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Re: [PATCH] Rewrite and fix grub_bufio_read()

From: Andrei Borzenkov
Subject: Re: [PATCH] Rewrite and fix grub_bufio_read()
Date: Fri, 29 Apr 2016 06:50:46 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.7.2

28.04.2016 22:52, Stefan Fritsch пишет:
> On Thu, 28 Apr 2016, Andrei Borzenkov wrote:
>> 28.04.2016 14:11, Stefan Fritsch пишет:
>>> We had problems downloading large (10-100 MB) gziped files via http
>>> with grub. After some debugging, I think I have found the reason in
>>> grub_bufio_read, which leads to wrong data being passed on. This patch
>>> fixes the issues for me.  I did my tests with a somewhat older
>>> snapshot, but the bufio.c file is the same.
>> Please send minimal patch that fixes bug in bufio so we can actually see
>> where the bug is and apply bug fix for 2.02. It is impossible to deduce
>> from your complete rewrite and this patch is too intrusive for 2.02
>> irrespectively of considerations below.
>>> Cheers,
>>> Stefan
>>> The grub_bufio_read() had some issues:
>>> * in the calculation of next_buf, it assumed that bufio->block_size is a
>>>   power of 2, which is not always the case (see code in grub_bufio_open()).
>> All current callers set it to power of 2, although you are right that it
>> should verify it.
> No:
>   if ((size < 0) || ((unsigned) size > io->size))
>     size = ((io->size > GRUB_BUFIO_MAX_SIZE) ? GRUB_BUFIO_MAX_SIZE :
>             io->size);
> ...
>   bufio->block_size = size;
> io->size is the file size. So at least for files < 32K (which is the size 
> that bufio is called with by the net layer), block_size is not a power of 
> 2 but the size of the file. Then next_buf typically gets a value from 0 
> to 8.

Ah, OK. Not sure why we need this check in the first place. Buffer size
is unrelated to file size, so this looks more like micro-optimization of
buffer size.

Although this means that we always have the whole file in buffer anyway
and so never hit power of 2 issue.

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