linux-kernel - Re: [PATCH net 1/1] ethtool: Fix mod state of verbose no

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Date:   Thu, 5 Oct 2023 15:36:33 +0200
From:   Simon Horman <horms@...nel.org>
To:     Köry Maincent <kory.maincent@...tlin.com>
Cc:     Michal Kubecek <mkubecek@...e.cz>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Maxime Chevallier <maxime.chevallier@...tlin.com>,
        stable@...r.kernel.org, thomas.petazzoni@...tlin.com,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net 1/1] ethtool: Fix mod state of verbose no_mask bitset

On Thu, Oct 05, 2023 at 10:03:49AM +0200, Köry Maincent wrote:
> Hello Simon,
> 
> Thank for your review.
> 
> On Wed, 4 Oct 2023 13:07:14 +0200
> Simon Horman <horms@...nel.org> wrote:
> 
> > On Tue, Oct 03, 2023 at 10:56:52AM +0200, Köry Maincent wrote:
> > > From: Kory Maincent <kory.maincent@...tlin.com>
> >
> > > @@ -448,8 +450,11 @@ ethnl_update_bitset32_verbose(u32 *bitmap, unsigned
> > > int nbits, }
> > >  
> > >  	no_mask = tb[ETHTOOL_A_BITSET_NOMASK];
> > > -	if (no_mask)
> > > -		ethnl_bitmap32_clear(bitmap, 0, nbits, mod);
> > > +	if (no_mask) {
> > > +		tmp = kcalloc(nbits, sizeof(u32), GFP_KERNEL);
> > > +		memcpy(tmp, bitmap, nbits);  
> > 
> > Hi Köry,
> > 
> > I'm no expert on etnhl bitmaps. But the above doesn't seem correct to me.
> > Given that sizeof(u32) == 4:
> > 
> > * The allocation is for nbits * 4 bytes
> > * The copy is for its for nbits bytes
> > * I believe that bitmap contains space for the value followed by a mask.
> >   So it seems to me the size of bitmap, in words, is
> >   DIV_ROUND_UP(nbits, 32) * 2
> >   And in bytes: DIV_ROUND_UP(nbits, 32) * 16
> >   But perhaps only half is needed if only the value part of tmp is used.
> > 
> > If I'm on the right track here I'd suggest helpers might be in order.
> 
> You are right I should use the same alloc as ethnl_update_bitset with tmp
> instead of bitmap32:
> 
>         u32 small_bitmap32[ETHNL_SMALL_BITMAP_WORDS];                      
>         u32 *bitmap32 = small_bitmap32; 
>         if (nbits > ETHNL_SMALL_BITMAP_BITS) {                             
>                 unsigned int dst_words = DIV_ROUND_UP(nbits, 32);          
>                                                                            
>                 bitmap32 = kmalloc_array(dst_words, sizeof(u32), GFP_KERNEL);
>                 if (!bitmap32)                                             
>                         return -ENOMEM;                                    
>         }   
> 
> But I am still wondering if it needs to be double as you said for the size of
> the value followed by the mask. Not sure about it, as ethnl_update_bitset does
> not do it. 

If you only need the value, then I don' think you need to x2 the allocation.
But I could be wrong.