Re: aranym bug, manifests as "ida_remove called for id=13" on recent kernels

[Geert Uytterhoeven]

On Thu, Oct 7, 2010 at 19:49, Al Viro <viro@xxxxxxxxxxxxxxxxxx> wrote:
> Â Â Â ÂI've spent quite a while hunting that crap down; reverting VFS fix
> mentioned in original thread *does* get rid of the symptoms, but so does the
> patch below.
>
> Â Â Â ÂWhat happens is this: if ->follow_link() (usually something like
> stat("/proc/2/fd", ...) done by pidof(8)) return ERR_PTR(-....), we return
> to __do_follow_link() and do the following:
> Â Â Â Â*p = dentry->d_inode->i_op->follow_link(dentry, nd);
> Â Â Â Âerror = PTR_ERR(*p);
> Â Â Â Âif (!IS_ERR(*p)) {
> Â Â Â Â Â Â Â Âchar *s = nd_get_link(nd);
> Â Â Â Â Â Â Â Âerror = 0;
> Â Â Â Â Â Â Â Âif (s)
> Â Â Â Â Â Â Â Â Â Â Â Âerror = __vfs_follow_link(nd, s);
> Â Â Â Â Â Â Â Âelse if (nd->last_type == LAST_BIND) {
> Â Â Â Â Â Â Â Â Â Â Â Âerror = force_reval_path(&nd->path, nd);
> Â Â Â Â Â Â Â Â Â Â Â Âif (error)
> Â Â Â Â Â Â Â Â Â Â Â Â Â Â Â Âpath_put(&nd->path);
> Â Â Â Â Â Â Â Â}
> Â Â Â Â}
> Â Â Â Âreturn error;
>
> We _should_ return non-zero value; IS_ERR(ERR_PTR(-n)) is 1 and
> PTR_ERR(ERR_PTR(n)) is -n. ÂWhat happens instead is that this thing
> actually returns 0. ÂAnd no, it's not a miscompile. ÂPatch below
> removes the symptoms of the bug, but only if both parts are present.
> I.e. *not* doing "report = 1" in proc_pid_follow_link() gives us
> visible breakage, despite the fact that report is initialized as
> 1 and nothing except proc_pid_follow_link() ever tries to assign
> anything to it. ÂSeeing that fs/namei.c and fs/proc/base.c are
> compiled separately, we can exclude gcc problems.
>
> The cheapest way to reproduce is to boot with init=/bin/sh, then
> mount /proc and have stat("/proc/2/exe", &st) called; if stat()
> returns 0, we are fscked. ÂThe critical part is between return
> from proc_exe_link() (we'll leave it via if (!mm) return -ENOENT;)
> to return from __do_follow_link() -> do_follow_link() -> link_path_walk().

I booted 2.6.36-rc7-atari-00360-g0dd2e6a (my current private test kernel) with
init=/bin/sh, mounted /proc, and tried

    for i in $(seq 1000); do stat /proc/2/exe; done

a few times, but I didn't see any ida_remove messages.
It cannot read the /proc/2/exe symlink, though.

This is on aranym-0.9.9-1 from Ubuntu/amd64.

Gr{oetje,eeting}s,

            Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
             Â Â -- Linus Torvalds
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