linux-kernel - Re: 2.6.19-rc4-mm1: writev() _functional

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Message-ID: <456A9115.3020101@yahoo.com.au>
Date:	Mon, 27 Nov 2006 18:17:41 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andrew Morton <akpm@...l.org>
CC:	Nick Orlov <bugfixer@...t.ru>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: 2.6.19-rc4-mm1: writev() _functional_ regression

Andrew Morton wrote:
> On Sun, 12 Nov 2006 17:30:24 -0500
> Nick Orlov <bugfixer@...t.ru> wrote:
> 
> 
>>Andrew,
>>
>>Somewhere in between 2.6.18-mm3 and 2.6.19-rc4-mm1 writev() got screwed.
>>It does not accept zero-length segments anymore.
>>
>>Bad thing that it is extremely easy to trigger (even w/o explicit writev calls).
>>For example the following innocent program will fail with 2.6.19-rc4-mm1:
>>
>>======================
>>#include <string.h>
>>#include <fstream>
>>
>>int main()
>>{
>>  char buf[1024];
>>  memset(buf, 'A', sizeof(buf));
>>  std::ofstream ofs("test");
>>  //ofs << 1 << '\n';
>>  ofs.write(buf, sizeof(buf));
>>  return 0;
>>}
>>======================
>>
>>
>>Here is the corresponding part if strace:
>>
>>======================
>>open("test", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
>>writev(3, [{NULL, 0}, {"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"..., 1024}], 2) = -1 EFAULT (Bad address)
>>close(3)                                = 0
>>======================
>>
>>
>>With 2.6.18-mm3 it works
>>
>>======================
>>open("test", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
>>writev(3, [{NULL, 0}, {"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"..., 1024}], 2) = 1024
>>close(3)                                = 0
>>======================
>>
>>
>>It works with 2.6.19-rc4-mm1 _if_ zero-length segments are eliminated
>>(by uncommenting ofs << 1 << '\n'):
>>
>>======================
>>open("test", O_WRONLY|O_CREAT|O_TRUNC|O_LARGEFILE, 0666) = 3
>>writev(3, [{"1\n", 2}, {"AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"..., 1024}], 2) = 1026
>>close(3)                                = 0
>>======================
>>
>>
>>Given that _all_ applications using C++ streams are potentially affected
>>I think it's better to preserve the previous behavior even if it is
>>something from "undefined behavior world" (or a plain bug).
>>
>>The bug is quite dangerous (I was really close to wipe out my mp3 collection).
>>
> 
> 
> OK, thanks.  Those patches do need more work.  I'll shelve them for a while.

Yeah, this just needs some kind of 0 size check in the access check
(arguably fault_in_pages_readable could check for zero size, but it could
go into the caller just as easily).

Thanks for reporting.

-- 
SUSE Labs, Novell Inc.
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