Understanding memcpy

[Kai Meyer]

I'm trying to poke around an ext4 file system. I can submit a bio for 
the correct block, and read in what seems to be the correct information, 
but when I try to memcpy my char *buffer to a reference to a struct I've 
made, it just doesn't seem to work. The relevant code looks like this:

typedef struct ext2_superblock {
         /* 00-03 */ uint32_t e2sb_inode_count;
         /* 04-07 */ uint32_t e2sb_block_count;
         /* 08-11 */ uint32_t e2sb_blocks_reserved;
         /* 12-15 */ uint32_t e2sb_unallocated_blocks;
         /* 16-19 */ uint32_t e2sb_unallocated_inodes;
         /* 20-23 */ uint32_t e2sb_sb_block;
         /* 24-27 */ uint32_t e2sb_log_block_size;
         /* 28-31 */ uint32_t e2sb_log_fragment_size;
         /* 32-35 */ uint32_t e2sb_num_blocks_per_group;
         /* 36-39 */ uint32_t e2sb_num_frag_per_group;
         /* 40-43 */ uint32_t e2sb_num_inodes_per_group;
         /* 44-47 */ uint32_t e2sb_last_mount_time;
         /* 48-51 */ uint32_t e2sb_last_written_time;
         /* 52-53 */ uint16_t e2sb_num_mounted;
         /* 54-55 */ uint16_t e2sb_num_allowed_mounts;
         /* 56-57 */ uint16_t e2sb_signature;
         /* 58-59 */ uint16_t e2sb_fs_state;
         /* 60-61 */ uint16_t e2sb_error_action;
         /* 62-63 */ uint16_t e2sb_ver_minor;
         /* 64-67 */ uint32_t e2sb_last_check;
         /* 68-71 */ uint32_t e2sb_time_between_checks;
         /* 72-75 */ uint32_t e2sb_os_id;
         /* 76-79 */ uint32_t e2sb_ver_major;
         /* 80-81 */ uint16_t e2sb_uid;
         /* 82-83 */ uint16_t e2sb_gid;
} e2sb;


char *buffer;
uint32_t *pointer;
e2sb sb;
buffer = __bio_kmap_atomic(bio, 0, KM_USER0);
pointer = (uint32_t *)buffer;
printk(KERN_DEBUG "sizeof pbd->sb %lu\n", sizeof(bpd->sb));
printk(KERN_DEBUG "Inode Count: %u\n", pointer[0]); /* Works! */
printk(KERN_DEBUG "Block Count: %u\n", pointer[1]); /* Works! */
printk(KERN_DEBUG "Block Reserved: %u\n", pointer[2]); /* Works! */
printk(KERN_DEBUG "Unallocated blocks: %u\n", pointer[3]); /* Works! */
printk(KERN_DEBUG "Unallocated inodes: %u\n", pointer[4]); /* Works! */
memcpy(buffer, &sb, sizeof(sb));
__bio_kunmap_atomic(bio, KM_USER0);
printk(KERN_DEBUG "e2sb_debug: Total number of inodes in file system 
%u\n", sb->e2sb_inode_count);/* Doesn't work! */
printk(KERN_DEBUG "e2sb_debug: Total number of blocks in file 
system%u\n", sb->e2sb_block_count); /* Doesn't work! */

My code is actually much more verbose. The values I get from indexing 
into pointer are correct, and match what I get from dumpe2fs. The values 
I get from the e2sb struct are not. They are usually 0. I would imagine 
that memcpy is the fastest way to copy data from buffer instead of 
casting the pointer to something else, and using array indexing to get 
the values.

I struggled to find where ext4 actually does this, so I'm making this up 
as I go along. Any thing that you see that I should be doing a different 
way that isn't actually part of my question is welcome too.