On Sun, 12 Nov 2006 17:30:24 -0500
Nick Orlov <[email protected]> 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.
-
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