linux-kernel - Re: [PATCH] arm64/debug: Fix registers on sleeping tasks

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Message-ID: <20180302181636.a2nqgm4ezrbcdxkc@lakrids.cambridge.arm.com>
Date:   Fri, 2 Mar 2018 18:16:37 +0000
From:   Mark Rutland <mark.rutland@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     Douglas Anderson <dianders@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        briannorris@...omium.org, linux-kernel@...r.kernel.org,
        evgreen@...omium.org, Ingo Molnar <mingo@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] arm64/debug: Fix registers on sleeping tasks

On Fri, Mar 02, 2018 at 06:01:31PM +0000, Will Deacon wrote:
> On Thu, Mar 01, 2018 at 11:38:03AM -0800, Douglas Anderson wrote:
> > This is the equivalent of commit 001bf455d206 ("ARM: 8428/1: kgdb: Fix
> > registers on sleeping tasks") but for arm64.  Nuff said.
> 
> It's a pity that 001bf455d206 doesn't explain *why* past_pt_regs doesn't
> work.

The task_pt_regs are the userspace regs at the highest address on the
kernel stack:

#define task_pt_regs(p) \
	((struct pt_regs *)(THREAD_SIZE + task_stack_page(p)) - 1)

... for kernel tasks, that's meaningless, and for user tasks, that won't
correspond to kernel state.

> > diff --git a/arch/arm64/kernel/kgdb.c b/arch/arm64/kernel/kgdb.c
> > index 2122cd187f19..01285d4dcdc3 100644
> > --- a/arch/arm64/kernel/kgdb.c
> > +++ b/arch/arm64/kernel/kgdb.c
> > @@ -138,14 +138,26 @@ int dbg_set_reg(int regno, void *mem, struct pt_regs *regs)
> >  void
> >  sleeping_thread_to_gdb_regs(unsigned long *gdb_regs, struct task_struct *task)
> >  {
> > -	struct pt_regs *thread_regs;
> > +	struct thread_struct *thread = &task->thread;
> > +	struct cpu_context *cpu_context = &thread->cpu_context;

We can do this in one go:

	struct cpu_context *cpu_context = &task->thread.cpu_context;

... since we don't need the thread variable otherwise.

> >  
> >  	/* Initialize to zero */
> >  	memset((char *)gdb_regs, 0, NUMREGBYTES);
> > -	thread_regs = task_pt_regs(task);
> > -	memcpy((void *)gdb_regs, (void *)thread_regs->regs, GP_REG_BYTES);
> > -	/* Special case for PSTATE (check comments in asm/kgdb.h for details) */
> > -	dbg_get_reg(33, gdb_regs + GP_REG_BYTES, thread_regs);
> > +
> > +	gdb_regs[19] = cpu_context->x19;
> > +	gdb_regs[20] = cpu_context->x20;
> > +	gdb_regs[21] = cpu_context->x21;
> > +	gdb_regs[22] = cpu_context->x22;
> > +	gdb_regs[23] = cpu_context->x23;
> > +	gdb_regs[24] = cpu_context->x24;
> > +	gdb_regs[25] = cpu_context->x25;
> > +	gdb_regs[26] = cpu_context->x26;
> > +	gdb_regs[27] = cpu_context->x27;
> > +	gdb_regs[28] = cpu_context->x28;
> > +	gdb_regs[29] = cpu_context->fp;
> > +
> > +	gdb_regs[31] = cpu_context->sp;
> > +	gdb_regs[32] = cpu_context->pc;

Are the other reg fields initialised elsewhere?

We might want to zero them here.

Thanks,
Mark.