Hello Michael, On Thu, 28 Jan 2021 21:50:23 +0100, "Michael Kerrisk (man-pages)" <mtk.manpa...@gmail.com> wrote: > Thanks for your patch revision. I've merged it, and have > done some light editing, but I still have a question: > > On 1/23/21 5:11 PM, Stephen Kitt wrote: > > [...] > > > +.SH ERRORS > > > +.TP > > +.B EMFILE > > +The per-process limit on the number of open file descriptors has been > > reached +(see the description of > > +.B RLIMIT_NOFILE > > +in > > +.BR getrlimit (2)). > > I think there was already a question about this error, but > I still have a doubt. > > A glance at the code tells me that indeed EMFILE can occur. > But how can the reason be because the limit on the number > of open file descriptors has been reached? I mean: no new > FDs are being opened, so how can we go over the limit. I think > the cause of this error is something else, but what is it?
Here’s how I understand the code that can lead to EMFILE:
* in __close_range(), if CLOSE_RANGE_UNSHARE is set, call unshare_fd() with
CLONE_FILES to clone the fd table
* unshare_fd() calls dup_fd()
* dup_fd() allocates a new fdtable, and if the resulting fdtable ends up
being too small to hold the number of fds calculated by
sane_fdtable_size(), fails with EMFILE
I suspect that, given that we’re starting with a valid fdtable, the only way
this can happen is if there’s a race with sysctl_nr_open being reduced.
Incidentally, isn’t this comment in file.c somewhat misleading?
/*
* If the requested range is greater than the current maximum,
* we're closing everything so only copy all file descriptors
* beneath the lowest file descriptor.
*/
As I understand it, dup_fd() will always copy any open file descriptor
anyway, it won’t stop at max_unshare_fds if that’s lower than the number of
open fds (thanks to save_fdtable_size())...
Regards,
Stephen
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