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epoll_ctl.2: BUGS: EPOLLWAKEUP is silently ignored without CAP_BLOCK_SUSPEND 

If the caller does not have CAP_BLOCK_SUSPEND, then EPOLLWAKEUP
is silently ignored.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2014-07-07 08:39:22 +02:00
parent d3695ae2c7
commit 8ab32b4785
1 changed files with 28 additions and 0 deletions
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man2/epoll_ctl.2
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@ -174,6 +174,7 @@ until the next call to
on the same on the same
.BR epoll (7) .BR epoll (7)
file descriptor. file descriptor.
See also BUGS.
.SH RETURN VALUE .SH RETURN VALUE
When successful, When successful,
.BR epoll_ctl () .BR epoll_ctl ()
@ -270,6 +271,33 @@ when using
Applications that need to be portable to kernels before 2.6.9 Applications that need to be portable to kernels before 2.6.9
should specify a non-null pointer in should specify a non-null pointer in
.IR event . .IR event .
If
.B EPOLLWAKEUP
is specified in
.IR flags ,
but the caller does not have the
.BR CAP_BLOCK_SUSPEND
capability, then the
.B EPOLLWAKEUP
flag is
.IR "silently ignored" .
This unfortunate behavior is necessary because no validity
checks were performed on the
.IR flags
argument in the original implementation, and the addition of the
.B EPOLLWAKEUP
with a check that caused the call to fail if the caller did not have the
.B CAP_BLOCK_SUSPEND
capability caused a breakage in at least one existing user-space
application that happened to randomly (and uselessly) specify this bit.
.\" commit a8159414d7e3af7233e7a5a82d1c5d85379bd75c (behavior change)
.\" https://lwn.net/Articles/520198/
A robust application should therefore double check that it has the
.B CAP_BLOCK_SUSPEND
capability if attempting to use the
.B EPOLLWAKEUP
flag.
.SH SEE ALSO .SH SEE ALSO
.BR epoll_create (2), .BR epoll_create (2),
.BR epoll_wait (2), .BR epoll_wait (2),