On 07/14/2017 04:03 AM, Mike Kravetz wrote: > On 07/13/2017 12:11 PM, Vlastimil Babka wrote: >> [+CC linux-api] >> >> On 07/13/2017 05:58 PM, Mike Kravetz wrote: >>> mremap will create a 'duplicate' mapping if old_size == 0 is >>> specified. Such duplicate mappings make no sense for private >>> mappings. If duplication is attempted for a private mapping, >>> mremap creates a separate private mapping unrelated to the >>> original mapping and makes no modifications to the original. >>> This is contrary to the purpose of mremap which should return >>> a mapping which is in some way related to the original. >>> >>> Therefore, return EINVAL in the case where if an attempt is >>> made to duplicate a private mapping. >>> >>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> >> Acked-by: Vlastimil Babka <vbabka@xxxxxxx> >> > In another e-mail thread, Andrea makes the case that mremap(old_size == 0) > of private file backed mappings could possibly be used for something useful. > For example to create a private COW mapping. Of course, a better way to do > this would be simply using the fd to create a private mapping. > > If returning EINVAL for all private mappings is too general, the following > patch adds a check to only return EINVAL for private anon mappings. > > mm/mremap: Fail map duplication attempts for private anon mappings > > mremap will create a 'duplicate' mapping if old_size == 0 is > specified. Such duplicate mappings make no sense for private > anonymous mappings. If duplication is attempted for a private > anon mapping, mremap creates a separate private mapping unrelated > to the original mapping and makes no modifications to the original. > This is contrary to the purpose of mremap which should return a > mapping which is in some way related to the original. > > Therefore, return EINVAL in the case where an attempt is made to > duplicate a private anon mapping. > > Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx> > --- > mm/mremap.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/mm/mremap.c b/mm/mremap.c > index cd8a1b1..586ea3d 100644 > --- a/mm/mremap.c > +++ b/mm/mremap.c > @@ -383,6 +383,14 @@ static struct vm_area_struct *vma_to_resize(unsigned long addr, > if (!vma || vma->vm_start > addr) > return ERR_PTR(-EFAULT); > > + /* > + * !old_len is a special case where a mapping is 'duplicated'. > + * Do not allow this for private anon mappings. > + */ > + if (!old_len && vma_is_anonymous(vma) && > + !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) > + return ERR_PTR(-EINVAL); Sounds better compared to rejecting everything private. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
