[LTP] [PATCH] setpgid02: Use pid_max as PGID for EPERM

[Teo Couprie Diaz]

In some simple systems (like Busybox), the login shell might be run
as init (PID 1).
This leads to a case where LTP is run in the same session as init,
thus setpgid is allowed to the PGID of init which results in a test fail.
Indeed, the test retrieves the PGID of init to try and generate EPERM.

Instead, get the PGID we use to generate EPERM from the kernel pid_max.
It should not be used by any process, guaranteeing a different session
and generating an EPERM error.

Signed-off-by: Teo Couprie Diaz <teo.coupriediaz@arm.com>
---
See discussion on the origin of this patch (end of thread):
https://lists.linux.it/pipermail/ltp/2023-April/033505.html

Apologies for the time it took to send this.

CI Build:
https://github.com/Teo-CD/ltp/actions/runs/4732620255

 testcases/kernel/syscalls/setpgid/setpgid02.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/testcases/kernel/syscalls/setpgid/setpgid02.c b/testcases/kernel/syscalls/setpgid/setpgid02.c
index 4b63afee8..5341774e1 100644
--- a/testcases/kernel/syscalls/setpgid/setpgid02.c
+++ b/testcases/kernel/syscalls/setpgid/setpgid02.c
@@ -21,7 +21,7 @@
 #include <unistd.h>
 #include "tst_test.h"
 
-static pid_t pgid, pid, ppid, init_pgid;
+static pid_t pgid, pid, ppid, inval_pgid;
 static pid_t negative_pid = -1;
 
 static struct tcase {
@@ -31,7 +31,7 @@ static struct tcase {
 } tcases[] = {
 	{&pid, &negative_pid, EINVAL},
 	{&ppid, &pgid, ESRCH},
-	{&pid, &init_pgid, EPERM}
+	{&pid, &inval_pgid, EPERM}
 };
 
 static void setup(void)
@@ -41,10 +41,10 @@ static void setup(void)
 	pgid = getpgrp();
 
 	/*
-	 * Getting pgid of init/systemd process to use it as a
-	 * process group from a different session for EPERM test
+	 * pid_max would not be in use by another process and guarantees that
+	 * it corresponds to "another session", generating EPERM.
 	 */
-	init_pgid = SAFE_GETPGID(1);
+	SAFE_FILE_SCANF("/proc/sys/kernel/pid_max", "%d\n", &inval_pgid);
 }
 
 static void run(unsigned int n)
-- 
2.34.1