On Wed, Apr 08, 2020 at 10:00:40PM +1000, Aleksa Sarai wrote: > On 2020-04-07, Josh Triplett <josh@xxxxxxxxxxxxxxxx> wrote: > > Some applications want to prevent the usual "lowest available fd" > > allocation from allocating certain file descriptors. For instance, they > > may want to prevent allocation of a closed fd 0, 1, or 2 other than via > > dup2/dup3, or reserve some low file descriptors for other purposes. > > > > Add a prctl to increase the minimum fd and return the previous minimum. > > > > System calls that allocate a specific file descriptor, such as > > dup2/dup3, ignore this minimum. > > > > exec resets the minimum fd, to prevent one program from interfering with > > another program's expectations about fd allocation. > > Why is it implemented as an "increase the value" interface? It feels > like this is meant to avoid some kind of security trap (with a library > reducing the value) but it means that if you want to temporarily raise > the minimum fd number it's not possible (without re-exec()ing yourself, > which is hardly a fun thing to do). > > Then again, this might've been discussed before and I missed it... It was: the previous version was a "get" and "set" interface. That interface didn't allow for the possibility that something else in the process had already set a minimum. This new atomic increase interface (which also serves as a "get" interface if you pass 0) makes it possible for a userspace library to reserve a range. (You have no guarantee about previously allocated descriptors in that range, but you know that no *new* automatically allocated descriptors will appear in that range, which suffices; userspace can do the rest.) - Josh Triplett
