Re: [PATCH v5 4/5] vvfat: Fix reading files with non-continuous clusters

[Kevin Wolf <kwolf@redhat.com>]

Am 12.06.2024 um 14:43 hat Amjad Alsharafi geschrieben:
> When reading with `read_cluster` we get the `mapping` with
> `find_mapping_for_cluster` and then we call `open_file` for this
> mapping.
> The issue appear when its the same file, but a second cluster that is
> not immediately after it, imagine clusters `500 -> 503`, this will give
> us 2 mappings one has the range `500..501` and another `503..504`, both
> point to the same file, but different offsets.
> 
> When we don't open the file since the path is the same, we won't assign
> `s->current_mapping` and thus accessing way out of bound of the file.
> 
> From our example above, after `open_file` (that didn't open anything) we
> will get the offset into the file with
> `s->cluster_size*(cluster_num-s->current_mapping->begin)`, which will
> give us `0x2000 * (504-500)`, which is out of bound for this mapping and
> will produce some issues.
> 
> Signed-off-by: Amjad Alsharafi <amjadsharafi10@gmail.com>
> ---
>  block/vvfat.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)
> 
> diff --git a/block/vvfat.c b/block/vvfat.c
> index b63ac5d045..fc570d0610 100644
> --- a/block/vvfat.c
> +++ b/block/vvfat.c
> @@ -1360,15 +1360,24 @@ static int open_file(BDRVVVFATState* s,mapping_t* mapping)
>  {
>      if(!mapping)
>          return -1;
> +    int new_path = 1;
>      if(!s->current_mapping ||
> -            strcmp(s->current_mapping->path,mapping->path)) {
> -        /* open file */
> -        int fd = qemu_open_old(mapping->path,
> +            s->current_mapping->info.file.offset
> +                != mapping->info.file.offset ||

I'm wondering if this couldn't just be s->current_mapping != mapping?

> +            (new_path = strcmp(s->current_mapping->path, mapping->path))) {

If both the path and the offset change, we still want to set new_path, I
think. And if we didn't already have a mapping, we also need to open the
file.

Actually, setting a variable inside the condition makes it kind of hard
to read, so if s->current_mapping != mapping works, we can do the check
only in the conditon below:

> +        if (new_path) {

if (!s->current_mapping ||
    strcmp(s->current_mapping->path, mapping->path))

> +            /* open file */
> +            int fd = qemu_open_old(mapping->path,
>                                 O_RDONLY | O_BINARY | O_LARGEFILE);
> -        if(fd<0)
> -            return -1;
> -        vvfat_close_current_file(s);
> -        s->current_fd = fd;
> +            if (fd < 0) {
> +                return -1;
> +            }
> +            vvfat_close_current_file(s);
> +
> +            s->current_fd = fd;
> +        }
> +        assert(s->current_fd);
>          s->current_mapping = mapping;
>      }

Kevin