uml: stop gdb from deleting breakpoints when running UML

Sometimes when UML is debugged gdb miss breakpoints.

When process traced by gdb do fork, debugger remove breakpoints from
child address space. There is possibility to trace more than one fork,
but this not work with UML, I guess (only guess) there is a deadlock -
gdb waits for UML and UML waits for gdb.

When clone() is called with SIGCHLD and CLONE_VM flags, gdb see this
as PTRACE_EVENT_FORK not as PTRACE_EVENT_CLONE and remove breakpoints
from child and at the same time from traced process, because either
have the same address space.

Maybe it is possible to do fix in gdb, but I'm not sure if there is
easy way to find out if traced and child processes share memory. So I
do fix for UML, it simply do not call clone() with both SIGCHLD and
CLONE_VM flags together.  Additionally __WALL flag is used for
waitpid() to assure not miss clone and normal process events.

[ jdike - checkpatch fixes ]

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
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, 7 insertions, 5 deletions

diff --git a/arch/um/os-Linux/skas/process.c b/arch/um/os-Linux/skas/process.c
index d77c81d7068..e8b7a97e83d 100644
--- a/arch/um/os-Linux/skas/process.c
+++ b/arch/um/os-Linux/skas/process.c
@@ -64,7 +64,7 @@ void wait_stub_done(int pid)
 	int n, status, err;
 
 	while (1) {
-		CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
+		CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED | __WALL));
 		if ((n < 0) || !WIFSTOPPED(status))
 			goto bad_wait;
 
@@ -153,7 +153,7 @@ static void handle_trap(int pid, struct uml_pt_regs *regs,
 			panic("handle_trap - continuing to end of syscall "
 			      "failed, errno = %d\n", errno);
 
-		CATCH_EINTR(err = waitpid(pid, &status, WUNTRACED));
+		CATCH_EINTR(err = waitpid(pid, &status, WUNTRACED | __WALL));
 		if ((err < 0) || !WIFSTOPPED(status) ||
 		   (WSTOPSIG(status) != SIGTRAP + 0x80)) {
                         err = ptrace_dump_regs(pid);
@@ -255,16 +255,18 @@ int start_userspace(unsigned long stub_stack)
 		panic("start_userspace : mmap failed, errno = %d", errno);
 	sp = (unsigned long) stack + UM_KERN_PAGE_SIZE - sizeof(void *);
 
-	flags = CLONE_FILES | SIGCHLD;
+	flags = CLONE_FILES;
 	if (proc_mm)
 		flags |= CLONE_VM;
+	else
+		flags |= SIGCHLD;
 
 	pid = clone(userspace_tramp, (void *) sp, flags, (void *) stub_stack);
 	if (pid < 0)
 		panic("start_userspace : clone failed, errno = %d", errno);
 
 	do {
-		CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED));
+		CATCH_EINTR(n = waitpid(pid, &status, WUNTRACED | __WALL));
 		if (n < 0)
 			panic("start_userspace : wait failed, errno = %d",
 			      errno);
@@ -314,7 +316,7 @@ void userspace(struct uml_pt_regs *regs)
 			      "pid=%d, ptrace operation = %d, errno = %d\n",
 			      pid, op, errno);
 
-		CATCH_EINTR(err = waitpid(pid, &status, WUNTRACED));
+		CATCH_EINTR(err = waitpid(pid, &status, WUNTRACED | __WALL));
 		if (err < 0)
 			panic("userspace - waitpid failed, errno = %d\n",
 			      errno);