Re: [REGRESSION][BISECTED] erroneous buffer overflow detected in bch2_xattr_validate

[Jan Hendrik Farr <kernel@jfarr.cc>]

Hi Nathan,

Do you want me to add a Co-Developed-by tag for you? I feel bad just
taking it.

For reference here is the current state of the patch, still waiting on
the merge into clang 19.1.x:

It needs three prerequisite commits on top of 6.6.x, but unfortunately
still requires a small amount of manual conflict resolution, but it's
easy enough

1. include/linux/compiler_types.h:
	use the incoming change until before (but not including) the
	"Apply __counted_by() when the Endianness matches to increase test coverage."
	comment)

2. lib/overflow_kunit.c: 
	HEAD is correct


From 6c667a43af0c57cd3f260fd75d5c4a198ba94220 Mon Sep 17 00:00:00 2001
From: Jan Hendrik Farr <kernel@jfarr.cc>
Date: Thu, 17 Oct 2024 04:39:40 +0200
Subject: [PATCH] Compiler Attributes: disable __counted_by for clang < 19.1.3

This patch disables __counted_by for clang versions < 19.1.3 because
of the two issues listed below. It does this by introducing
CONFIG_CC_HAS_COUNTED_BY.

1. clang < 19.1.2 has a bug that can lead to __bdos returning 0:
https://github.com/llvm/llvm-project/pull/110497

2. clang < 19.1.3 has a bug that can lead to __bdos being off by 4:
https://github.com/llvm/llvm-project/pull/112636

Fixes: c8248faf3ca2 ("Compiler Attributes: counted_by: Adjust name and identifier expansion")
Cc: stable@vger.kernel.org # 6.6.x: 16c31dd7fdf6: Compiler Attributes: counted_by: bump min gcc version
Cc: stable@vger.kernel.org # 6.6.x: 2993eb7a8d34: Compiler Attributes: counted_by: fixup clang URL
Cc: stable@vger.kernel.org # 6.6.x: 231dc3f0c936: lkdtm/bugs: Improve warning message for compilers without counted_by support
Cc: stable@vger.kernel.org # 6.6.x
Reported-by: Nathan Chancellor <nathan@kernel.org>
Closes: https://lore.kernel.org/all/20240913164630.GA4091534@thelio-3990X/
Reported-by: kernel test robot <oliver.sang@intel.com>
Closes: https://lore.kernel.org/oe-lkp/202409260949.a1254989-oliver.sang@intel.com
Link: https://lore.kernel.org/all/Zw8iawAF5W2uzGuh@archlinux/T/#m204c09f63c076586a02d194b87dffc7e81b8de7b
Signed-off-by: Jan Hendrik Farr <kernel@jfarr.cc>
---
 drivers/misc/lkdtm/bugs.c           |  2 +-
 include/linux/compiler_attributes.h | 13 -------------
 include/linux/compiler_types.h      | 19 +++++++++++++++++++
 init/Kconfig                        |  8 ++++++++
 lib/overflow_kunit.c                |  2 +-
 5 files changed, 29 insertions(+), 15 deletions(-)

diff --git a/drivers/misc/lkdtm/bugs.c b/drivers/misc/lkdtm/bugs.c
index 62ba01525479..376047beea3d 100644
--- a/drivers/misc/lkdtm/bugs.c
+++ b/drivers/misc/lkdtm/bugs.c
@@ -445,7 +445,7 @@ static void lkdtm_FAM_BOUNDS(void)
 
 	pr_err("FAIL: survived access of invalid flexible array member index!\n");
 
-	if (!__has_attribute(__counted_by__))
+	if (!IS_ENABLED(CONFIG_CC_HAS_COUNTED_BY))
 		pr_warn("This is expected since this %s was built with a compiler that does not support __counted_by\n",
 			lkdtm_kernel_info);
 	else if (IS_ENABLED(CONFIG_UBSAN_BOUNDS))
diff --git a/include/linux/compiler_attributes.h b/include/linux/compiler_attributes.h
index 32284cd26d52..c16d4199bf92 100644
--- a/include/linux/compiler_attributes.h
+++ b/include/linux/compiler_attributes.h
@@ -94,19 +94,6 @@
 # define __copy(symbol)
 #endif
 
-/*
- * Optional: only supported since gcc >= 15
- * Optional: only supported since clang >= 18
- *
- *   gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
- * clang: https://github.com/llvm/llvm-project/pull/76348
- */
-#if __has_attribute(__counted_by__)
-# define __counted_by(member)		__attribute__((__counted_by__(member)))
-#else
-# define __counted_by(member)
-#endif
-
 /*
  * Optional: not supported by gcc
  * Optional: only supported since clang >= 14.0
diff --git a/include/linux/compiler_types.h b/include/linux/compiler_types.h
index 1a957ea2f4fe..639be0f30b45 100644
--- a/include/linux/compiler_types.h
+++ b/include/linux/compiler_types.h
@@ -323,6 +323,25 @@ struct ftrace_likely_data {
 #define __no_sanitize_or_inline __always_inline
 #endif
 
+/*
+ * Optional: only supported since gcc >= 15
+ * Optional: only supported since clang >= 18
+ *
+ *   gcc: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108896
+ * clang: https://github.com/llvm/llvm-project/pull/76348
+ *
+ * __bdos on clang < 19.1.2 can erroneously return 0:
+ * https://github.com/llvm/llvm-project/pull/110497
+ *
+ * __bdos on clang < 19.1.3 can be off by 4:
+ * https://github.com/llvm/llvm-project/pull/112636
+ */
+#ifdef CONFIG_CC_HAS_COUNTED_BY
+# define __counted_by(member)		__attribute__((__counted_by__(member)))
+#else
+# define __counted_by(member)
+#endif
+
 /*
  * Apply __counted_by() when the Endianness matches to increase test coverage.
  */
diff --git a/init/Kconfig b/init/Kconfig
index 530a382ee0fe..5f1fe3583f20 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -116,6 +116,14 @@ config CC_HAS_ASM_INLINE
 config CC_HAS_NO_PROFILE_FN_ATTR
 	def_bool $(success,echo '__attribute__((no_profile_instrument_function)) int x();' | $(CC) -x c - -c -o /dev/null -Werror)
 
+# clang needs to be at least 19.1.3 to avoid __bdos miscalculations
+# https://github.com/llvm/llvm-project/pull/110497
+# https://github.com/llvm/llvm-project/pull/112636
+# TODO: when gcc 15 is released remove the build test and add gcc version check
+config CC_HAS_COUNTED_BY
+	def_bool $(success,echo 'struct flex { int count; int array[] __attribute__((__counted_by__(count))); };' | $(CC) $(CLANG_FLAGS) -x c - -c -o /dev/null -Werror)
+	depends on !(CC_IS_CLANG && CLANG_VERSION < 190103)
+
 config PAHOLE_VERSION
 	int
 	default $(shell,$(srctree)/scripts/pahole-version.sh $(PAHOLE))
diff --git a/lib/overflow_kunit.c b/lib/overflow_kunit.c
index 2abc78367dd1..5222c6393f11 100644
--- a/lib/overflow_kunit.c
+++ b/lib/overflow_kunit.c
@@ -1187,7 +1187,7 @@ static void DEFINE_FLEX_test(struct kunit *test)
 {
 	/* Using _RAW_ on a __counted_by struct will initialize "counter" to zero */
 	DEFINE_RAW_FLEX(struct foo, two_but_zero, array, 2);
-#if __has_attribute(__counted_by__)
+#ifdef CONFIG_CC_HAS_COUNTED_BY
 	int expected_raw_size = sizeof(struct foo);
 #else
 	int expected_raw_size = sizeof(struct foo) + 2 * sizeof(s16);
-- 
2.47.0