Re: [PATCH v3 2/3] mmap: Fix error paths with dup_anon_vma()

[Lorenzo Stoakes <lstoakes@gmail.com>]

On Mon, Oct 02, 2023 at 09:26:03AM +0200, Vlastimil Babka wrote:
> On 9/30/23 00:28, Lorenzo Stoakes wrote:
> > On Fri, Sep 29, 2023 at 02:30:40PM -0400, Liam R. Howlett wrote:
> >> When the calling function fails after the dup_anon_vma(), the
> >> duplication of the anon_vma is not being undone.  Add the necessary
> >> unlink_anon_vma() call to the error paths that are missing them.
> >>
> >> This issue showed up during inspection of the error path in vma_merge()
> >> for an unrelated vma iterator issue.
> >>
> >> Users may experience increased memory usage, which may be problematic as
> >> the failure would likely be caused by a low memory situation.
> >>
> >> Fixes: d4af56c5c7c6 ("mm: start tracking VMAs with maple tree")
> >> Cc: stable@vger.kernel.org
> >> Cc: Jann Horn <jannh@google.com>
> >> Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
> >> ---
> >>  mm/mmap.c | 30 ++++++++++++++++++++++--------
> >>  1 file changed, 22 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/mm/mmap.c b/mm/mmap.c
> >> index acb7dea49e23..f9f0a5fe4db4 100644
> >> --- a/mm/mmap.c
> >> +++ b/mm/mmap.c
> >> @@ -583,11 +583,12 @@ static inline void vma_complete(struct vma_prepare *vp,
> >>   * dup_anon_vma() - Helper function to duplicate anon_vma
> >>   * @dst: The destination VMA
> >>   * @src: The source VMA
> >> + * @dup: Pointer to the destination VMA when successful.
> >>   *
> >>   * Returns: 0 on success.
> >
> > Being a bit nitpicky/refactory here, but anon_vma_clone() appears to have
> > two possible return values - 0 for success, and -ENOMEM.
> >
> > As a result, it's not really gaining us much passing through this value.
> >
> > It'd be nice if dup_anon_vma() and anon_vma_clone() were therefore updated
> > to instead return NULL on ENOMEM and the dst otherwise.
>
> But we also need to represent that dup_anon_vma() had nothing to do, because
> "(src->anon_vma && !dst->anon_vma)" was false, and in that case we should
> not be returning dst from there?
>
> So maybe we could return NULL for that case and ERR_PTR(ret) for the -ENOMEM
> from anon_vma_clone() ?

Yeah, you're right, actually I think that would probably be the best
approach as you'd both eliminate the awkward out parameter but retain the
fact that there's 3 possible return states (dup'd, no need to dup, error).

>
> > Then we could de-clunk this whole code path, and the quite natural fact of
> > 'thing didn't return a pointer therefore had no memory to allocate it' fals
> > out.
> >
> > But this isn't exactly an earth-shattering concern :)
> >
>