On Mon, Jul 18, 2022 at 04:52:59PM +0800, Guozihua (Scott) wrote: > On 2022/7/14 15:33, Guozihua (Scott) wrote: > > Hi Community, > > > > Recently we noticed the removal of flag O_NONBLOCK on /dev/random by > > commit 30c08efec888 ("random: make /dev/random be almost like > > /dev/urandom"), it seems that some of the open_source packages e.g. > > random_get_fd() of util-linux and __getrandom() of glibc. The man page > > for random() is not updated either. > > Correction: I mean various open source packages are still using O_NONBLOCK > flag while accessing /dev/random > > > > Would anyone please kindly provide some background knowledge of this > > flag and it's removal? Thanks! > > This was changed a while ago, in v5.6. /dev/random no longer recognizes O_NONBLOCK because it no longer blocks, except before the entropy pool has been initialized. (I don't know why O_NONBLOCK stopped being recognized *before* the entropy pool has been initialized; it's either an oversight, or it was decided it doesn't matter. Probably the latter, since I can't think of a real use case for using O_NONBLOCK on /dev/random.) The random(4) man page is indeed in need of an update, not just for this reason but for some other reasons too. The util-linux code which you mentioned is opening /dev/random with O_NONBLOCK if opening /dev/urandom fails, which is pretty much pointless. Perhaps the author thought that /dev/random with O_NONBLOCK is equivalent to /dev/urandom (it's not). The glibc code, if you mean sysdeps/mach/hurd/getrandom.c, is actually code written for GNU Hurd, not for Linux, so it's not relevant. - Eric