Re: [PATCH 2/2] powerpc: FULL_REGS on exec

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Mon, 2007-09-24 at 16:52 -0700, Roland McGrath wrote:
> When PTRACE_O_TRACEEXEC is used, a ptrace call to fetch the registers at
> the PTRACE_EVENT_EXEC stop (PTRACE_PEEKUSR) will oops in CHECK_FULL_REGS.
> With recent versions, "gdb --args /bin/sh -c 'exec /bin/true'" and "run" at
> the (gdb) prompt is sufficient to produce this.  I also have written an
> isolated test case, see https://bugzilla.redhat.com/show_bug.cgi?id=301791#c15.
> 
> This change fixes the problem by clearing the low bit of pt_regs.trap in
> start_thread so that FULL_REGS is true again.  This is correct since all of
> the GPRs that "full" refers to are cleared in start_thread.

Looks good, nice catch.

> Signed-off-by: Roland McGrath <roland@redhat.com>

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

> ---
>  arch/powerpc/kernel/process.c |    7 +++++++
>  1 files changed, 7 insertions(+), 0 deletions(-)
> 
> diff --git a/arch/powerpc/kernel/process.c b/arch/powerpc/kernel/process.c
> index e477c9d..fd799d2 100644
> --- a/arch/powerpc/kernel/process.c
> +++ b/arch/powerpc/kernel/process.c
> @@ -605,6 +605,13 @@ void start_thread(struct pt_regs *regs, unsigned long start, unsigned long sp)
>  	regs->ccr = 0;
>  	regs->gpr[1] = sp;
>  
> +	/*
> +	 * We have just cleared all the nonvolatile GPRs, so make
> +	 * FULL_REGS(regs) return true.  This is necessary to allow
> +	 * ptrace to examine the thread immediately after exec.
> +	 */
> +	regs->trap &= ~1UL;
> +
>  #ifdef CONFIG_PPC32
>  	regs->mq = 0;
>  	regs->nip = start;
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