Re: [Qemu-devel] [PATCH 2/3] dmg: use pread

[Kevin Wolf <kwolf@redhat.com>]

Am 07.05.2010 16:56, schrieb Christoph Hellwig:
> Use pread instead of lseek + read in preparation of using the qemu
> block API.  Note that dmg actually uses the implicit file offset
> a lot in dmg_open, and we had to replace it with an offset variable.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> 
> Index: qemu-kevin/block/dmg.c
> ===================================================================
> --- qemu-kevin.orig/block/dmg.c	2010-05-03 13:17:40.696287171 +0200
> +++ qemu-kevin/block/dmg.c	2010-05-03 13:29:39.560024006 +0200
> @@ -58,18 +58,18 @@ static int dmg_probe(const uint8_t *buf,
>      return 0;
>  }
>  
> -static off_t read_off(int fd)
> +static off_t read_off(int fd, int64_t offset)
>  {
>  	uint64_t buffer;
> -	if(read(fd,&buffer,8)<8)
> +	if (pread(fd, &buffer, 8, offset) < 8)
>  		return 0;
>  	return be64_to_cpu(buffer);
>  }
>  
> -static off_t read_uint32(int fd)
> +static off_t read_uint32(int fd, int64_t offset)
>  {
>  	uint32_t buffer;
> -	if(read(fd,&buffer,4)<4)
> +	if (pread(fd, &buffer, 4, offset) < 4)
>  		return 0;
>  	return be32_to_cpu(buffer);
>  }
> @@ -80,6 +80,7 @@ static int dmg_open(BlockDriverState *bs
>      off_t info_begin,info_end,last_in_offset,last_out_offset;
>      uint32_t count;
>      uint32_t max_compressed_size=1,max_sectors_per_chunk=1,i;
> +    int64_t offset;
>  
>      s->fd = open(filename, O_RDONLY | O_BINARY);
>      if (s->fd < 0)
> @@ -89,38 +90,45 @@ static int dmg_open(BlockDriverState *bs
>      s->offsets = s->lengths = s->sectors = s->sectorcounts = NULL;
>  
>      /* read offset of info blocks */
> -    if(lseek(s->fd,-0x1d8,SEEK_END)<0) {
> +    offset = lseek(s->fd, -0x1d8, SEEK_END);
> +    if (offset < 0) {
>          goto fail;
>      }
>  
> -    info_begin=read_off(s->fd);
> -    if(info_begin==0)
> -	goto fail;
> -    if(lseek(s->fd,info_begin,SEEK_SET)<0)
> -	goto fail;

We seek to info_begin.

> -    if(read_uint32(s->fd)!=0x100)
> -	goto fail;

Now we are at info_begin + 4

> -    if((count = read_uint32(s->fd))==0)
> -	goto fail;

info_begin + 8

> -    info_end = info_begin+count;
> -    if(lseek(s->fd,0xf8,SEEK_CUR)<0)

info_begin + 0x100

> +    info_begin = read_off(s->fd, offset);
> +    if (info_begin == 0) {
>  	goto fail;
> +    }
> +
> +    if (read_uint32(s->fd, info_begin) != 0x100) {
> +        goto fail;
> +    }
> +
> +    count = read_uint32(s->fd, info_begin + 4);
> +    if (count == 0) {
> +        goto fail;
> +    }
> +    info_end = info_begin + count;
> +
> +    offset = info_begin + 0xfc;

So, wrong offset here?

>  
>      /* read offsets */
>      last_in_offset = last_out_offset = 0;
> -    while(lseek(s->fd,0,SEEK_CUR)<info_end) {
> +    while (offset < info_end) {
>          uint32_t type;
>  
> -	count = read_uint32(s->fd);
> +	count = read_uint32(s->fd, offset);
>  	if(count==0)
>  	    goto fail;
> -	type = read_uint32(s->fd);
> -	if(type!=0x6d697368 || count<244)
> -	    lseek(s->fd,count-4,SEEK_CUR);
> -	else {
> +        offset += 4;
> +
> +	type = read_uint32(s->fd, offset);
> +	if (type == 0x6d697368 && count >= 244) {
>  	    int new_size, chunk_count;
> -	    if(lseek(s->fd,200,SEEK_CUR)<0)
> -	        goto fail;
> +
> +            offset += 4;

Isn't this needed in the else case, too?

> +            offset += 200;
> +
>  	    chunk_count = (count-204)/40;
>  	    new_size = sizeof(uint64_t) * (s->n_chunks + chunk_count);
>  	    s->types = qemu_realloc(s->types, new_size/2);
> @@ -130,7 +138,8 @@ static int dmg_open(BlockDriverState *bs
>  	    s->sectorcounts = qemu_realloc(s->sectorcounts, new_size);
>  
>  	    for(i=s->n_chunks;i<s->n_chunks+chunk_count;i++) {
> -		s->types[i] = read_uint32(s->fd);
> +		s->types[i] = read_uint32(s->fd, offset);
> +		offset += 4;
>  		if(s->types[i]!=0x80000005 && s->types[i]!=1 && s->types[i]!=2) {
>  		    if(s->types[i]==0xffffffff) {
>  			last_in_offset = s->offsets[i-1]+s->lengths[i-1];
> @@ -138,15 +147,24 @@ static int dmg_open(BlockDriverState *bs
>  		    }
>  		    chunk_count--;
>  		    i--;
> -		    if(lseek(s->fd,36,SEEK_CUR)<0)
> -			goto fail;
> +		    offset += 36;
>  		    continue;
>  		}
> -		read_uint32(s->fd);
> -		s->sectors[i] = last_out_offset+read_off(s->fd);
> -		s->sectorcounts[i] = read_off(s->fd);
> -		s->offsets[i] = last_in_offset+read_off(s->fd);
> -		s->lengths[i] = read_off(s->fd);
> +		read_uint32(s->fd, offset);

This read is useless. offset += 4 alone should be enough.

> +		offset += 4;
> +
> +		s->sectors[i] = last_out_offset+read_off(s->fd, offset);
> +		offset += 8;
> +
> +		s->sectorcounts[i] = read_off(s->fd, offset);
> +		offset += 8;
> +
> +		s->offsets[i] = last_in_offset+read_off(s->fd, offset);
> +		offset += 8;
> +
> +		s->lengths[i] = read_off(s->fd, offset);
> +		offset += 8;
> +
>  		if(s->lengths[i]>max_compressed_size)
>  		    max_compressed_size = s->lengths[i];
>  		if(s->sectorcounts[i]>max_sectors_per_chunk)
> @@ -210,15 +228,12 @@ static inline int dmg_read_chunk(BDRVDMG
>  	case 0x80000005: { /* zlib compressed */
>  	    int i;
>  
> -	    ret = lseek(s->fd, s->offsets[chunk], SEEK_SET);
> -	    if(ret<0)
> -		return -1;
> -
>  	    /* we need to buffer, because only the chunk as whole can be
>  	     * inflated. */
>  	    i=0;
>  	    do {
> -		ret = read(s->fd, s->compressed_chunk+i, s->lengths[chunk]-i);
> +		ret = pread(s->fd, s->compressed_chunk+i, s->lengths[chunk]-i,
> +                            s->offsets[chunk]);

This is in a loop, whereas the lseek was outside the loop. From the
second iteration on you'll repeat the first read instead of advancing.

Kevin