linux-kernel - [PATCH][next] xfs: Replace one-element arrays with flexible-array members

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Message-ID: <Y9xiYmVLRIKdpJcC@work>
Date:   Thu, 2 Feb 2023 19:24:50 -0600
From:   "Gustavo A. R. Silva" <gustavoars@...nel.org>
To:     "Darrick J. Wong" <djwong@...nel.org>
Cc:     linux-xfs@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        linux-hardening@...r.kernel.org
Subject: [PATCH][next] xfs: Replace one-element arrays with flexible-array
 members

One-element arrays are deprecated, and we are replacing them with flexible
array members instead. So, replace one-element arrays with flexible-array
members in structures xfs_attr_leaf_name_local and
xfs_attr_leaf_name_remote.

The only binary differences reported after the changes are all like
these:

fs/xfs/libxfs/xfs_attr_leaf.o
_@@ -435,7 +435,7 @@
      3b8:      movzbl 0x2(%rbx),%eax
      3bc:      rol    $0x8,%bp
      3c0:      movzwl %bp,%ebp
-     3c3:      lea    0x2(%rax,%rbp,1),%ebx
+     3c3:      lea    0x3(%rax,%rbp,1),%ebx
      3c7:      call   3cc <xfs_attr_leaf_entsize+0x8c>
                        3c8: R_X86_64_PLT32     __tsan_func_exit-0x4
      3cc:      or     $0x3,%ebx
_@@ -454,7 +454,7 @@
      3ea:      movzbl 0x8(%rbx),%ebx
      3ee:      call   3f3 <xfs_attr_leaf_entsize+0xb3>
                        3ef: R_X86_64_PLT32     __tsan_func_exit-0x4
-     3f3:      add    $0xa,%ebx
+     3f3:      add    $0xb,%ebx
      3f6:      or     $0x3,%ebx
      3f9:      add    $0x1,%ebx
      3fc:      mov    %ebx,%eax

similar changes in fs/xfs/scrub/attr.o and fs/xfs/xfs.o object files.

And the reason for this is because of the round_up() macro called in
functions xfs_attr_leaf_entsize_remote() and xfs_attr_leaf_entsize_local(),
which is compensanting for the one-byte reduction in size (due to the
flex-array transformation) of structures xfs_attr_leaf_name_remote and
xfs_attr_leaf_name_local. So, sizes remain the same before and after
changes.

This helps with the ongoing efforts to tighten the FORTIFY_SOURCE
routines on memcpy() and help us make progress towards globally
enabling -fstrict-flex-arrays=3 [1].

Link: https://github.com/KSPP/linux/issues/79
Link: https://github.com/KSPP/linux/issues/251
Link: https://gcc.gnu.org/pipermail/gcc-patches/2022-October/602902.html [1]
Signed-off-by: Gustavo A. R. Silva <gustavoars@...nel.org>
---
 fs/xfs/libxfs/xfs_da_format.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/xfs/libxfs/xfs_da_format.h b/fs/xfs/libxfs/xfs_da_format.h
index 25e2841084e1..e1e62ebb0c44 100644
--- a/fs/xfs/libxfs/xfs_da_format.h
+++ b/fs/xfs/libxfs/xfs_da_format.h
@@ -620,14 +620,14 @@ typedef struct xfs_attr_leaf_entry {	/* sorted on key, not name */
 typedef struct xfs_attr_leaf_name_local {
 	__be16	valuelen;		/* number of bytes in value */
 	__u8	namelen;		/* length of name bytes */
-	__u8	nameval[1];		/* name/value bytes */
+	__u8	nameval[];		/* name/value bytes */
 } xfs_attr_leaf_name_local_t;
 
 typedef struct xfs_attr_leaf_name_remote {
 	__be32	valueblk;		/* block number of value bytes */
 	__be32	valuelen;		/* number of bytes in value */
 	__u8	namelen;		/* length of name bytes */
-	__u8	name[1];		/* name bytes */
+	__u8	name[];			/* name bytes */
 } xfs_attr_leaf_name_remote_t;
 
 typedef struct xfs_attr_leafblock {
@@ -747,13 +747,13 @@ xfs_attr3_leaf_name_local(xfs_attr_leafblock_t *leafp, int idx)
  */
 static inline int xfs_attr_leaf_entsize_remote(int nlen)
 {
-	return round_up(sizeof(struct xfs_attr_leaf_name_remote) - 1 +
+	return round_up(sizeof(struct xfs_attr_leaf_name_remote) +
 			nlen, XFS_ATTR_LEAF_NAME_ALIGN);
 }
 
 static inline int xfs_attr_leaf_entsize_local(int nlen, int vlen)
 {
-	return round_up(sizeof(struct xfs_attr_leaf_name_local) - 1 +
+	return round_up(sizeof(struct xfs_attr_leaf_name_local) +
 			nlen + vlen, XFS_ATTR_LEAF_NAME_ALIGN);
 }
 
-- 
2.34.1