On Fri, Dec 04, 2020 at 03:11:54PM +0100, David Hildenbrand wrote:
On 03.12.20 09:53, Liu Zixian wrote:
On success, mmap should return the begin address of newly mapped area, but patch "mm: mmap: merge vma after call_mmap() if possible" set vm_start of newly merged vma to return value addr. Users of mmap will get wrong address if vma is merged after call_mmap(). We fix this by moving the assignment to addr before merging vma.
Fixes: d70cec898324 ("mm: mmap: merge vma after call_mmap() if possible") Signed-off-by: Liu Zixian liuzixian4@huawei.com v2: We want to do "addr = vma->vm_start;" unconditionally, so move assignment to addr before if(unlikely) block. mm/mmap.c | 26 ++++++++++++-------------- 1 file changed, 12 insertions(+), 14 deletions(-)
diff --git a/mm/mmap.c b/mm/mmap.c index d91ecb00d38c..5c8b4485860d 100644 +++ b/mm/mmap.c @@ -1808,6 +1808,17 @@ unsigned long mmap_region(struct file *file, unsigned long addr, if (error) goto unmap_and_free_vma;
/* Can addr have changed??** Answer: Yes, several device drivers can do it in their* f_op->mmap method. -DaveM* Bug: If addr is changed, prev, rb_link, rb_parent should* be updated for vma_link()*/Why do we tolerate device drivers doing such stuff at all? WARN_ON_ONCE() is just as good as BUG_ON() in many environments. If we support it, then no warning. If we don't support it, then why tolerate it?
The commit that introduced this seemed pretty clear it is to catch possibly wrong drivers. I suppose the idea was to give a migration time where things would "work" and drivers could be fixed. Since it has now been 8 years it should be either dropped or turned into:
/* Drivers are not permitted to change vm_start */ if (WARN_ON(addr != vma->vm_start)) { err = EINVAL goto unmap_and_free_vma }
commit 2897b4d29d9fca82a57b09b8a216a5d604966e4b Author: Joonsoo Kim js1304@gmail.com Date: Wed Dec 12 13:51:53 2012 -0800
mm: WARN_ON_ONCE if f_op->mmap() change vma's start address
During reviewing the source code, I found a comment which mention that after f_op->mmap(), vma's start address can be changed. I didn't verify that it is really possible, because there are so many f_op->mmap() implementation. But if there are some mmap() which change vma's start address, it is possible error situation, because we already prepare prev vma, rb_link and rb_parent and these are related to original address.
So add WARN_ON_ONCE for finding that this situtation really happens.
Jason