Re: [PATCH] fs/coredump: open coredump file in O_WRONLY instead of O_RDWR

[Vladimir Sementsov-Ogievskiy]

On 15.05.23 22:13, Linus Torvalds wrote:
> On Mon, May 15, 2023 at 11:50 AM Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxx> wrote:
> > It's strange, because the "O_WRONLY" -> "2" change that changes to a
> > magic raw number is right next to changing "(unsigned short) 0x10" to
> > "KERNEL_DS", so we're getting *rid* of a magic raw number there.
>
> Oh, no, never mind. I see what is going on.
>
> Back then, "open_namei()" didn't actually take O_RDWR style flags AT ALL.
>
> The O_RDONLY flags are broken, because you cannot say "open with no
> permissions", which we used internally. You have
>
>   0 - read-only
>   1 - write-only
>   2 - read-write
>
> but the internal code actually wants to match that up with the
> read-write permission bits (FMODE_READ etc).
>
> And then we've long had a special value for "open for special
> accesses" (format etc), which (naturally) was 3.
>
> So then the open code would do
>
>          f->f_flags = flag = flags;
>          f->f_mode = (flag+1) & O_ACCMODE;
>          if (f->f_mode)
>                  flag++;
>
> which means that "f_mode" now becomes that FMODE_READ | FMODE_WRITE
> mask, and "flag" ends up being a translation from that O_RDWR space
> (0/1/2/3) into the FMODE_READ/WRITE space (1/2/3/3, where "special"
> required read-write permissions, and 0 was only used for symlinks).
>
> We still have that, although the code looks different.
>
> So back then, "open_namei()" took that FMODE_READ/WRITE flag as an
> argument, and the  "O_WRONLY" -> "2" change is actually a bugfix and
> makes sense. The O_WRONLY thing was wrong, because it was 1, which
> actuall ymeant FMODE_READ.
>
> And back then, we didn't *have* FMODE_READ and FMODE_WRITE.
>
> So just writing it as "2" made sense, even if it was horrible. We
> added FMODE_WRITE later, but never fixed up those core file writers.
>
> So that 0.99pl10 commit from 1993 is actually correct, and the bug
> happened *later*.
>
> I think the real bug may have been in 2.2.4pre4 (February 16, 1999),
> when this happened:
>
> -       dentry = open_namei(corefile,O_CREAT | 2 | O_TRUNC | O_NOFOLLOW, 0600);
> ...
> +       file = filp_open(corefile,O_CREAT | 2 | O_TRUNC | O_NOFOLLOW, 0600);
>
> without realizing that the "2" in open_namei() should have become a
> O_WRONLY for filp_open().
>
> So I think this explains it all.
>
> Very understandable mistake after all.
>
>                      Linus

Wow that's became a detective story, great thanks! [took note to check history myself next time]

--
Best regards,
Vladimir