On Sun, Apr 30, 2023 at 3:26 PM Lorenzo Stoakes <lstoakes@xxxxxxxxx> wrote: > > In order for a F_SEAL_WRITE sealed memfd mapping to have an opportunity to > clear VM_MAYWRITE, we must be able to invoke the appropriate vm_ops->mmap() > handler to do so. We would otherwise fail the mapping_map_writable() check > before we had the opportunity to avoid it. Is there any reason this can't go before patch 3? If I'm understanding correctly, a comment like the following might make this a lot more comprehensible: > > This patch moves this check after the call_mmap() invocation. Only memfd > actively denies write access causing a potential failure here (in > memfd_add_seals()), so there should be no impact on non-memfd cases. > > This patch makes the userland-visible change that MAP_SHARED, PROT_READ > mappings of an F_SEAL_WRITE sealed memfd mapping will now succeed. > > Link: https://bugzilla.kernel.org/show_bug.cgi?id=217238 > Signed-off-by: Lorenzo Stoakes <lstoakes@xxxxxxxxx> > --- > mm/mmap.c | 12 ++++++------ > 1 file changed, 6 insertions(+), 6 deletions(-) > > diff --git a/mm/mmap.c b/mm/mmap.c > index 646e34e95a37..1608d7f5a293 100644 > --- a/mm/mmap.c > +++ b/mm/mmap.c > @@ -2642,17 +2642,17 @@ unsigned long mmap_region(struct file *file, unsigned long addr, > vma->vm_pgoff = pgoff; > > if (file) { > - if (is_shared_maywrite(vm_flags)) { > - error = mapping_map_writable(file->f_mapping); > - if (error) > - goto free_vma; > - } > - > vma->vm_file = get_file(file); > error = call_mmap(file, vma); > if (error) > goto unmap_and_free_vma; > /* vm_ops->mmap() may have changed vma->flags. Check for writability now. */ > + if (vma_is_shared_maywrite(vma)) { > + error = mapping_map_writable(file->f_mapping); > + if (error) > + goto close_and_free_vma; > + } > + Alternatively, if anyone is nervous about the change in ordering here, there could be a whole new vm_op like adjust_vma_flags() that happens before any of this. --Andy
