Re: [PATCH net-next 3/4] net: ipa: introduce ipa_assert()

On Fri, Mar 19, 2021 at 07:38:26AM -0500, Alex Elder wrote:

On 3/18/21 11:55 PM, Leon Romanovsky wrote:

quoted

On Thu, Mar 18, 2021 at 11:29:22PM -0500, Alex Elder wrote:

quoted

Create a new macro ipa_assert() to verify that a condition is true.
This produces a build-time error if the condition can be evaluated
at build time; otherwise __ipa_assert_runtime() is called (described
below).

Another macro, ipa_assert_always() verifies that an expression
yields true at runtime, and if it does not, reports an error
message.

When IPA validation is enabled, __ipa_assert_runtime() becomes a
call to ipa_assert_always(), resulting in runtime verification of
the asserted condition.  Otherwise __ipa_assert_runtime() has no
effect, so such ipa_assert() calls are effectively ignored.

IPA assertion errors will be reported using the IPA device if
possible.

Signed-off-by: Alex Elder <redacted>
---
  drivers/net/ipa/ipa_assert.h | 50 ++++++++++++++++++++++++++++++++++++
  1 file changed, 50 insertions(+)
  create mode 100644 drivers/net/ipa/ipa_assert.h

diff --git a/drivers/net/ipa/ipa_assert.h b/drivers/net/ipa/ipa_assert.h
new file mode 100644
index 0000000000000..7e5b9d487f69d
--- /dev/null
+++ b/drivers/net/ipa/ipa_assert.h

@@ -0,0 +1,50 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (C) 2021 Linaro Ltd.
+ */
+#ifndef _IPA_ASSERT_H_
+#define _IPA_ASSERT_H_
+
+#include <linux/compiler.h>
+#include <linux/printk.h>
+#include <linux/device.h>
+
+/* Verify the expression yields true, and fail at build time if possible */
+#define ipa_assert(dev, expr) \
+	do { \
+		if (__builtin_constant_p(expr)) \
+			compiletime_assert(expr, __ipa_failure_msg(expr)); \
+		else \
+			__ipa_assert_runtime(dev, expr); \
+	} while (0)
+
+/* Report an error if the given expression evaluates to false at runtime */
+#define ipa_assert_always(dev, expr) \
+	do { \
+		if (unlikely(!(expr))) { \
+			struct device *__dev = (dev); \
+			\
+			if (__dev) \
+				dev_err(__dev, __ipa_failure_msg(expr)); \
+			else  \
+				pr_err(__ipa_failure_msg(expr)); \
+		} \
+	} while (0)

It will be much better for everyone if you don't obfuscate existing
kernel primitives and don't hide constant vs. dynamic expressions.

I don't agree with this characterization.

Yes, there is some complexity in this one source file, where
ipa_assert() is defined.  But as a result, callers are simple
one-line statements (similar to WARN_ON()).

It is not complexity but being explicit vs. implicit. The coding
style that has explicit flows will be always better than implicit
one. By adding your custom assert, you are hiding the flows and
makes unclear what can be evaluated at compilation and what can't.

Existing kernel primitives don't achieve these objectives:
- Don't check things at run time under normal conditions
- Do check things when validation is enabled
- If you can check it at compile time, check it regardless
If there is something that helps me do that, suggest it because
I will be glad to use it.

Separate checks to two flows and it will be natural to achieve what you
want.

quoted

So any random kernel developer will be able to change the code without
investing too much time to understand this custom logic.

There should be almost no need to change the definition of
ipa_assert().  Even so, this custom logic is not all that
complicated in my view; it's broken into a few macros that
are each pretty simple.  It was actuallyl a little simpler
before I added some things to satisfy checkpatch.pl.

Every obfuscation is simple, but together it is nightmare for the person
who does some random kernel job and needs to change such obfuscated code.

quoted

And constant expressions are checked with BUILD_BUG_ON().

BUILD_BUG_ON() is great.  But it doesn't work for
non-constant expressions.

Of course, be explicit and use BUILD_BUG_ON() for constants and write
the code that don't need such constructions.

Thanks