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Date: Thu, 1 Oct 2015 12:39:14 +0200
From: Ingo Molnar <mingo@kernel.org>
To: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Kees Cook <keescook@chromium.org>, Thomas Gleixner <tglx@linutronix.de>,
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Subject: Re: [PATCH v3] fs/proc, core/debug: Don't expose absolute kernel
 addresses via wchan
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* Andrey Ryabinin <ryabinin.a.a@gmail.com> wrote:

> I think you misunderstood me.
> Yes, this code currently doesn't use %pX, but it could:
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index b25eee4..f58f66e 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -425,18 +425,7 @@ static int proc_pid_auxv(struct seq_file *m, struct pid_namespace *ns,
>  static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
>                           struct pid *pid, struct task_struct *task)
>  {
> -       unsigned long wchan;
> -       char symname[KSYM_NAME_LEN];
> -
> -       wchan = get_wchan(task);
> -
> -       if (lookup_symbol_name(wchan, symname) < 0) {
> -               if (!ptrace_may_access(task, PTRACE_MODE_READ))
> -                       return 0;
> -               seq_printf(m, "%lu", wchan);
> -       } else {
> -               seq_printf(m, "%s", symname);
> -       }
> +       seq_printf(m, "%ps", get_wchan(task));
>  
>         return 0;
>  }
> 
> 
> There is a problem here, though. %ps will print absolute kernel address instead of symbol name
> if KALLSYMS=n or if resolution of address failed.
> So I was wondering, may be should just fix %ps ?
> i.e. print 0 instead of absolute address if KALLSYMS=n or lookup failure?

There's another problem as well: your change loses the PTRACE_MODE_READ permission 
check.

But ... I think I like it open coded, which is good precisely because it will stay 
invariant even if we change details in the %ps/etc. debug output.

Thanks,

	Ingo