Re: Large modules with 6.15 [was: [PATCH v4 6/6] percpu/x86: Enable strict percpu checks via named AS qualifiers]

[Dave Hansen <dave.hansen@intel.com>]

[-- Attachment #1: Type: text/plain, Size: 453 bytes --]

On 6/6/25 02:17, Jiri Slaby wrote:
> Given this is the second time I hit a bug with this, perhaps introduce
> an EXPERIMENTAL CONFIG option, so that random users can simply disable
> it if an issue occurs? Without the need of patching random userspace and
> changing random kernel headers?

What about something like the attached (untested) patch? That should at
least get folks back to the old, universal working behavior even when
using new compilers.

[-- Attachment #2: CC_USE_TYPEOF_UNQUAL1.patch --]
[-- Type: text/x-patch, Size: 2128 bytes --]

From 08d98b4fa08ba76be96e406b53ae69946842f6a9 Mon Sep 17 00:00:00 2001
From: Dave Hansen <dave.hansen@linux.intel.com>
Date: Fri, 6 Jun 2025 08:33:29 -0700
Subject: [PATCH] compiler.h: Enable config choice for using unqualified casts

TYPEOF_UNQUAL() has a noble goal of letting normal compilers do more
of the job normally reserved for sparse. But it has caused (or
exposed) a number of nasty bugs and is not quite ready for prime time.
Even nastier, some of these issues need separate userspace fixes.

Right now, __typeof_unqual__ will be whenever the compiler supports
it. Restrict it to cases where users have opted in with a new Kconfig
option. This option can either be removed or have its default polarity
flipped when userspace is widely fixed up.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
---
 include/linux/compiler.h |  2 +-
 init/Kconfig             | 11 +++++++++++
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 27725f1ab5abc..3efa93f8eca66 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -232,7 +232,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
  * XXX: Remove test for __CHECKER__ once
  * sparse learns about __typeof_unqual__().
  */
-#if CC_HAS_TYPEOF_UNQUAL && !defined(__CHECKER__)
+#if defined(CONFIG_CC_USE_TYPEOF_UNQUAL) && CC_HAS_TYPEOF_UNQUAL && !defined(__CHECKER__)
 # define USE_TYPEOF_UNQUAL 1
 #endif
 
diff --git a/init/Kconfig b/init/Kconfig
index 63f5974b9fa6e..74e5e8d640750 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1489,6 +1489,17 @@ config CC_OPTIMIZE_FOR_SIZE
 
 endchoice
 
+config CC_USE_TYPEOF_UNQUAL
+	bool "Use compiler-provided unqualified casts (EXPERIMENTAL)"
+	depends on EXPERT
+	help
+	  Newer compilers have the ability to do "unqualified" casts which
+	  strip out type qualifiers like 'const'. Kernel builds can
+	  leverage these to do more strict type checking with normal
+	  compilers instead of resorting to using sparse.
+
+	  If unsure, say N here.
+
 config HAVE_LD_DEAD_CODE_DATA_ELIMINATION
 	bool
 	help
-- 
2.34.1