Re: [RFC PATCH 6/6] riscv: ptrace: Add hw breakpoint support

["Clément Léger" <cleger@rivosinc.com>]

On 22/07/2025 19:38, Jesse Taube wrote:
> Add ability to setup hw breakpoints to ptrace. Call defines a new
> structure of (ulong[3]){bp_addr, bp_len, bp_type} with
> bp_type being one of HW_BREAKPOINT_LEN_X and
> bp_len being one of HW_BREAKPOINT_X with a value of
> zero dissabling the breakpoint.
> 
> Signed-off-by: Jesse Taube <jesse@rivosinc.com>
> ---
>  arch/riscv/include/asm/processor.h   |  4 ++
>  arch/riscv/include/uapi/asm/ptrace.h |  3 +-
>  arch/riscv/kernel/hw_breakpoint.c    | 14 ++++-
>  arch/riscv/kernel/process.c          |  4 ++
>  arch/riscv/kernel/ptrace.c           | 93 ++++++++++++++++++++++++++++
>  5 files changed, 116 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> index 5f56eb9d114a..488d956a951f 100644
> --- a/arch/riscv/include/asm/processor.h
> +++ b/arch/riscv/include/asm/processor.h
> @@ -12,6 +12,7 @@
>  
>  #include <vdso/processor.h>
>  
> +#include <asm/hw_breakpoint.h>
>  #include <asm/ptrace.h>
>  
>  #define arch_get_mmap_end(addr, len, flags)			\
> @@ -108,6 +109,9 @@ struct thread_struct {
>  	struct __riscv_v_ext_state vstate;
>  	unsigned long align_ctl;
>  	struct __riscv_v_ext_state kernel_vstate;
> +#ifdef CONFIG_HAVE_HW_BREAKPOINT
> +	struct perf_event *ptrace_bps[RV_MAX_TRIGGERS];
> +#endif
>  #ifdef CONFIG_SMP
>  	/* Flush the icache on migration */
>  	bool force_icache_flush;
> diff --git a/arch/riscv/include/uapi/asm/ptrace.h b/arch/riscv/include/uapi/asm/ptrace.h
> index a38268b19c3d..a7998ed41913 100644
> --- a/arch/riscv/include/uapi/asm/ptrace.h
> +++ b/arch/riscv/include/uapi/asm/ptrace.h
> @@ -14,7 +14,8 @@
>  
>  #define PTRACE_GETFDPIC_EXEC	0
>  #define PTRACE_GETFDPIC_INTERP	1
> -
> +#define PTRACE_GETHBPREGS	2
> +#define PTRACE_SETHBPREGS	3
>  /*
>   * User-mode register state for core dumps, ptrace, sigcontext
>   *
> diff --git a/arch/riscv/kernel/hw_breakpoint.c b/arch/riscv/kernel/hw_breakpoint.c
> index 437fd82b9590..c58145464539 100644
> --- a/arch/riscv/kernel/hw_breakpoint.c
> +++ b/arch/riscv/kernel/hw_breakpoint.c
> @@ -633,7 +633,19 @@ void arch_uninstall_hw_breakpoint(struct perf_event *event)
>  		pr_warn("%s: Failed to uninstall trigger %d. error: %ld\n", __func__, i, ret.error);
>  }
>  
> -void flush_ptrace_hw_breakpoint(struct task_struct *tsk) { }
> +/*
> + * Release the user breakpoints used by ptrace
> + */
> +void flush_ptrace_hw_breakpoint(struct task_struct *tsk)
> +{
> +	int i;
> +	struct thread_struct *t = &tsk->thread;
> +
> +	for (i = 0; i < dbtr_total_num; i++) {
> +		unregister_hw_breakpoint(t->ptrace_bps[i]);
> +		t->ptrace_bps[i] = NULL;
> +	}
> +}

Hi Jesse,

You can discard my comment in the previous patch about implementing this
function ;)

Thanks,

Clément

>  
>  void hw_breakpoint_pmu_read(struct perf_event *bp) { }
>  
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index 15d8f75902f8..9cf07ecfb523 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -9,6 +9,7 @@
>  
>  #include <linux/bitfield.h>
>  #include <linux/cpu.h>
> +#include <linux/hw_breakpoint.h>
>  #include <linux/kernel.h>
>  #include <linux/sched.h>
>  #include <linux/sched/debug.h>
> @@ -164,6 +165,7 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
>  
>  void flush_thread(void)
>  {
> +	flush_ptrace_hw_breakpoint(current);
>  #ifdef CONFIG_FPU
>  	/*
>  	 * Reset FPU state and context
> @@ -218,6 +220,8 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
>  		set_bit(MM_CONTEXT_LOCK_PMLEN, &p->mm->context.flags);
>  
>  	memset(&p->thread.s, 0, sizeof(p->thread.s));
> +	if (IS_ENABLED(CONFIG_HAVE_HW_BREAKPOINT))
> +		memset(p->thread.ptrace_bps, 0, sizeof(p->thread.ptrace_bps));
>  
>  	/* p->thread holds context to be restored by __switch_to() */
>  	if (unlikely(args->fn)) {
> diff --git a/arch/riscv/kernel/ptrace.c b/arch/riscv/kernel/ptrace.c
> index ea67e9fb7a58..b78cfb0f1c0e 100644
> --- a/arch/riscv/kernel/ptrace.c
> +++ b/arch/riscv/kernel/ptrace.c
> @@ -9,11 +9,13 @@
>  
>  #include <asm/vector.h>
>  #include <asm/ptrace.h>
> +#include <asm/hw_breakpoint.h>
>  #include <asm/syscall.h>
>  #include <asm/thread_info.h>
>  #include <asm/switch_to.h>
>  #include <linux/audit.h>
>  #include <linux/compat.h>
> +#include <linux/hw_breakpoint.h>
>  #include <linux/ptrace.h>
>  #include <linux/elf.h>
>  #include <linux/regset.h>
> @@ -336,12 +338,103 @@ void ptrace_disable(struct task_struct *child)
>  {
>  }
>  
> +#ifdef CONFIG_HAVE_HW_BREAKPOINT
> +static void ptrace_hbptriggered(struct perf_event *bp,
> +				struct perf_sample_data *data,
> +				struct pt_regs *regs)
> +{
> +	struct arch_hw_breakpoint *bkpt = counter_arch_bp(bp);
> +	int num = 0;
> +
> +	force_sig_ptrace_errno_trap(num, (void __user *)bkpt->address);
> +}
> +
> +/*
> + * idx selects the breakpoint index.
> + * Both PTRACE_GETHBPREGS and PTRACE_SETHBPREGS transfer three 32-bit words:
> + * address (0), length (1), type (2).
> + * Instruction breakpoint length is one of HW_BREAKPOINT_LEN_X or 0. 0 will
> + * disable the breakpoint.
> + * Instruction breakpoint type is one of HW_BREAKPOINT_X.
> + */
> +
> +static long ptrace_gethbpregs(struct task_struct *child, unsigned long idx,
> +			      unsigned long __user *datap)
> +{
> +	struct perf_event *bp;
> +	unsigned long user_data[3] = {0};
> +
> +	if (idx >= RV_MAX_TRIGGERS)
> +		return -EINVAL;
> +
> +	bp = child->thread.ptrace_bps[idx];
> +
> +	if (!IS_ERR_OR_NULL(bp)) {
> +		user_data[0] = bp->attr.bp_addr;
> +		user_data[1] = bp->attr.disabled ? 0 : bp->attr.bp_len;
> +		user_data[2] = bp->attr.bp_type;
> +	}
> +
> +	if (copy_to_user(datap, user_data, sizeof(user_data)))
> +		return -EFAULT;
> +
> +	return 0;
> +}
> +
> +static long ptrace_sethbpregs(struct task_struct *child, unsigned long idx,
> +			      unsigned long __user *datap)
> +{
> +	struct perf_event *bp;
> +	struct perf_event_attr attr;
> +	unsigned long user_data[3];
> +
> +	if (idx >= RV_MAX_TRIGGERS)
> +		return -EINVAL;
> +
> +	if (copy_from_user(user_data, datap, sizeof(user_data)))
> +		return -EFAULT;
> +
> +	bp = child->thread.ptrace_bps[idx];
> +	if (IS_ERR_OR_NULL(bp))
> +		attr = bp->attr;
> +	else
> +		ptrace_breakpoint_init(&attr);
> +
> +	attr.bp_addr = user_data[0];
> +	attr.bp_len = user_data[1];
> +	attr.bp_type = user_data[2];
> +	attr.disabled = !attr.bp_len;
> +
> +	if (IS_ERR_OR_NULL(bp)) {
> +		bp = register_user_hw_breakpoint(&attr, ptrace_hbptriggered, NULL,
> +					   child);
> +		if (IS_ERR(bp))
> +			return PTR_ERR(bp);
> +
> +		child->thread.ptrace_bps[idx] = bp;
> +		return 0;
> +	} else {
> +		return modify_user_hw_breakpoint(bp, &attr);
> +	}
> +}
> +#endif
> +
>  long arch_ptrace(struct task_struct *child, long request,
>  		 unsigned long addr, unsigned long data)
>  {
>  	long ret = -EIO;
> +	unsigned long __user *datap = (unsigned long __user *) data;
>  
>  	switch (request) {
> +#ifdef CONFIG_HAVE_HW_BREAKPOINT
> +	case PTRACE_GETHBPREGS:
> +		ret = ptrace_gethbpregs(child, addr, datap);
> +		break;
> +
> +	case PTRACE_SETHBPREGS:
> +		ret = ptrace_sethbpregs(child, addr, datap);
> +		break;
> +#endif
>  	default:
>  		ret = ptrace_request(child, request, addr, data);
>  		break;