linux-kernel - Re: mmap

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Message-ID: <1322837127.2762.11.camel@edumazet-laptop>
Date:	Fri, 02 Dec 2011 15:45:27 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Sébastien Paumier 
	<sebastien.paumier@...v-mlv.fr>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: mmap

Le vendredi 02 décembre 2011 à 13:20 +0100, Sébastien Paumier a écrit :
> Hi,
> I have a question about mmap's behavior when one tries to map a file asking for 
> a length greater than the actual file size. When I run the attached code on a 
> 100 bytes file, I have the following output:
> 
> (... file content followed by zeros...)
> n=4096
> write: Bad address
> 
> So, it seems that the actual memory area provided by mmap is one page large and 
> not the requested length of filesize+10000. I guess that 'write' writes less 
> than requested because it was interrupted by the SIGBUS signal. And my question is:
> 
> shouldn't mmap either complain about the requested length or provide an 
> accessible area of the requested length, instead of silently failing ?


Accessing non existing memory leads to SIGBUS signal, not a silent
failure. Its documented behavior.

man mmap

       SIGBUS Attempted access to a portion of the buffer that does not corre‐
              spond to the file (for example, beyond  the  end  of  the  file,
              including  the  case  where  another  process  has truncated the
              file).


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