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); + if (is_vm_hugetlb_page(vma)) return ERR_PTR(-EINVAL); -- 2.7.5 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
