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Date: Sun, 31 Jan 2016 12:19:01 -0800
From: Andy Lutomirski <>
To: Kees Cook <>
Cc: LSM List <>, 
	"" <>, Oleg Nesterov <>, 
	Linux API <>, Will Drewry <>, 
	"" <>, 
	"" <>, Jeffrey Vander Stoep <>
Subject: Re: [RFC][PATCH] seccomp: add SECCOMP_RET_ACK for non-fatal SIGSYS

On Jan 28, 2016 6:33 PM, "Andy Lutomirski" <> wrote:
> On Thu, Jan 28, 2016 at 5:06 PM, Kees Cook <> wrote:
> > Tracing processes for syscall usage can be done one step at a time with
> > SECCOMP_RET_TRAP, but this will block the syscall. Alternatively, using
> > a ptrace manager to handle SECCOMP_RET_TRACE returns can be used but is
> > heavy weight and depends on the ptrace infrastructure. A light-weight
> > method to learn syscalls is needed, which can reuse the existing delivery
> > of SIGSYS but without skipping the syscall. This is implemented as
> > SECCOMP_RET_ACK which is as permissive as SECCOMP_RET_ALLOW but delivers
> > SIGSYS after syscall completion, as long as the SECCOMP_RET_DATA is
> > non-zero. A signal handler can install a new rule for each syscall as
> > they are signaled with SECCOMP_RET_DATA set to 0 to disable reporting
> > for that syscall in the future (which is required for restarting syscalls
> > that are signal-sensitive like nanosleep).
> >
> > Registers from the signal will reflect registers after the syscall returns
> > rather than before. Signal-sensitive syscalls will trigger EINTR, so they
> > must be whitelisted before they are resumed. Not allowing the sigreturn
> > syscall (and likely prctl to whitelist) will make using SECCOMP_RET_ACK
> > useless.
> >
> > Signed-off-by: Kees Cook <>
> Could this use task_work to queue the signal on return to user mode
> instead?  Would that solve the EINTR issues?

As another option, use the existing TRAP option but add a way for a
process to set a flag such that it can delete and re-add a filter.
Then you get SIGSYS, delete the old filter, add a new one that allows
the current syscall, and resume.  No funny business with EINTR or
clobbered regs.


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