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From Jeff Trawick <traw...@gmail.com>
Subject Re: buffered files read after apr_file_close()'ed
Date Tue, 03 Dec 2013 13:18:16 GMT
On Tue, Dec 3, 2013 at 3:16 AM, Stefan Ruppert <sr@myarm.com> wrote:

> Am 03.12.2013 00:37, schrieb William A. Rowe Jr.:
>
>  On Mon, 02 Dec 2013 01:34:58 +0100
>> Branko ─îibej <brane@apache.org> wrote:
>>
>>  On 02.12.2013 01:29, Eric Covener wrote:
>>>
>>>> I am looking at a httpd bug that causes a hang on windows but
>>>> succeeds on unix.
>>>>
>>>> It seems that (short) files are opened w/ buffering, read,
>>>> apr_file_closed, and read again [succesfully on unix]
>>>>
>>>> On Unix, they sare satisfied out of the buffer.  file->fileset is
>>>> -1. On Windows, the destroyed apr_thread_mutex causes a hang.
>>>>
>>>> Is reading from the closed file on the extreme bogus end of the
>>>> spectrum as I suspect and just getting lucky on the unix case?
>>>>
>>>
>>> I'd certainly call a successful read from a closed file a bug.
>>>
>>>  Should they blow up in 2.0 during a read if they've been closed?
>>>>
>>>
>>> Dunno ... my gut feeling in cases like this is to just leave it be.
>>> Developers should have some fun finding heisenbugs, too. :)
>>>
>>
>> If we fix this, it needs to be minimal impact.  Zero'ing out the
>> buffer on close seems like the lowest cpu impact.
>>
>>
> A minimal impact and IMHO the correct fix is to return an error in
> buffered I/O if the file was closed.
>
> An application should not call a read or write function on a closed file.
> If it does its a bug and it should be notified about that fact.
>

I keep telling myself over the years that APR's propensity to crash when
called in situations as wrong as this (quite different from proprietary
libraries with other considerations) is actually very good for code quality
in the long run.

What's the least we can do for this case that avoids having to check good
calls for validity and yet give unmistakable feedback?  Clear the buffer
pointer during close?



> Thus I would propose the following fix in apr_file_read() and any other
> read/write function which uses buffered I/O:
>
> APR_DECLARE(apr_status_t) apr_file_read(apr_file_t *thefile, void *buf,
> apr_size_t *nbytes)
> {
>     apr_ssize_t rv;
>     apr_size_t bytes_read;
>
> +    if (thefile->filedes < 0) {
> +        return APR_EBADF;
> +    }
>
>     if (*nbytes <= 0) {
>         *nbytes = 0;
>         return APR_SUCCESS;
>     }
>
> Regards,
> Stefan
>
>


-- 
Born in Roswell... married an alien...
http://emptyhammock.com/

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