[PATCH] uml: Move signal handlers to arch code

Have most signals go through an arch-provided handler which recovers the
sigcontext and then calls a generic handler.  This replaces the
ARCH_GET_SIGCONTEXT macro, which was somewhat fragile.  On x86_64, recovering
%rdx (which holds the sigcontext pointer) must be the first thing that
happens.  sig_handler duly invokes that first, but there is no guarantee that
I can see that instructions won't be reordered such that %rdx is used before
that.  Having the arch provide the handler seems much more robust.

Some signals in some parts of UML require their own handlers - these places
don't call set_handler any more.  They call sigaction or signal themselves.

Signed-off-by: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 28 insertions, 6 deletions

diff --git a/arch/um/os-Linux/main.c b/arch/um/os-Linux/main.c
index 90912aaca7aa..d1c5670787dc 100644
--- a/arch/um/os-Linux/main.c
+++ b/arch/um/os-Linux/main.c
@@ -67,13 +67,32 @@ static __init void do_uml_initcalls(void)
 
 static void last_ditch_exit(int sig)
 {
-        signal(SIGINT, SIG_DFL);
-        signal(SIGTERM, SIG_DFL);
-        signal(SIGHUP, SIG_DFL);
         uml_cleanup();
         exit(1);
 }
 
+static void install_fatal_handler(int sig)
+{
+        struct sigaction action;
+
+        /* All signals are enabled in this handler ... */
+        sigemptyset(&action.sa_mask);
+
+        /* ... including the signal being handled, plus we want the
+         * handler reset to the default behavior, so that if an exit
+         * handler is hanging for some reason, the UML will just die
+         * after this signal is sent a second time.
+         */
+        action.sa_flags = SA_RESETHAND | SA_NODEFER;
+        action.sa_restorer = NULL;
+        action.sa_handler = last_ditch_exit;
+        if(sigaction(sig, &action, NULL) < 0){
+                printf("failed to install handler for signal %d - errno = %d\n",
+                       errno);
+                exit(1);
+        }
+}
+
 #define UML_LIB_PATH    ":/usr/lib/uml"
 
 static void setup_env_path(void)
@@ -158,9 +177,12 @@ int main(int argc, char **argv, char **envp)
         }
         new_argv[argc] = NULL;
 
-        set_handler(SIGINT, last_ditch_exit, SA_ONESHOT | SA_NODEFER, -1);
-        set_handler(SIGTERM, last_ditch_exit, SA_ONESHOT | SA_NODEFER, -1);
-        set_handler(SIGHUP, last_ditch_exit, SA_ONESHOT | SA_NODEFER, -1);
+        /* Allow these signals to bring down a UML if all other
+         * methods of control fail.
+         */
+        install_fatal_handler(SIGINT);
+        install_fatal_handler(SIGTERM);
+        install_fatal_handler(SIGHUP);
 
         scan_elf_aux( envp);