linux-hardening - [PATCH 1/2] bcachefs: Remove incorrect __counted

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Message-ID: <20250501200132.216859-1-mmpgouride@gmail.com>
Date: Fri,  2 May 2025 04:01:31 +0800
From: Alan Huang <mmpgouride@...il.com>
To: kent.overstreet@...ux.dev,
	kees@...nel.org,
	gustavoars@...nel.org,
	thorsten.blum@...lux.com
Cc: linux-bcachefs@...r.kernel.org,
	linux-hardening@...r.kernel.org,
	Alan Huang <mmpgouride@...il.com>
Subject: [PATCH 1/2] bcachefs: Remove incorrect __counted_by annotation

This actually reverts 86e92eeeb237 ("bcachefs: Annotate struct bch_xattr
with __counted_by()").

After the x_name, there is a value. According to the disscussion[1],
__counted_by assumes that the flexible array member contains exactly
the amount of elements that are specified. Now there are users came across
a false positive detection of an out of bounds write caused by
the __counted_by here[2], so revert that.

[1] https://lore.kernel.org/lkml/Zv8VDKWN1GzLRT-_@archlinux/T/#m0ce9541c5070146320efd4f928cc1ff8de69e9b2
[2] https://privatebin.net/?a0d4e97d590d71e1#9bLmp2Kb5NU6X6cZEucchDcu88HzUQwHUah8okKPReEt

Signed-off-by: Alan Huang <mmpgouride@...il.com>
---
 fs/bcachefs/xattr_format.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/bcachefs/xattr_format.h b/fs/bcachefs/xattr_format.h
index c7916011ef34..67426e33d04e 100644
--- a/fs/bcachefs/xattr_format.h
+++ b/fs/bcachefs/xattr_format.h
@@ -13,7 +13,13 @@ struct bch_xattr {
 	__u8			x_type;
 	__u8			x_name_len;
 	__le16			x_val_len;
-	__u8			x_name[] __counted_by(x_name_len);
+	/*
+	 * x_name contains the name and value counted by
+	 * x_name_len + x_val_len. The introduction of
+	 * __counted_by(x_name_len) caused a false positive
+	 * detection of an out of bounds write.
+	 */
+	__u8			x_name[];
 } __packed __aligned(8);
 
 #endif /* _BCACHEFS_XATTR_FORMAT_H */
-- 
2.48.1