[PATCH wayland 2/2 v2] tests: Fix FAIL in sanity-test (*timeout*) when Yama LSM enabled

[Bryce Harrington]

This fixes a regression in the testsuite since c3653f7f, where four of
the timeout tests fail with "Timeouts suppressed" messages.

The timeouts are being suppressed because the testsuite is erroneously
detecting that a debugger is attached.  This detection mechanism
(adopted from libinput) uses ptrace to test if there is a debugger
parent process that can be attached.  Unfortunately, this is an
unreliable test: Kernel security policies exist to restrict the scope of
ptrace to prevent processes from snooping on one another.[1] This
security policy is set as the default on Ubuntu, and potentially other
Linux distributions.[2]

The Yama documentation suggests, "For software that has defined
application-specific relationships between a debugging process and its
inferior (crash handlers, etc), prctl(PR_SET_PTRACER, pid, ...) can be
used.  An inferior can declare which other process (and its descendents)
are allowed to call PTRACE_ATTACH against it."

The child needs to be synchronized to the client to prevent a race
condition where the child might try to operate before the parent has
finished its prctl call.  This synchronization is done via pipes.

1: http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=2d514487faf188938a4ee4fb3464eeecfbdcf8eb
2: https://wiki.ubuntu.com/SecurityTeam/Roadmap/KernelHardening#ptrace_Protection

Signed-off-by: Bryce Harrington <bryce at osg.samsung.com>
---
 tests/test-runner.c | 35 ++++++++++++++++++++++++++++++++---
 1 file changed, 32 insertions(+), 3 deletions(-)

diff --git a/tests/test-runner.c b/tests/test-runner.c
index b813b69..11d064e 100644
--- a/tests/test-runner.c
+++ b/tests/test-runner.c
@@ -34,6 +34,7 @@
 #include <errno.h>
 #include <limits.h>
 #include <sys/ptrace.h>
+#include <linux/prctl.h>
 
 #include "test-runner.h"
 
@@ -255,13 +256,33 @@ is_debugger_attached(void)
 {
 	int status;
 	int rc;
-	pid_t pid = fork();
+	pid_t pid;
+	int pipefd[2];
 
-	if (pid == -1)
+	if (pipe(pipefd) == -1) {
+		perror("pipe");
 		return 0;
+	}
+
+	pid = fork();
+	if (pid == -1) {
+		perror("fork");
+		close(pipefd[0]);
+		close(pipefd[1]);
+		return 0;
+	}
 
 	if (pid == 0) {
-		pid_t ppid = getppid();
+		char buf;
+		pid_t ppid;
+
+		/* Wait until parent is ready */
+		close(pipefd[1]); /* Close unused write end */
+		while (read(pipefd[0], &buf, 1) > 0);
+		close(pipefd[0]);
+
+		/* Attempt to ptrace the parent */
+		ppid = getppid();
 		rc = ptrace(PTRACE_ATTACH, ppid, NULL, NULL);
 		if (rc == 0) {
 			waitpid(ppid, NULL, 0);
@@ -273,6 +294,14 @@ is_debugger_attached(void)
 		}
 		_exit(rc);
 	} else {
+		close(pipefd[0]);
+
+		/* Enable child to ptrace the parent process */
+		prctl(PR_SET_PTRACER, pid);
+
+		/* Signal to client that parent is ready */
+		close(pipefd[1]);
+
 		waitpid(pid, &status, 0);
 		rc = WEXITSTATUS(status);
 	}
-- 
1.9.1