Re: [PATCH 03/10] tools/libxg: Fix uninitialised variable in meminit()

[Julien Grall <julien@xen.org>]

Hi Andrew,

On 12/02/2021 15:39, Andrew Cooper wrote:
> Various version of gcc, when compiling with -Og, complain:
> 
>    xg_dom_arm.c: In function 'meminit':
>    xg_dom_arm.c:420:19: error: 'p2m_size' may be used uninitialized in this function [-Werror=maybe-uninitialized]
>      420 |     dom->p2m_size = p2m_size;
>          |     ~~~~~~~~~~~~~~^~~~~~~~~~
> 
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>

This was reported nearly 3 years ago (see [1]) and it is pretty sad this 
was never merged :(.

> ---
> CC: Ian Jackson <iwj@xenproject.org>
> CC: Wei Liu <wl@xen.org>
> CC: Stefano Stabellini <sstabellini@kernel.org>
> CC: Julien Grall <julien@xen.org>
> 
> Julien/Stefano: I can't work out how this variable is supposed to work, and
> the fact that it isn't a straight accumulation across the RAM banks looks
> suspect.

It looks buggy, but the P2M is never used on Arm. In fact, you sent a 
patch a year ago to drop it (see [2]). It would be nice to revive it.

> ---
>   tools/libs/guest/xg_dom_arm.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/tools/libs/guest/xg_dom_arm.c b/tools/libs/guest/xg_dom_arm.c
> index 94948d2b20..f1b8d06f75 100644
> --- a/tools/libs/guest/xg_dom_arm.c
> +++ b/tools/libs/guest/xg_dom_arm.c
> @@ -373,7 +373,7 @@ static int meminit(struct xc_dom_image *dom)
>       const uint64_t modsize = dtb_size + ramdisk_size;
>       const uint64_t ram128mb = bankbase[0] + (128<<20);
>   
> -    xen_pfn_t p2m_size;
> +    xen_pfn_t p2m_size = 0;
>       uint64_t bank0end;
>   
>       assert(dom->rambase_pfn << XC_PAGE_SHIFT == bankbase[0]);
>

If your original series is too risky for 4.15, I would consider to 
remote p2m_size completely and always 0 dom->p2m_size.

Cheers,

[1] 
https://lore.kernel.org/xen-devel/20180314123203.30646-1-wei.liu2@citrix.com/
[2] 
https://patchwork.kernel.org/project/xen-devel/patch/20191217201550.15864-3-andrew.cooper3@citrix.com/

-- 
Julien Grall