On Sat, Jul 24, 2021 at 12:52:34PM -0700, Linus Torvalds wrote: > ... > > + if (fault_in_user_pages(start, len, true) != len) > > + return -EFAULT; > > Looking at this once more, I think this is likely wrong. > > Why? > > Because any user can/should only care about at least *part* of the > area being writable. > > Imagine that you're doing a large read. If the *first* page is > writable, you should still return the partial read, not -EFAULT. Agreed. > So I think the code needs to return 0 if _any_ fault was successful. s/any/the first/... The same goes for fault-in for read, of course; I've a half-baked conversion to such semantics (-EFAULT vs. 0; precise length is unreliable anyway, especially if you have sub-page failure areas), need to finish and post it...
