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

[Anshuman Khandual <anshuman.khandual@arm.com>]

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.