On 24/08/17 19:03, Michael Kerrisk (man-pages) wrote: > Hi Neil, > > On 24 August 2017 at 06:07, NeilBrown <neilb@xxxxxxxx> wrote: >> On Wed, Aug 23 2017, Ian Kent wrote: >> >>> >>> That inconsistency has bothered me for quite a while now. >>> >>> It was carried over from the autofs module behavior when automounting >>> support was added to the VFS. What's worse is it prevents the use of >>> the AT_NO_AUTOMOUNT flag from working properly with fstatat(2) and with >>> statx(). >>> >>> There is some risk in changing that so it does work but it really does >>> need to work to enable userspace to not trigger an automount by using >>> this flag. >>> >>> So that's (hopefully) going to change soonish, see: >>> http://ozlabs.org/~akpm/mmotm/broken-out/autofs-fix-at_no_automount-not-being-honored.patch >>> >>> The result should be that stat family calls don't trigger automounts except >>> for fstatat(2) and statx() which will require the AT_NO_AUTOMOUNT flag. >>> >> >> oooh, yes. That's much better - thanks. >> >> We should make sure that change gets into the man pages... >> >> First however, we should probably correct the man page! >> stat.2 says: >> >> >> NOTES >> On Linux, lstat() will generally not trigger automounter >> action, whereas stat() will (but see the description of >> fstatat() AT_NO_AUTOMOUNT fag, above). >> >> which is wrong: lstat and stat treat automounts the same. >> @Michael: do you recall why you inserted that text? The commit message >> in commit 1ef5b2805471 ("stat.2: Cosmetic reworking of timestamp >> discussion in NOTES") is not very helpful. > > That commit really was just cosmetic changes. The change that > introduced the text was 82d2be3d9d66b7, based on a note from Peter > Anvin: Indeed, that was correct for autofs v3 but we're at autofs v5 now and a lot has changed over time (the commit is from 2008). All I can do is apologize for not also checking the man pages and trying to keep them up to date. Let's just work on making them accurate now. > > [[ > > > Additionally, you may want to make a note in the stat/lstat man page tha > t on > > > Linux, lstat(2) will generally not trigger automounter action, whereas > > > stat(2) will. > > > > I don't understand this last piece. Can you say some more. (I'm not > > familiar with automounter details.) > > An automounter (either an explicit one, like autofs, or an implicit > one, such as are used by AFS or NFSv4) is something that triggers > a mount when something is touched. > > However, it's undesirable to automount, say, everyone's home > directory just because someone opened up /home in their GUI > browser or typed "ls -l /home". The early automounters simply > didn't list the contents until you accessed it by name; > this is still the case when you can't enumerate a mapping > (say, all DNS names under /net). However, this is extremely > inconvenient, too. > > The solution we ended up settling on is to create something > that looks like a directory (i.e. reports S_IFDIR in stat()), > but behaves somewhat like a symlink. In particular, when it is > accessed in a way where a symlink would be dereferenced, > the automount triggers and the directory is mounted. However, > system calls which do *not* cause a symlink to be dereferenced, > like lstat(), also do not cause the automounter to trigger. > This means that "ls -l", or a GUI file browser, can see a list > of directories without causing each one of them to be automounted. > > -hpa > ]] > > Cheers, > > Michael > >> I propose correcting to >> >> NOTES: >> On Linux, lstat() nor stat() act as though AT_NO_AUTOMOUNT was set >> and will not trigger automounter action for direct automount >> points, though they may (prior to 4.14) for indirect automount >> points. >> >> >> The more precise details, that automount action for indirect automount >> points is not triggered when the 'browse' option is used, is probably >> not necessary. >> >> Ian: if you agree with that text, and Michael doesn't provide alternate >> evidence, I'll submit a formal patch for the man page.... or should we >> just wait until the patch actually lands? >> >> Thanks, >> NeilBrown >> > > >
