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SubjectRe: [PATCH 11/28] ext4: Calculate and verify inode checksums
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On 2011-10-08, at 12:54 AM, Darrick J. Wong wrote:
> This patch introduces to ext4 the ability to calculate and verify inode
> checksums. This requires the use of a new ro compatibility flag and some
> accompanying e2fsprogs patches to provide the relevant features in tune2fs and
> e2fsck.
>
> +static __u32 ext4_inode_csum(struct inode *inode, struct ext4_inode *raw,
> + struct ext4_inode_info *ei)
> +{
> + struct ext4_sb_info *sbi = EXT4_SB(inode->i_sb);
> + __u16 crc_lo;
> + __u16 crc_hi = 0;
> + __u32 crc;
> +
> + crc_lo = raw->i_checksum_lo;
> + raw->i_checksum_lo = 0;
> + if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE &&
> + EXT4_FITS_IN_INODE(raw, ei, i_checksum_hi)) {
> + crc_hi = raw->i_checksum_hi;
> + raw->i_checksum_hi = 0;
> + }
> +
> + crc = ext4_chksum(sbi, ei->i_uuid_inum_crc, (__u8 *)raw,
> + EXT4_INODE_SIZE(inode->i_sb));
> +
> + raw->i_checksum_lo = crc_lo;
> + if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE &&
> + EXT4_FITS_IN_INODE(raw, ei, i_checksum_hi))
> + raw->i_checksum_hi = crc_hi;
> +
> + return crc;
> +}

This computes both the _lo and _hi parts of the checksum and overwrites what is in the inode...

> +static int ext4_inode_csum_verify(struct inode *inode, struct ext4_inode *raw,
> + struct ext4_inode_info *ei)
> +{
> + __u32 provided, calculated;
> +
> + if (EXT4_SB(inode->i_sb)->s_es->s_creator_os !=
> + cpu_to_le32(EXT4_OS_LINUX) ||
> + !EXT4_HAS_RO_COMPAT_FEATURE(inode->i_sb,
> + EXT4_FEATURE_RO_COMPAT_METADATA_CSUM))
> + return 1;
> +
> + provided = le16_to_cpu(raw->i_checksum_lo);
> + calculated = ext4_inode_csum(inode, raw, ei);

This only saves the _lo part of the checksum before computing the new
checksum (which overwrites both _lo and _hi fields), so the _hi part
of the checksum is never properly validated below.

> + if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE &&
> + EXT4_FITS_IN_INODE(raw, ei, i_checksum_hi))
> + provided |= ((__u32)le16_to_cpu(raw->i_checksum_hi)) << 16;

This should be moved up to save _hi before calling ext4_inode_csum().

> + else
> + calculated &= 0xFFFF;
> +
> + return provided == calculated;
> +}

> +static void ext4_inode_csum_set(struct inode *inode, struct ext4_inode *raw,
> + struct ext4_inode_info *ei)
> +{
> + __u32 crc;
> +
> + if (EXT4_SB(inode->i_sb)->s_es->s_creator_os !=
> + cpu_to_le32(EXT4_OS_LINUX) ||
> + !EXT4_HAS_RO_COMPAT_FEATURE(inode->i_sb,
> + EXT4_FEATURE_RO_COMPAT_METADATA_CSUM))
> + return;
> +
> + crc = ext4_inode_csum(inode, raw, ei);
> + raw->i_checksum_lo = cpu_to_le16(crc & 0xFFFF);
> + if (EXT4_INODE_SIZE(inode->i_sb) > EXT4_GOOD_OLD_INODE_SIZE &&
> + EXT4_FITS_IN_INODE(raw, ei, i_checksum_hi))
> + raw->i_checksum_hi = cpu_to_le16(crc >> 16);

What is the point of storing the returned crc into raw->i_checksum_lo
and raw->i_checksum_hi, if this is done internal to ext4_inode_csum()
already?

Also, would it be better to call the temporary variable "csum" instead
of "crc", since we may use something other than crc32c as the hash
function in the future.

Cheers, Andreas







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