On Thu, Jan 07, 2021 at 10:47:07AM -0800, Linus Torvalds wrote: > On Thu, Jan 7, 2021 at 10:34 AM Al Viro <v...@zeniv.linux.org.uk> wrote: > > > > I'm not sure it's the best approach, TBH. How about simply > > for (walk = head; walk; ufds += walk->len, walk = walk->next) { > > if (copy_to_user(ufds, walk->entries, > > walk->len * sizeof(struct pollfd)) > > goto out_fds; > > } > > in there? It's both simpler (obviously matches the copyin side) and > > might very well be faster... > > I started doing that, but .. Nope. > > It's not copying the whole entry. It's literally just modifying one > 16-bit word in each entry. > > Now, the "whole entry" is just 8 bytes, so it's possible that it would > actually be faster to do a copy of the whole thing rather than write > just the 16 bits. But I got very nervous about it, because I could > easily see some threaded app actually changing the 'fd' (or the > 'event' field) in place (ie writing -1 to it as they close and re-use > it)
Point... Pity, that.