Re: [PATCH 1/1] mm/madvise: initialize prev pointer in madvise_walk_vmas

[Lance Yang]

On 2025/6/17 15:54, David Hildenbrand wrote:
> On 17.06.25 04:05, Lance Yang wrote:
> > From: Lance Yang <lance.yang@xxxxxxxxx>
> >
> > The prev pointer was uninitialized, which could lead to undefined 
> > behavior
> > where its address is taken and passed to the visit() callback without 
> > being
> > assigned a value.
>
> So, we are passing the pointer value to visit(), which is not undefined 
> behavior.
>
> The issue would be if anybody takes a look at the value stored at that 
> pointer. Because, already passing an uninitialized value to a (non- 
> inlined) function is undefined behavior according to C.

Yes, that is precisely is what I am concerned about ;)

> In madvise_update_vma()->vma_modify_flags_name() we do exactly that, 
> correct?
>
>      vma = vma_modify_flags_name(&vmi, *prev, ...
>
> We should use Fixes: then.

Exactly, I missed that. Will add the "Fixes" tag and send out a new version.

> Acked-by: David Hildenbrand <david@xxxxxxxxxx>

Thanks!
Lance

> > Initializing it to NULL makes the code safer and prevents potential bugs
> > if a future callback function attempts to read from it.
> >
> > Signed-off-by: Lance Yang <lance.yang@xxxxxxxxx>
> > ---
> >   mm/madvise.c | 4 ++--
> >   1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/madvise.c b/mm/madvise.c
> > index 267d8e4adf31..c87325000303 100644
> > --- a/mm/madvise.c
> > +++ b/mm/madvise.c
> > @@ -1536,10 +1536,10 @@ int madvise_walk_vmas(struct mm_struct *mm, 
> > unsigned long start,
> >                      struct vm_area_struct **prev, unsigned long start,
> >                      unsigned long end, void *arg))
> >   {
> > +    struct vm_area_struct *prev = NULL;
> >       struct vm_area_struct *vma;
> > -    struct vm_area_struct *prev;
> > -    unsigned long tmp;
> >       int unmapped_error = 0;
> > +    unsigned long tmp;
> >       int error;
> >       /*