linux-kernel - Re: [PATCH] arm64: hw_breakpoint: don't clear debug registers in halt mode

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Message-ID: <20200330134218.GB10633@willie-the-truck>
Date:   Mon, 30 Mar 2020 14:42:18 +0100
From:   Will Deacon <will@...nel.org>
To:     Mark Rutland <mark.rutland@....com>
Cc:     Tingwei Zhang <tingwei@...eaurora.org>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64: hw_breakpoint: don't clear debug registers in
 halt mode

On Mon, Mar 30, 2020 at 01:39:46PM +0100, Mark Rutland wrote:
> On Sat, Mar 28, 2020 at 04:32:09PM +0800, Tingwei Zhang wrote:
> > If external debugger sets a breakpoint for one Kernel function
> > when device is in bootloader mode and loads Kernel, this breakpoint
> > will be wiped out in hw_breakpoint_reset(). To fix this, check
> > MDSCR_EL1.HDE in hw_breakpoint_reset(). When MDSCR_EL1.HDE is
> > 0b1, halting debug is enabled. Don't reset debug registers in this case.
> 
> I don't think this is sufficient, because the kernel can still
> subsequently mess with breakpoints, and the HW debugger might not be
> attached at this point in time anyhow.
> 
> I reckon this should hang off the existing "nodebumon" command line
> option, and we shouldn't use HW breakpoints at all when that is passed.
> Then you can pass that to prevent the kernel stomping on the external
> debugger.
> 
> Will, thoughts?

I was going to suggest the same thing, although we will also need to take
care to reset the registers if "nodebugmon" is toggled at runtime via the
"debug_enabled" file in debugfs.

Will