[RFC][PATCH v2 3/5] clone: Disallown CLONE_THREAD with a shared sighand_struct

[Eric W. Biederman]

Old threading libraries used CLONE_SIGHAND without clone thread.
Modern threadding libraries always use CLONE_SIGHAND | CLONE_THREAD.

Therefore let's simplify our lives and stop supporting a case no one cares about.

Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
---
 kernel/fork.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/fork.c b/kernel/fork.c
index fe6f1bf32bb9..0632ac1180be 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1515,6 +1515,13 @@ static __latent_entropy struct task_struct *copy_process(
 	if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND))
 		return ERR_PTR(-EINVAL);
 
+	/* Disallow CLONE_THREAD with a shared SIGHAND structure.  No
+	 * one cares and supporting it leads to unnecessarily complex
+	 * code.
+	 */
+	if ((clone_flags & CLONE_THREAD) && (atomic_read(&current->sighand->count) > 1))
+		return ERR_PTR(-EINVAL);
+
 	/*
 	 * Shared signal handlers imply shared VM. By way of the above,
 	 * thread groups also imply shared VM. Blocking this case allows
-- 
2.10.1