[LTP] [PATCH v1 1/1] thp04: Skip when FOLL_FORCE writes are disabled

[Jan Polensky]

Since commit 41e8149c8892 ("proc: add config & param to block forcing
mem writes"), /proc/self/mem writes using FOLL_FORCE can be blocked by
CONFIG_PROC_MEM_NO_FORCE or proc_mem_force_write=0 as a security
hardening measure.

On such systems, thp04 fails with EIO during the /proc/self/mem write.
This is expected kernel behavior, not a real test failure.

Add a pre-flight write check in setup() and return TCONF on EIO, with
an explanation that FOLL_FORCE writes are disabled by kernel hardening.

This makes thp04 skip on hardened systems instead of reporting a false
failure.

Signed-off-by: Jan Polensky <japo@linux.ibm.com>
---
 testcases/kernel/mem/thp/thp04.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/testcases/kernel/mem/thp/thp04.c b/testcases/kernel/mem/thp/thp04.c
index 16d766c349b7..a5748c09cb70 100644
--- a/testcases/kernel/mem/thp/thp04.c
+++ b/testcases/kernel/mem/thp/thp04.c
@@ -73,6 +73,7 @@ static void *alloc_zero_page(void *baseaddr)
 static void setup(void)
 {
 	size_t i;
+	int test_val = 0;
 
 	thp_size = tst_get_hugepage_size();
 
@@ -93,6 +94,28 @@ static void setup(void)
 	writefd = SAFE_OPEN("/proc/self/mem", O_RDWR);
 	readfd = SAFE_OPEN("/proc/self/mem", O_RDWR);
 
+	/*
+	 * Test if /proc/self/mem write with FOLL_FORCE works. Since kernel commit
+	 * 41e8149c8892 ("proc: add config & param to block forcing mem writes")
+	 * (Aug 2024), writes can be blocked by CONFIG_PROC_MEM_NO_FORCE or
+	 * proc_mem_force_write=0 boot param.
+	 */
+	TEST(lseek(writefd, (off_t)write_ptr, SEEK_SET));
+	if (TST_RET == -1)
+		tst_brk(TBROK | TTERRNO, "lseek on /proc/self/mem failed");
+
+	TEST(write(writefd, &test_val, sizeof(test_val)));
+	if (TST_RET == -1) {
+		if (TST_ERR == EIO) {
+			tst_brk(TCONF,
+				"/proc/self/mem write with FOLL_FORCE is disabled. "
+				"This is a kernel security feature (commit 41e8149c8892). "
+				"To enable: boot with proc_mem_force_write=1 or "
+				"rebuild kernel with CONFIG_PROC_MEM_FORCE_WRITE=y");
+		}
+		tst_brk(TBROK | TTERRNO, "test write to /proc/self/mem failed");
+	}
+
 	fzsync_pair.exec_loops = 100000;
 	tst_fzsync_pair_init(&fzsync_pair);
 }
-- 
2.54.0