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seccomp_unotify.2: Add caveats regarding emulation of blocking system calls 

Reported-by: Sargun Dhillon <sargun@sargun.me>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2020-11-02 14:10:44 +01:00
parent 1b5592f534
commit 911789ee76
1 changed files with 46 additions and 0 deletions
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man2/seccomp_unotify.2
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@ -1048,6 +1048,52 @@ a write by the supervisor into the target's memory can
.B never
be considered safe.
.\"
.SS Caveats regarding blocking system calls
Suppose that the target performs a blocking system call (e.g.,
.BR accept (2))
that the supervisor should handle.
The supervisor might then in turn execute the same blocking system call.
.PP
In this scenario,
it is important to note that if the target's system call is now
interrupted by a signal, the supervisor is
.I not
informed of this.
If the supervisor does not take suitable steps to
actively discover that the target's system call has been canceled,
various difficulties can occur.
Taking the example of
.BR accept (2),
the supervisor might remain blocked in its
.BR accept (2)
holding a port number that the target
(which, after the interruption by the signal handler,
perhaps closed its listening socket) might expect to be able to reuse in a
.BR bind (2)
call.
.PP
Therefore, when the supervisor wishes to emulate a blocking system call,
it must do so in such a way that it gets informed if the target's
system call is interrupted by a signal handler.
For example, if the supervisor itself executes the same
blocking system call, then it could employ a separate thread
that uses the
.B SECCOMP_IOCTL_NOTIF_ID_VALID
operation to check if the target is still blocked in its system call.
Alternatively, in the
.BR accept (2)
example, the supervisor might use
.BR poll (2)
to monitor both the notification file descriptor
(so as as to discover when the target's
.BR accept (2)
call has been interrupted) and the listening file descriptor
(so as to know when a connection is available).
.PP
If the target's system call is interrupted,
the supervisor must take care to release resources (e.g., file descriptors)
that it acquired on behalf of the target.
.\"
.SS Interaction with SA_RESTART signal handlers
Consider the following scenario:
.IP \(bu 2