Re: [Xen-devel] [PATCH] xen/arm: add warning if memory modules overlap

[Brian Woods <brian.woods@xxxxxxxxxx>]

On Fri, Oct 11, 2019 at 05:58:35PM +0100, Julien Grall wrote:
> Hi,
> 
> Please at least remove the signature in the e-mail you reply to. The best
> would be to trim the e-mail and answer right below the specific paragraph.
> 
> >
> >To make sure the module is going to get added, you'd need to do the
> >check after the for loop.  This means there's going to be multiple for
> >loops just spread over the course of adding the boot modules rather than
> >one place.
> 
> I don't think you need to do the check after the loop. The only way to go
> out of the loop in add_boot_module() is when i reached mods->nr_mods.

See below.

> >
> >I had this before but decided against it but after changing it to both
> >starts rather than the stand and end (ends look much uglier), it looks
> >cleaner.
> >
> >     for ( i = 0 ; i < mods->nr_mods-1; i++ )
> >         for ( j = i+1 ; j < mods->nr_mods; j++ )
> >             if ( ((mods->module[i].start >= mods->module[j].start) &&
> >                   (mods->module[i].start <=
> >                    mods->module[j].start + mods->module[j].size)) ||
> >                  ((mods->module[j].start >= mods->module[i].start) &&
> >                   (mods->module[j].start <=
> >                    mods->module[i].start + mods->module[i].size)) )
> >                 printk("WARNING: modules %-12s and %-12s overlap\n",
> >                        boot_module_kind_as_string(mods->module[i].kind),
> >                        boot_module_kind_as_string(mods->module[j].kind));
> >
> >That's also a possibility.
> >
> >I just don't see a way around it, computationally.  You can split where
> >the loops are executed but in the end the same amount of checks/total
> >iterations have to be run.
> >
> >I was talking to someone and he suggested you could just check Xen at
> >early boot and then check other modules later.
> 
> What's wrong with:
> 
> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> index 705a917abf..ecd09ec698 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -254,6 +254,10 @@ struct bootmodule __init
> *add_boot_module(bootmodule_kind kind,
>                  mod->domU = false;
>              return mod;
>          }
> +
> +        if ((mod->start >= start) &&
> +            (mod->start < (start + size)))
> +            printk("WARNING: modules...\n");
>      }
> 
>      mod = &mods->module[mods->nr_mods++];
> 
> Cheers,
> 
> -- 
> Julien Grall

For that, you'd need to either check the start and end of the added
module or the start of both.  So something like:

if ( ((mod->start >= start) && (mod->start < (start + size))) ||
     ((start >= mod->start) && (start < (mod->start + mod->size))) )
    printk("WARNING: ...");

If you don't you run the risk of having a module overlap but not at the
start of the added module (unless there's a guaranteed order).  You're
still running all of the checks from what I can tell, just not in nested
for loop so. Plus I'm not sure how many times add_boot_module gets run
and the "mod->kind == kind .." if statement gets run and is true.
If the above is true, wouldn't that cause extra checks for the for loop
iterations before it was true?

Brian

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