Re: zram: block_state returns premature EOF with short read buffers

[Sergey Senozhatsky <senozhatsky@chromium.org>]

Sorry for the delay in response, your email somehow fell through
the cracks.

On (26/09/20 18:27), Pooyan Azadparvar wrote:
[..]
> However, reading the same file with a one-byte buffer returns zero
> bytes:
> 
>         # python3 - <<'PY'
>         import os
> 
>         path = "/sys/kernel/debug/zram/zram0/block_state"
>         fd = os.open(path, os.O_RDONLY)
> 
>         try:
>             for attempt in range(3):
>                 data = os.read(fd, 1)
>                 offset = os.lseek(fd, 0, os.SEEK_CUR)
>                 print(
>                     f"read {attempt}: bytes={len(data)}, "
>                     f"data={data!r}, offset={offset}"
>                 )
>         finally:
>             os.close(fd)
>         PY
> 
>         read 0: bytes=0, data=b'', offset=0
>         read 1: bytes=0, data=b'', offset=0
>         read 2: bytes=0, data=b'', offset=0
> 
> The file contains a valid record, but the short-buffer reads return zero
> bytes repeatedly and the file position remains unchanged.
> 
> This does not appear to be a memory-safety or security issue. The
> observed impact is that userspace readers using a short buffer receive
> zero bytes and cannot make progress through this debugfs file.
> 
> Could you please confirm whether this short-read behavior is expected
> for block_state?

I don't think I tend to see this as a problem.  Why would anyone do
a 1-byte (or any other tiny buffer) read in a loop?  block_state holds
a lot of data, just pass a huge buffer maybe?