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List:       user-mode-linux-devel
Subject:    [uml-devel] Signal handling: deliver segfault immediately
From:       "Stroesser, Bodo" <Bodo.Stroesser () fujitsu-siemens ! com>
Date:       2004-09-30 15:29:06
Message-ID: 8B6FF516CBA0194AB0996705076B02520F7C3F () ABGEX01E ! abg ! fsc ! net
[Download RAW message or body]

This one covers the fact, that the SIGSEGV signal, which is created by
force_sigsegv() in case of an error in handle_signal(), is not delivered
to the user immediately. In the worst case it even could be masked if a
sigprocmask() systemcall follows immediately after return from kernel.
The patch is relevant for other architectures, too.

As a further consequence of the patch the first instruction of a signal
handler will be checked for a system call, if the process should be
singlestepped (not a big risk, but better close the hole, I think). If
you don't want this, replace the "break" by "return 1".

Bodo


--- arch/um/kernel/signal_kern.c.orig	2004-09-30 16:34:07.610109219
+0200
+++ arch/um/kernel/signal_kern.c	2004-09-30 16:51:51.743255023
+0200
@@ -38,7 +38,7 @@
 /*
  * OK, we're invoking a handler
  */	
-static void handle_signal(struct pt_regs *regs, unsigned long signr, 
+static int  handle_signal(struct pt_regs *regs, unsigned long signr, 
 			  struct k_sigaction *ka, siginfo_t *info, 
 			  sigset_t *oldset)
 {
@@ -103,23 +103,25 @@
 		recalc_sigpending();
 		spin_unlock_irq(&current->sighand->siglock);
 	}
+
+	return err;
 }
 
 static int kern_do_signal(struct pt_regs *regs, sigset_t *oldset)
 {
 	struct k_sigaction ka_copy;
 	siginfo_t info;
-	int sig;
+	int sig, handled_sig = 0;
 
-	sig = get_signal_to_deliver(&info, &ka_copy, regs, NULL);
-	if(sig > 0){
+	while ( (sig = get_signal_to_deliver(&info, &ka_copy, regs,
NULL)) ) {
+		handled_sig = 1;
 		/* Whee!  Actually deliver the signal.  */
-		handle_signal(regs, sig, &ka_copy, &info, oldset);
-		return(1);
+		if ( ! handle_signal(regs, sig, &ka_copy, &info, oldset)
)
+			break;
 	}
 
 	/* Did we come from a system call? */
-	if(PT_REGS_SYSCALL_NR(regs) >= 0){
+	if(!handled_sig && PT_REGS_SYSCALL_NR(regs) >= 0){
 		/* Restart the system call - no handlers present */
 		if(PT_REGS_SYSCALL_RET(regs) == -ERESTARTNOHAND ||
 		   PT_REGS_SYSCALL_RET(regs) == -ERESTARTSYS ||
@@ -143,7 +145,8 @@
 	if((current->ptrace & PT_DTRACE) && 
 	   is_syscall(PT_REGS_IP(&current->thread.regs)))
  		(void)
CHOOSE_MODE(current->thread.mode.tt.singlestep_syscall = 1, 0);
-	return(0);
+
+	return( handled_sig);
 }
 
 int do_signal(void)


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