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

Hui Su
2020-10-08T12:12:55Z

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@163.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
> >
> >
> >