Thank you for looking at this. I appreciate the scrutiny.
* David Hildenbrand <david@xxxxxxxxxx> [201216 09:58]:
> On 15.12.20 16:54, Liam R. Howlett wrote:
> > do_mmap() will unlock the necessary VMAs. There is also a bug in the
> > loop which will evaluate as false and not unlock any VMAs anyways.
>
> If there is a BUG, do we have a Fixes: tag? Also
The bug would never show up as it is masked by do_mmap() unlocking the
necessary range. Although there is a bug in this code, the code does
not cause an issue as it won't execute so should I have a Fixes tag?
The code works and what I've done is remove a chunk of code that never
runs.
>
> 1. Can we fix the bug separately first?
I think it is safer to remove unexecuted code than enable it and then
remove it.
> 2. Can we have a better description on what the bug actually is
> "evaluate as false"? What is the result of the bug?
The bug is in the for loop test expression that I removed in the patch.
Here is the long explaination of why the loop has never run.
Line 2982: if (start + size <= start
Line 2983: goto out;
size is positive.
Line 2992: vma = find_vma(mm, start);
Look up the first VMA which satisfies start < vm_end
Line 2997: if (start < vma->vm_start)
Line 2998: goto out;
So now vma->vm_start >= start.
If vma->vm_start > start, then there are no VMAs in that area, otherwise
it would have been returned by find_vma().
So we can say that vma->vm_start == start.
Line 3033: for (tmp = vma; tmp->vm_start >= start + size;
Line 3034: tmp = tmp->vm_next) {
This is the for loop with the error in the test expression.
tmp->vm_start == start which cannot be >= (start + size).
I believe the intention was to loop through vmas in the range of start
to (start + size) and unlock them.
The result of the bug is no VMA is unlocked in this fuction. But that
doesn't matter as they are unlocked later in the call chain - which is
why this code works as intended.
>
> CCing some people that might know if this is actually a sane change.
> Skimming over do_mmap(), it's not immediately clear to me that
> "do_mmap() will unlock the necessary VMAs".
Ah, yes. That is understandable.
do_mmap() L1583 -> mmap_region() L1752 -> munmap_vma_range() ->
do_munmap() -> __do_munmap() loop at 2891 to unlock the range.
Would you like me to add this call chain to the changelog?
>
> >
> > Signed-off-by: Liam R. Howlett <Liam.Howlett@xxxxxxxxxx>
> > ---
> > mm/mmap.c | 18 +-----------------
> > 1 file changed, 1 insertion(+), 17 deletions(-)
> >
> > diff --git a/mm/mmap.c b/mm/mmap.c
> > index 5c8b4485860de..f7fecb77f84fd 100644
> > --- a/mm/mmap.c
> > +++ b/mm/mmap.c
> > @@ -3025,25 +3025,9 @@ SYSCALL_DEFINE5(remap_file_pages, unsigned long, start, unsigned long, size,
> >
> > flags &= MAP_NONBLOCK;
> > flags |= MAP_SHARED | MAP_FIXED | MAP_POPULATE;
> > - if (vma->vm_flags & VM_LOCKED) {
> > - struct vm_area_struct *tmp;
> > + if (vma->vm_flags & VM_LOCKED)
> > flags |= MAP_LOCKED;
> >
> > - /* drop PG_Mlocked flag for over-mapped range */
> > - for (tmp = vma; tmp->vm_start >= start + size;
This should probably be less than ---^
> > - tmp = tmp->vm_next) {
> > - /*
> > - * Split pmd and munlock page on the border
> > - * of the range.
> > - */
> > - vma_adjust_trans_huge(tmp, start, start + size, 0);
> > -
> > - munlock_vma_pages_range(tmp,
> > - max(tmp->vm_start, start),
> > - min(tmp->vm_end, start + size));
> > - }
> > - }
> > -
> > file = get_file(vma->vm_file);
> > ret = do_mmap(vma->vm_file, start, size,
> > prot, flags, pgoff, &populate, NULL);
> >
>
>
> --
> Thanks,
>
> David / dhildenb
>
>
