linux-kernel - [PATCH v3 1/3] selftests: Fix errno checking in syscall_user

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Message-ID: <af6a04dbfef9af8570f5bab43e3ef1416b62699a.1747839857.git.dvyukov@google.com>
Date: Wed, 21 May 2025 17:04:28 +0200
From: Dmitry Vyukov <dvyukov@...gle.com>
To: krisman@...labora.com, tglx@...utronix.de, luto@...nel.org, 
	peterz@...radead.org, keescook@...omium.org, gregory.price@...verge.com
Cc: Dmitry Vyukov <dvyukov@...gle.com>, Marco Elver <elver@...gle.com>, linux-kernel@...r.kernel.org
Subject: [PATCH v3 1/3] selftests: Fix errno checking in syscall_user_dispatch test

Successful syscalls don't change errno, so checking errno is wrong
to ensure that a syscall has failed. For example for the following
sequence:

	prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0xff, 0);
	EXPECT_EQ(EINVAL, errno);
	prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0x0, &sel);
	EXPECT_EQ(EINVAL, errno);

only the first syscall may fail and set errno, but the second may succeed
and keep errno intact, and the check will falsely pass.
Or if errno happened to be EINVAL before, even the first check may falsely
pass.

Also use EXPECT/ASSERT consistently. Currently there is an inconsistent mix
without obvious reasons for usage of one or another.

Signed-off-by: Dmitry Vyukov <dvyukov@...gle.com>
Fixes: 179ef035992e ("selftests: Add kselftest for syscall user dispatch")
Cc: Gabriel Krisman Bertazi <krisman@...labora.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Andy Lutomirski <luto@...nel.org>
Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
Cc: Kees Cook <keescook@...omium.org>
Cc: Gregory Price <gregory.price@...verge.com>
Cc: Marco Elver <elver@...gle.com>
Cc: linux-kernel@...r.kernel.org

---

Changes in v3:
 - Added Fixes tag
 - Moved this patch as the first in the series
 - Added prctl_valid/invalid helpers
---
 .../syscall_user_dispatch/sud_test.c          | 50 +++++++++----------
 1 file changed, 25 insertions(+), 25 deletions(-)

diff --git a/tools/testing/selftests/syscall_user_dispatch/sud_test.c b/tools/testing/selftests/syscall_user_dispatch/sud_test.c
index d975a67673299..48cf01aeec3e7 100644
--- a/tools/testing/selftests/syscall_user_dispatch/sud_test.c
+++ b/tools/testing/selftests/syscall_user_dispatch/sud_test.c
@@ -79,6 +79,21 @@ TEST_SIGNAL(dispatch_trigger_sigsys, SIGSYS)
 	}
 }
 
+static void prctl_valid(struct __test_metadata *_metadata,
+			unsigned long op, unsigned long off,
+			unsigned long size, void *sel)
+{
+	EXPECT_EQ(0, prctl(PR_SET_SYSCALL_USER_DISPATCH, op, off, size, sel));
+}
+
+static void prctl_invalid(struct __test_metadata *_metadata,
+			  unsigned long op, unsigned long off,
+			  unsigned long size, void *sel, int err)
+{
+	EXPECT_EQ(-1, prctl(PR_SET_SYSCALL_USER_DISPATCH, op, off, size, sel));
+	EXPECT_EQ(err, errno);
+}
+
 TEST(bad_prctl_param)
 {
 	char sel = SYSCALL_DISPATCH_FILTER_ALLOW;
@@ -86,57 +101,42 @@ TEST(bad_prctl_param)
 
 	/* Invalid op */
 	op = -1;
-	prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0, 0, &sel);
-	ASSERT_EQ(EINVAL, errno);
+	prctl_invalid(_metadata, op, 0, 0, &sel, EINVAL);
 
 	/* PR_SYS_DISPATCH_OFF */
 	op = PR_SYS_DISPATCH_OFF;
 
 	/* offset != 0 */
-	prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x1, 0x0, 0);
-	EXPECT_EQ(EINVAL, errno);
+	prctl_invalid(_metadata, op, 0x1, 0x0, 0, EINVAL);
 
 	/* len != 0 */
-	prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0xff, 0);
-	EXPECT_EQ(EINVAL, errno);
+	prctl_invalid(_metadata, op, 0x0, 0xff, 0, EINVAL);
 
 	/* sel != NULL */
-	prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0x0, &sel);
-	EXPECT_EQ(EINVAL, errno);
+	prctl_invalid(_metadata, op, 0x0, 0x0, &sel, EINVAL);
 
 	/* Valid parameter */
-	errno = 0;
-	prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0x0, 0x0);
-	EXPECT_EQ(0, errno);
+	prctl_valid(_metadata, op, 0x0, 0x0, 0x0);
 
 	/* PR_SYS_DISPATCH_ON */
 	op = PR_SYS_DISPATCH_ON;
 
 	/* Dispatcher region is bad (offset > 0 && len == 0) */
-	prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x1, 0x0, &sel);
-	EXPECT_EQ(EINVAL, errno);
-	prctl(PR_SET_SYSCALL_USER_DISPATCH, op, -1L, 0x0, &sel);
-	EXPECT_EQ(EINVAL, errno);
+	prctl_invalid(_metadata, op, 0x1, 0x0, &sel, EINVAL);
+	prctl_invalid(_metadata, op, -1L, 0x0, &sel, EINVAL);
 
 	/* Invalid selector */
-	prctl(PR_SET_SYSCALL_USER_DISPATCH, op, 0x0, 0x1, (void *) -1);
-	ASSERT_EQ(EFAULT, errno);
+	prctl_invalid(_metadata, op, 0x0, 0x1, (void *) -1, EFAULT);
 
 	/*
 	 * Dispatcher range overflows unsigned long
 	 */
-	prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON, 1, -1L, &sel);
-	ASSERT_EQ(EINVAL, errno) {
-		TH_LOG("Should reject bad syscall range");
-	}
+	prctl_invalid(_metadata, PR_SYS_DISPATCH_ON, 1, -1L, &sel, EINVAL);
 
 	/*
 	 * Allowed range overflows usigned long
 	 */
-	prctl(PR_SET_SYSCALL_USER_DISPATCH, PR_SYS_DISPATCH_ON, -1L, 0x1, &sel);
-	ASSERT_EQ(EINVAL, errno) {
-		TH_LOG("Should reject bad syscall range");
-	}
+	prctl_invalid(_metadata, PR_SYS_DISPATCH_ON, -1L, 0x1, &sel, EINVAL);
 }
 
 /*
-- 
2.49.0.1143.g0be31eac6b-goog