Re: [PATCH] arm64/ptrace: Make user_hwdebug_state.dbg_regs[] array size as ARM_MAX_BRP

[Will Deacon <will@kernel.org>]

On Fri, May 30, 2025 at 09:19:53AM +0530, Anshuman Khandual wrote:
> 
> 
> On 5/16/25 19:29, Will Deacon wrote:
> > On Mon, Apr 21, 2025 at 11:22:12AM +0530, Anshuman Khandual wrote:
> >> Array elements inside 'struct user_hwdebug_state.dbg_regs[]' are inherently
> >> coupled with maximum breakpoints or watchpoints which could be present on a
> >> platform and which are defined with macros ARM_MAX_[BRP|WRP].
> >>
> >> Rather than explicitly trying to keep the array elements in sync with these
> >> macros and then adding a BUILD_BUG_ON() just to ensure continued compliance
> >> , move these two macros into the uapi ptrace header itself thus making them
> >> available both for user space and kernel.
> >>
> >> While here also ensure that ARM_MAX_BRP and ARM_MAX_WRP are always the same
> >> via a new BUILD_BUG_ON(). This helps in making sure that user_hwdebug_state
> >> structure remains usable both for breakpoint and watchpoint registers set
> >> via ptrace() system call interface.
> >>
> >> Cc: Will Deacon <will@kernel.org>
> >> Cc: Mark Rutland <mark.rutland@arm.com>
> >> Cc: Catalin Marinas <catalin.marinas@arm.com>
> >> Cc: linux-arm-kernel@lists.infradead.org
> >> Cc: linux-perf-users@vger.kernel.org
> >> Cc: linux-kernel@vger.kernel.org
> >> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
> >> ---
> >> This patch applies on v6.15-rc3
> >>
> >>  arch/arm64/include/asm/hw_breakpoint.h |  7 -------
> >>  arch/arm64/include/uapi/asm/ptrace.h   | 10 +++++++++-
> >>  arch/arm64/kernel/hw_breakpoint.c      |  9 +++++++++
> >>  3 files changed, 18 insertions(+), 8 deletions(-)
> >>
> >> diff --git a/arch/arm64/include/asm/hw_breakpoint.h b/arch/arm64/include/asm/hw_breakpoint.h
> >> index bd81cf17744a..63c21b515647 100644
> >> --- a/arch/arm64/include/asm/hw_breakpoint.h
> >> +++ b/arch/arm64/include/asm/hw_breakpoint.h
> >> @@ -75,13 +75,6 @@ static inline void decode_ctrl_reg(u32 reg,
> >>  #define ARM_KERNEL_STEP_ACTIVE	1
> >>  #define ARM_KERNEL_STEP_SUSPEND	2
> >>  
> >> -/*
> >> - * Limits.
> >> - * Changing these will require modifications to the register accessors.
> >> - */
> >> -#define ARM_MAX_BRP		16
> >> -#define ARM_MAX_WRP		16
> >> -
> >>  /* Virtual debug register bases. */
> >>  #define AARCH64_DBG_REG_BVR	0
> >>  #define AARCH64_DBG_REG_BCR	(AARCH64_DBG_REG_BVR + ARM_MAX_BRP)
> >> diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
> >> index 0f39ba4f3efd..8683f541a467 100644
> >> --- a/arch/arm64/include/uapi/asm/ptrace.h
> >> +++ b/arch/arm64/include/uapi/asm/ptrace.h
> >> @@ -99,6 +99,14 @@ struct user_fpsimd_state {
> >>  	__u32		__reserved[2];
> >>  };
> >>  
> >> +/*
> >> + * Maximum number of breakpoint and watchpoint registers
> >> + * on the platform. These macros get used both in kernel
> >> + * and user space as well.
> >> + */
> >> +#define ARM_MAX_BRP		16
> >> +#define ARM_MAX_WRP		16
> >> +
> >>  struct user_hwdebug_state {
> >>  	__u32		dbg_info;
> >>  	__u32		pad;
> >> @@ -106,7 +114,7 @@ struct user_hwdebug_state {
> >>  		__u64	addr;
> >>  		__u32	ctrl;
> >>  		__u32	pad;
> >> -	}		dbg_regs[16];
> >> +	}		dbg_regs[ARM_MAX_BRP];	/* Or ARM_MAX_WRP */
> >>  };
> >>  
> >>  /* SVE/FP/SIMD state (NT_ARM_SVE & NT_ARM_SSVE) */
> >> diff --git a/arch/arm64/kernel/hw_breakpoint.c b/arch/arm64/kernel/hw_breakpoint.c
> >> index 722ac45f9f7b..9bc51682713d 100644
> >> --- a/arch/arm64/kernel/hw_breakpoint.c
> >> +++ b/arch/arm64/kernel/hw_breakpoint.c
> >> @@ -981,6 +981,15 @@ static int __init arch_hw_breakpoint_init(void)
> >>  {
> >>  	int ret;
> >>  
> >> +	/*
> >> +	 * Maximum supported breakpoint and watchpoint registers must
> >> +	 * always be the same - regardless of actual register numbers
> >> +	 * found on a given platform. This is because the user facing
> >> +	 * ptrace structure 'user_hwdebug_state' actually depends on
> >> +	 * these macros to be the same.
> >> +	 */
> >> +	BUILD_BUG_ON(ARM_MAX_BRP != ARM_MAX_WRP);
> > 
> > Sorry, but I don't understand why this patch is an improvement over what
> > we have.
> This is an improvement because
> 
> - user_hwdebug_state.dbg_regs[16] is hard coded for size without context
>   requiring explicit sync up when ARM_MAX_WRP/BRP changes to 64 later on
>   with FEAT_Debugv8p9. Defining the array size in terms of ARM_MAX_WRP/
>   BRP provides required context and also avoids the need for an explicit
>   hard coded sync up when those macros change.
> 
> - user_hwdebug_state.dbg_regs[16] gets used both for breakpoint registers
>   and watchpoint registers. Hence there is an inherent assumption that
>   ARM_MAX_BRP == ARM_MAX_WRP which should be ensured with a corresponding
>   assert.

Sorry, but I still don't see it. Plus, this is a UAPI header so I think
it's no bad thing to require an "explicit sync up", assuming we can
actually change this structure at all.

Let's leave the code as it is, please.

Will