Linus Torvalds wrote on Thu, Jan 24, 2019: > I've reverted the 'let's try to just remove the code' part in my tree. > But I didn't apply the two other patches yet. Any final comments > before that should happen?
I mentionned when sending the updated version that just checking file permission might not be enough, e.g. a git tree is full of read-only objects that someone might want to preload and think we might really want to check both despite the overhead in the denied case. Josh agreed and I meant to send a new version since nothing was happening but work priorities got the better of me, and I was kind of waiting for the ltp testcases[1] as well because aside from the few tests I ran by hand I'm not sure the few hours of ltp/xfstests Jiri ran did much but this is probably going to be a chicken-or-egg problem.. [1] https://github.com/linux-test-project/ltp/issues/461 Jiri Kosina wrote on Thu, Jan 24, 2019: > On Thu, 24 Jan 2019, Linus Torvalds wrote: > > > Side note: the inode_permission() addition to can_do_mincore() in that > > patch 0002, seems to be questionable. We do > > > > +static inline bool can_do_mincore(struct vm_area_struct *vma) > > +{ > > + return vma_is_anonymous(vma) > > + || (vma->vm_file && (vma->vm_file->f_mode & FMODE_WRITE)) > > + || inode_permission(file_inode(vma->vm_file), MAY_WRITE) == > > 0; > > +} > > > > note how it tests whether vma->vm_file is NULL for the FMODE_WRITE > > test, but not for the inode_permission() test. > > > > So either we test unnecessarily in the second line, or we don't > > properly test it in the third one. > > > > I think the "test vm_file" thing may be unnecessary, because a > > non-anonymous mapping should always have a file pointer and an inode. > > But I could imagine some odd case (vdso mapping, anyone?) that > > doesn't have a vm_file, but also isn't anonymous. > > Hmm, good point. > > So dropping the 'vma->vm_file' test and checking whether given vma is > special mapping should hopefully provide the desired semantics, shouldn't > it? I think it's probably better to keep this simple, if we're going to check something before accessing vm_file we might as well directly check it. I was thinking of something along the lines of: return vma_is_anonymous(vma) || (vma->vm_file && (inode_owner_or_capable(file_inode(vma->vm_file)) || inode_permission(file_inode(vma->vm_file), MAY_WRITE) == 0)); I dropped the first f_mode check because none of the known mincore users open the files read-write, and the check is redundant with inode_permission() so while it would probably be an optimisation in some cases I do not think it is useful in practice. On the other hand, I have no idea how expensive the inode_permission and owner checks really are - do they try to refresh attributes on a networked filesystem or would it trust the cache or is it fs dependant? Honestly this is more a case of "the people who's be interested in seeing this have no idea what they're doing" than lack of interest.. I wouldn't mind if there were tests doing mincore on a bunch of special files/mappings but I just tried on a few regular files by hand, this isn't proper coverage; I'll try to take more time to test various mappings today (JST). Thanks, -- Dominique
