[PATCH v2] selftests/mm: fix ptrace PEEKDATA check in memfd_secret test

[Hongfu Li <hongfu.li@linux.dev>]

From: Hongfu Li <lihongfu@kylinos.cn>

try_ptrace() treats PTRACE_PEEKDATA return value as a boolean
check. A successful read returns non-zero data (memory filled with
0x55), causing the test to incorrectly report PASS when secret memory
protection is broken.

Check the return value against -1 instead. The test should only pass
when PTRACE_PEEKDATA fails, which means secret memory protection works.

Fixes: 76fe17ef588a ("secretmem: test: add basic selftest for memfd_secret(2)")
Signed-off-by: Hongfu Li <lihongfu@kylinos.cn>
Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
---
v2:
- Add a comment explaining why PEEKDATA on secret memory is expected to
  fail, as suggested during review.
- Add Acked-by from Lorenzo Stoakes and Mike Rapoport
---
 tools/testing/selftests/mm/memfd_secret.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/memfd_secret.c b/tools/testing/selftests/mm/memfd_secret.c
index c55d84c5e613..aef774be87f1 100644
--- a/tools/testing/selftests/mm/memfd_secret.c
+++ b/tools/testing/selftests/mm/memfd_secret.c
@@ -145,7 +145,8 @@ static void try_ptrace(int fd, int pipefd[2])
 		exit(KSFT_FAIL);
 	}
 
-	if (ptrace(PTRACE_PEEKDATA, ppid, mem, 0))
+	/* PEEKDATA on secret memory must fail, else protection is broken. */
+	if (ptrace(PTRACE_PEEKDATA, ppid, mem, 0) == -1)
 		exit(KSFT_PASS);
 
 	exit(KSFT_FAIL);
-- 
2.54.0