[PATCH] generic signal code (small new feature - userspace signal mask), kernel 2.6.16

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I looked in MAINTAINERS for a suitable person for the generic signal code, but couldn't find anyone in particular. Please Cc me on comments, which are most welcome, as I am not on LKML, although I do peruse the archives. 

Gareth.


Userspace signal mask
==================

Inspiration and rationale:
This is a proposed addition to the linux kernel to reduce the overhead required to mask signals. The intended usage is an application with critical sections that need to be guarded against deadlock by preventing signals from being delivered whilst inside one of the critical sections. Currently such applications may be very heavy users of the sigprocmask syscall, this proposal provides an additional signal mask stored in userspace that can be updated with a simple store rather than a syscall.
The userspace sigprocmask is activated via a new subcommand of the sigprocmask or rt_sigprocmask syscall. The first argument is currently an integer indicating which operation to perform. Currently the operations are SIG_BLOCK, SIG_UNBLOCK and SIG_SETMASK; the new operation I've called SIG_SETUSERMASKADDRESS. 

Usage is:

sigprocmask( SIG_SETUSERMASKADDRESS, sigset_t *address, NULL);
The *address pointer points to wherever you've decided to keep the userspace sigmask in your thread. Any non-zero address enables the use of the userspace sigprocmask and conversely, setting the address to zero disables it. On a kernel without support, this operation can be expected to return -EINVAL. 

Behaviour:
Each thread has its own address, but there is no restriction on threads in a process pointing to the same address.
By default, a process does not have a userspace sigmask. When a new thread is created or a process is forked, the new thread inherits any userspace sigmask address from its parent.
At any time the userspace sigmask is enabled, the effective sigmask is the union of the userspace sigmask and the traditional sigmask in the kernel for that thread. The traditional kernel sigmask is set and controlled by the SIG_BLOCK etc operations as it was before.
As soon as the relevant bit is set in the userspace sigmask, that signal is blocked in the same way it would be as if you'd blocked it via a traditional sigprocmask call. If a signal is both blocked and also pending for delivery, clearing the bit in the userspace sigmask does not guarantee immediate delivery as a SIG_UNBLOCK call would - instead delivery typically occurs at the next pre-emption point (any syscall for example).
If a bad address is passed to the kernel for the location of the userspace mask, it is discarded. 

Patch:
This patch is against a vanilla 2.6.16 kernel from kernel.org. The modifications are entirely architecture independent. It has been tested on i386, x86_64, ia64 and ppc64 kernels:
--- pristine-linux-2.6.16/include/linux/sched.h 2006-03-19 21:53:29.000000000 -0800 +++ linux-2.6.16-userspacesigmask/include/linux/sched.h 2006-10-09 18:09:24.000000000 -0700 
@@ -807,6 +807,7 @@
	struct sighand_struct *sighand;
	sigset_t blocked, real_blocked;
+ sigset_t __user *userspace_blocked; /* If non-zero, task has opted to maintain an additional blocked mask in userspace */ 
	sigset_t saved_sigmask;		/* To be restored with TIF_RESTORE_SIGMASK */
	struct sigpending pending;
--- pristine-linux-2.6.16/kernel/fork.c 2006-03-19 21:53:29.000000000 -0800 +++ linux-2.6.16-userspacesigmask/kernel/fork.c 2006-10-05 22:10:13.000000000 -0700 
@@ -987,6 +987,7 @@
	spin_lock_init(&p->alloc_lock);
	spin_lock_init(&p->proc_lock);
+  p->userspace_blocked = current->userspace_blocked;
	clear_tsk_thread_flag(p, TIF_SIGPENDING);
	init_sigpending(&p->pending);
--- pristine-linux-2.6.16/kernel/signal.c 2006-03-19 21:53:29.000000000 -0800 +++ linux-2.6.16-userspacesigmask/kernel/signal.c 2006-10-14 21:08:38.000000000 -0700 
@@ -24,6 +24,7 @@
#include <linux/ptrace.h>
#include <linux/posix-timers.h>
#include <linux/signal.h>
+#include <linux/mm.h>
#include <linux/audit.h>
#include <linux/capability.h>
#include <asm/param.h>
@@ -224,7 +225,30 @@
void recalc_sigpending(void)
{
-	recalc_sigpending_tsk(current);
+  sigset_t usermask, combinedblocked;
+
+  sigemptyset(&usermask);
+
+  if (current->userspace_blocked != 0)
+  {
+ /* Task is using the optional userspace signal mask, fetch it from the address given to us. */ + /* If the address is bad, we don't mind and the mask remains empty */ + if (copy_from_user(&usermask, current->userspace_blocked, sizeof (sigset_t))) 
+    {
+      /* If we have a bad address, disable the userspace mask */
+      current->userspace_blocked = 0;
+    }
+  }
+
+  sigorsets(&combinedblocked,&usermask,&current->blocked);
+
+	if (current->signal->group_stop_count > 0 ||
+	    (freezing(current)) ||
+	    PENDING(&current->pending, &combinedblocked) ||
+	    PENDING(&current->signal->shared_pending, &combinedblocked))
+		set_thread_flag(TIF_SIGPENDING);
+	else
+		clear_thread_flag(TIF_SIGPENDING);
}
/* Given the mask, find the first available signal that should be serviced. */ 
@@ -914,6 +938,7 @@
	if (sigismember(&t->blocked, sig)) {
		sigdelset(&t->blocked, sig);
	}
+
	recalc_sigpending_tsk(t);
	ret = specific_send_sig_info(sig, info, t);
	spin_unlock_irqrestore(&t->sighand->siglock, flags);
@@ -1921,7 +1946,7 @@
{
	sigset_t *mask = &current->blocked;
	int signr = 0;
-
+
relock:
	spin_lock_irq(&current->sighand->siglock);
	for (;;) {
@@ -2136,6 +2161,15 @@
	if (set) {
		error = -EFAULT;
+
+ /* SIG_SETUSERMASK just needs to record the address of the extra mask in userspace and return */ 
+    if ( how == SIG_SETUSERMASKADDRESS )
+    {
+      current->userspace_blocked = set;
+      error = 0;
+      goto out;
+		}
+
		if (copy_from_user(&new_set, set, sizeof(*set)))
			goto out;
		sigdelsetmask(&new_set, sigmask(SIGKILL)|sigmask(SIGSTOP));
@@ -2607,6 +2641,9 @@
			break;
		case SIG_SETMASK:
			current->blocked.sig[0] = new_set;
+      break;
+    case SIG_SETUSERMASKADDRESS:
+      current->userspace_blocked = (sigset_t *) set;
			break;
		}
--- pristine-linux-2.6.16/include/asm-generic/signal.h 2006-03-19 21:53:29.000000000 -0800 +++ linux-2.6.16-userspacesigmask/include/asm-generic/signal.h 2006-10-09 18:13:36.000000000 -0700 
@@ -7,6 +7,9 @@
#ifndef SIG_SETMASK
#define SIG_SETMASK        2	/* for setting the signal mask */
#endif
+#ifndef SIG_SETUSERMASKADDRESS
+#define SIG_SETUSERMASKADDRESS 8 /* for setting the optional userspace signal mask address */ 
+#endif
#ifndef __ASSEMBLY__
typedef void __signalfn_t(int);

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