linux-kernel - Re: [PATCH] mm/vmalloc.c: check the addr first

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Date:   Thu, 8 Oct 2020 20:11:25 +0800
From:   Hui Su <sh_def@....com>
To:     Ira Weiny <ira.weiny@...el.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/vmalloc.c: check the addr first

On Mon, Sep 28, 2020 at 11:04:34AM -0700, Ira Weiny wrote:
> On Mon, Sep 28, 2020 at 12:33:37AM +0800, Hui Su wrote:
> > As the comments said, if @addr is NULL, no operation
> > is performed, check the addr first in vfree() and
> > vfree_atomic() maybe a better choice.
> 
> I don't see how this change helps anything.  kmemleak_free() checks addr so no
> danger there.  Also kmemleak_free() contains a pr_debug() which some might find
> useful.
> 
> Ira
> 
Thanks for your explanations, Ira.

Firstly, as you have said, the kmemleak_free() only unregister a previously
registered object(which shouldn't be NULL), and kmemleak_free() alse check the
ptr whether is NULL, it does no danger here.

Secondly, as you said kmemleak_free() contains a pr_debug() which will be useful,
and i don't think so because this patch only filter out the NULL pointer case, which
may not be useful?(maybe not right?)

Thirdly, let's see kimage_file_prepare_segments() and kimage_file_post_load_cleanup()
in kernel/kexec_file.c.

kimage_file_prepare_segments() uses 'out' label in several places, when reading
files from kernel_fd or initrd_fd or memdup_user cmdline_buf.
The out label will run kimage_file_post_load_cleanup(), which will do vfree()
for ->kernel_buf and ->initrd_buf and ->cmdline_buf and so on...

Supposing the arch_kexec_kernel_image_probe() failed, and it goto out, then it
still vfree(->initrd_buf). In this case we can return faster if we check param
addr first in vfree().

In this case maybe we should use more Precise labels?

All above is my opinion, thanks.

> > 
> > Signed-off-by: Hui Su <sh_def@....com>
> > ---
> >  mm/vmalloc.c | 11 ++++++-----
> >  1 file changed, 6 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> > index be4724b916b3..1cf50749a209 100644
> > --- a/mm/vmalloc.c
> > +++ b/mm/vmalloc.c
> > @@ -2305,10 +2305,11 @@ void vfree_atomic(const void *addr)
> >  {
> >  	BUG_ON(in_nmi());
> >  
> > -	kmemleak_free(addr);
> > -
> >  	if (!addr)
> >  		return;
> > +
> > +	kmemleak_free(addr);
> > +
> >  	__vfree_deferred(addr);
> >  }
> >  
> > @@ -2340,13 +2341,13 @@ void vfree(const void *addr)
> >  {
> >  	BUG_ON(in_nmi());
> >  
> > +	if (!addr)
> > +		return;
> > +
> >  	kmemleak_free(addr);
> >  
> >  	might_sleep_if(!in_interrupt());
> >  
> > -	if (!addr)
> > -		return;
> > -
> >  	__vfree(addr);
> >  }
> >  EXPORT_SYMBOL(vfree);
> > -- 
> > 2.25.1
> > 
> > 
> >