linux-kernel - Re: [PATCH] arch_ptrace

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Date:	Tue, 11 Dec 2007 21:51:35 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Roland McGrath <roland@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-ia64@...r.kernel.org,
	Petr Tesarik <ptesarik@...e.cz>, tony.luck@...el.com
Subject: Re: [PATCH] arch_ptrace_stop

On Fri,  7 Dec 2007 17:11:52 -0800 (PST) Roland McGrath <roland@...hat.com> wrote:

> 
> This adds support to allow asm/ptrace.h to define two new macros,
> arch_ptrace_stop_needed and arch_ptrace_stop.  These control special
> machine-specific actions to be done before a ptrace stop.  The new
> code compiles away to nothing when the new macros are not defined.
> This is the case on all machines to begin with.
> 
> On ia64, these macros will be defined to solve the long-standing
> issue of ptrace vs register backing store.
> 
> Signed-off-by: Roland McGrath <roland@...hat.com>
> CC: Petr Tesarik <ptesarik@...e.cz>
> CC: Tony Luck <tony.luck@...el.com>
> ---
>  include/linux/ptrace.h |   35 +++++++++++++++++++++++++++++++++++
>  kernel/signal.c        |   33 ++++++++++++++++++++++++++++++++-
>  2 files changed, 67 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/ptrace.h b/include/linux/ptrace.h
> index ae8146a..7168757 100644
> --- a/include/linux/ptrace.h
> +++ b/include/linux/ptrace.h
> @@ -128,6 +128,41 @@ int generic_ptrace_pokedata(struct task_struct *tsk, long addr, long data);
>  #define force_successful_syscall_return() do { } while (0)
>  #endif
>  
> +#ifndef arch_ptrace_stop_needed
> +/**
> + * arch_ptrace_stop_needed - Decide whether arch_ptrace_stop() should be called
> + * @code:	current->exit_code value ptrace will stop with
> + * @info:	siginfo_t pointer (or %NULL) for signal ptrace will stop with
> + *
> + * This is called with the siglock held, to decide whether or not it's
> + * necessary to release the siglock and call arch_ptrace_stop() with the
> + * same @code and @info arguments.  It can be defined to a constant if
> + * arch_ptrace_stop() is never required, or always is.  On machines where
> + * this makes sense, it should be defined to a quick test to optimize out
> + * calling arch_ptrace_stop() when it would be superfluous.  For example,
> + * if the thread has not been back to user mode since the last stop, the
> + * thread state might indicate that nothing needs to be done.
> + */
> +#define arch_ptrace_stop_needed(code, info)	(0)
> +#endif
> +
> +#ifndef arch_ptrace_stop
> +/**
> + * arch_ptrace_stop - Do machine-specific work before stopping for ptrace
> + * @code:	current->exit_code value ptrace will stop with
> + * @info:	siginfo_t pointer (or %NULL) for signal ptrace will stop with
> + *
> + * This is called with no locks held when arch_ptrace_stop_needed() has
> + * just returned nonzero.  It is allowed to block, e.g. for user memory
> + * access.  The arch can have machine-specific work to be done before
> + * ptrace stops.  On ia64, register backing store gets written back to user
> + * memory here.  Since this can be costly (requires dropping the siglock),
> + * we only do it when the arch requires it for this particular stop, as
> + * indicated by arch_ptrace_stop_needed().
> + */
> +#define arch_ptrace_stop(code, info)		do { } while (0)

Mutter.  These would be better as static inlines.  A macro just invites
variable-unused warnings on non-ia64 and outright compilation errors on
ia64.  Speaking from experience...

static inline void arch_ptrace_stop(int exit_code, siginfo_t *info)
{
}
#define arch_ptrace_stop arch_ptrace_stop

should work?

>  /*
> + * Return nonzero if there is a SIGKILL that should be waking us up.
> + * Called with the siglock held.
> + */
> +static int sigkill_pending(struct task_struct *tsk)
> +{
> +	return ((sigismember(&tsk->pending.signal, SIGKILL) ||
> +		 sigismember(&tsk->signal->shared_pending.signal, SIGKILL)) &&
> +		!unlikely(sigismember(&tsk->blocked, SIGKILL)));
> +}

Could you please take a peek at the infrastructure added by
ftp://ftp.kernel.org/pub/linux/kernel/people/akpm/patches/2.6/2.6.24-rc4/2.6.24-rc4-mm1/broken-out/add-lock_page_killable.patch
and see if there is exploitable commonality?


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