linux-ext4 - Re: [PATCH, REWORKED 01/11] Add initial checksum support for the gdt_checksum/uninit

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Message-id: <20080317172254.GF3542@webber.adilger.int>
Date:	Tue, 18 Mar 2008 01:22:54 +0800
From:	Andreas Dilger <adilger@....com>
To:	"Theodore Ts'o" <tytso@....edu>
Cc:	linux-ext4@...r.kernel.org, "Jose R. Santos" <jrs@...ibm.com>,
	Andreas Dilger <adilger@...sterfs.com>
Subject: Re: [PATCH, REWORKED 01/11] Add initial checksum support for the
	gdt_checksum/uninit_group feature

On Mar 17, 2008  09:28 -0400, Theodore Ts'o wrote:
> +STATIC __u16 ext2fs_group_desc_csum(ext2_filsys fs, dgrp_t group)
> +{
> +	__u16 crc = 0;
> +	struct ext2_group_desc *desc;
> +
> +	desc = &fs->group_desc[group];
> +
> +	if (fs->super->s_feature_ro_compat & EXT4_FEATURE_RO_COMPAT_GDT_CSUM) {
> +		int offset = offsetof(struct ext2_group_desc, bg_checksum);
> +
> +#ifdef WORDS_BIGENDIAN
> +		struct ext2_group_desc swabdesc = *desc;
> +
> +		/* Have to swab back to little-endian to do the checksum */
> +		ext2fs_swap_group_desc(&swabdesc);
> +		desc = &swabdesc;
> +
> +		group = ext2fs_swab32(group);
> +#endif
> +		crc = crc16(~0, fs->super->s_uuid, sizeof(fs->super->s_uuid));
> +		crc = crc16(crc, &group, sizeof(group));
> +		crc = crc16(crc, desc, offset);
> +		offset += sizeof(desc->bg_checksum); /* skip checksum */
> +		assert(offset == sizeof(*desc));

Note that this assertion needs to be removed when the group descriptor
becomes larger, unless ext2_group_desc never changes in the future..

Cheers, Andreas
--
Andreas Dilger
Sr. Staff Engineer, Lustre Group
Sun Microsystems of Canada, Inc.

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