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

[Kevin Wolf]

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 <address@hidden>
> 
> 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