approach would be 
to turn off to turn off -Werror for some version of GCC. So we can 
continue to benefit from the newer compiler diagnostics.

Cheers,

-- 
Julien Grall