Re: 2.6.19-rc4-mm1: writev() _functional_ regression

[Nick Piggin]

Andrew Morton wrote:
> On Sun, 12 Nov 2006 17:30:24 -0500
> Nick Orlov <bugfixer@xxxxxxx> 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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