uml: simplify SIGSEGV handling

Simplify the page fault stub by not masking signals while it is running.  This
allows it to signal that it is done by executing an instruction which will
generate a SIGTRAP (int3 on x86) rather than running sigreturn by hand after
queueing a blocked SIGUSR1.

userspace_tramp now no longer puts anything in the SIGSEGV sa_mask, but it
does add SA_NODEFER to sa_flags so that SIGSEGV is still enabled after the
signal handler fails to run sigreturn.

SIGWINCH is just blocked so that we don't have to deal with it and the signal
masks used by wait_stub_done are updated to reflect the smaller number of
signals that it has to worry about.

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

1 files changed, 4 insertions, 7 deletions

diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index 765cfa6ddbcd..2cc2071112bc 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -55,10 +55,10 @@ static int ptrace_dump_regs(int pid)
  * Signals that are OK to receive in the stub - we'll just continue it.
  * SIGWINCH will happen when UML is inside a detached screen.
  */
-#define STUB_SIG_MASK ((1 << SIGVTALRM) | (1 << SIGWINCH))
+#define STUB_SIG_MASK (1 << SIGVTALRM)
 
 /* Signals that the stub will finish with - anything else is an error */
-#define STUB_DONE_MASK ((1 << SIGUSR1) | (1 << SIGTRAP))
+#define STUB_DONE_MASK (1 << SIGTRAP)
 
 void wait_stub_done(int pid)
 {
@@ -179,6 +179,7 @@ static int userspace_tramp(void *stack)
         ptrace(PTRACE_TRACEME, 0, 0, 0);
 
         signal(SIGTERM, SIG_DFL);
+        signal(SIGWINCH, SIG_IGN);
         err = set_interval();
         if (err)
                 panic("userspace_tramp - setting timer failed, errno = %d\n",
@@ -222,11 +223,7 @@ static int userspace_tramp(void *stack)
 
                 set_sigstack((void *) STUB_DATA, UM_KERN_PAGE_SIZE);
                 sigemptyset(&sa.sa_mask);
-                sigaddset(&sa.sa_mask, SIGIO);
-                sigaddset(&sa.sa_mask, SIGWINCH);
-                sigaddset(&sa.sa_mask, SIGVTALRM);
-                sigaddset(&sa.sa_mask, SIGUSR1);
-                sa.sa_flags = SA_ONSTACK;
+                sa.sa_flags = SA_ONSTACK | SA_NODEFER;
                 sa.sa_handler = (void *) v;
                 sa.sa_restorer = NULL;
                 if (sigaction(SIGSEGV, &sa, NULL) < 0)