Re: [Patch 03/12] x86 architecture implementation of HardwareBreakpoint interfaces

[David Gibson]

On Mon, May 11, 2009 at 05:23:03PM +0530, K.Prasad wrote:
> This patch introduces the arch-specific implementation of
> hw_breakpoint.c inside x86 specific directories. They contain
> functions which help validate and serve requests for using Hardware
> Breakpoint registers on x86 processors.

[snip]
> Index: arch/x86/kernel/hw_breakpoint.c
> ===================================================================
> --- /dev/null
> +++ arch/x86/kernel/hw_breakpoint.c
> @@ -0,0 +1,358 @@
> +/*
> + * This program is free software; you can redistribute it and/or modify
> + * it under the terms of the GNU General Public License as published by
> + * the Free Software Foundation; either version 2 of the License, or
> + * (at your option) any later version.
> + *
> + * This program is distributed in the hope that it will be useful,
> + * but WITHOUT ANY WARRANTY; without even the implied warranty of
> + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + * GNU General Public License for more details.
> + *
> + * You should have received a copy of the GNU General Public License
> + * along with this program; if not, write to the Free Software
> + * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
> + *
> + * Copyright (C) 2007 Alan Stern
> + * Copyright (C) 2009 IBM Corporation
> + */
> +
> +/*
> + * HW_breakpoint: a unified kernel/user-space hardware breakpoint facility,
> + * using the CPU's debug registers.
> + */
> +
> +#include <linux/irqflags.h>
> +#include <linux/notifier.h>
> +#include <linux/kallsyms.h>
> +#include <linux/kprobes.h>
> +#include <linux/percpu.h>
> +#include <linux/kdebug.h>
> +#include <linux/kernel.h>
> +#include <linux/module.h>
> +#include <linux/sched.h>
> +#include <linux/init.h>
> +#include <linux/smp.h>
> +
> +#include <asm/hw_breakpoint.h>
> +#include <asm/processor.h>
> +#include <asm/debugreg.h>
> +
> +/* Unmasked kernel DR7 value */
> +static unsigned long kdr7;
> +
> +/*
> + * Masks for the bits corresponding to registers DR0 - DR3 in DR7 register.
> + * Used to clear and verify the status of bits corresponding to DR0 - DR3
> + */
> +static const unsigned long	dr7_masks[HB_NUM] = {
> +	0x000f0003,	/* LEN0, R/W0, G0, L0 */
> +	0x00f0000c,	/* LEN1, R/W1, G1, L1 */
> +	0x0f000030,	/* LEN2, R/W2, G2, L2 */
> +	0xf00000c0	/* LEN3, R/W3, G3, L3 */
> +};
> +
> +
> +/*
> + * Encode the length, type, Exact, and Enable bits for a particular breakpoint
> + * as stored in debug register 7.
> + */
> +static unsigned long encode_dr7(int drnum, unsigned len, unsigned type)
> +{
> +	unsigned long temp;
> +
> +	temp = (len | type) & 0xf;
> +	temp <<= (DR_CONTROL_SHIFT + drnum * DR_CONTROL_SIZE);
> +	temp |= (DR_GLOBAL_ENABLE << (drnum * DR_ENABLE_SIZE)) |
> +				DR_GLOBAL_SLOWDOWN;
> +	return temp;
> +}
> +
> +void arch_update_kernel_hw_breakpoints(void *unused)
> +{
> +	struct hw_breakpoint *bp;
> +	int i, cpu = get_cpu();
> +
> +	/* Don't allow debug exceptions while we update the registers */
> +	set_debugreg(0UL, 7);
> +
> +	/* Clear all kernel-space breakpoints in kdr7 */
> +	kdr7 = 0;
> +	for (i = hbp_kernel_pos; i < HB_NUM; i++) {
> +		per_cpu(this_hbp_kernel[i], cpu) = bp = hbp_kernel[i];
> +		if (bp) {
> +			kdr7 |= encode_dr7(i, bp->info.len, bp->info.type);
> +			set_debugreg(hbp_kernel[i]->info.address, i);
> +		}
> +	}
> +
> +	/* No need to set DR6. Update the debug registers with kernel-space
> +	 * breakpoint values from kdr7 and user-space requests from the
> +	 * current process
> +	 */
> +	set_debugreg(kdr7 | current->thread.debugreg7, 7);
> +	put_cpu_no_resched();
> +}
> +
> +/*
> + * Install the thread breakpoints in their debug registers.
> + */
> +void arch_install_thread_hw_breakpoint(struct task_struct *tsk)
> +{
> +	struct thread_struct *thread = &(tsk->thread);
> +
> +	switch (hbp_kernel_pos) {
> +	case 4:
> +		set_debugreg(thread->debugreg[3], 3);
> +	case 3:
> +		set_debugreg(thread->debugreg[2], 2);
> +	case 2:
> +		set_debugreg(thread->debugreg[1], 1);
> +	case 1:
> +		set_debugreg(thread->debugreg[0], 0);
> +	default:
> +		break;
> +	}
> +
> +	/* No need to set DR6 */
> +	set_debugreg((kdr7 | thread->debugreg7), 7);
> +}
> +
> +/*
> + * Install the debug register values for just the kernel, no thread.
> + */
> +void arch_uninstall_thread_hw_breakpoint()
> +{
> +	/* Clear the user-space portion of debugreg7 by setting only kdr7 */
> +	set_debugreg(kdr7, 7);
> +
> +}
> +
> +static int get_hbp_len(u8 hbp_len)
> +{
> +	unsigned int len_in_bytes = 0;
> +
> +	switch (hbp_len) {
> +	case HW_BREAKPOINT_LEN_1:
> +		len_in_bytes = 1;
> +		break;
> +	case HW_BREAKPOINT_LEN_2:
> +		len_in_bytes = 2;
> +		break;
> +	case HW_BREAKPOINT_LEN_4:
> +		len_in_bytes = 4;
> +		break;
> +#ifdef CONFIG_X86_64
> +	case HW_BREAKPOINT_LEN_8:
> +		len_in_bytes = 8;
> +		break;
> +#endif

Hrm, the fact that you have to do this nasty back-conversion again
makes me wonder at the wisdom of having per-arch encodings for
breakpoint length.

> +	}
> +	return len_in_bytes;
> +}
> +
> +/*
> + * Check for virtual address in user space.
> + */
> +int arch_check_va_in_userspace(unsigned long va, u8 hbp_len)
> +{
> +	unsigned int len;
> +
> +	len = get_hbp_len(hbp_len);
> +
> +	return (va <= TASK_SIZE - len);
> +}
> +
> +/*
> + * Check for virtual address in kernel space.
> + */
> +int arch_check_va_in_kernelspace(unsigned long va, u8 hbp_len)
> +{
> +	unsigned int len;
> +
> +	len = get_hbp_len(hbp_len);
> +
> +	return (va >= TASK_SIZE) && ((va + len - 1) >= TASK_SIZE);
> +}
> +
> +/*
> + * Store a breakpoint's encoded address, length, and type.
> + */
> +static int arch_store_info(struct hw_breakpoint *bp)

This function doesn't look very arch specific.  Nor is the name very
meaningful.

> +{
> +	/*
> +	 * User-space requests will always have the address field populated
> +	 * For kernel-addresses, either the address or symbol name can be
> +	 * specified.
> +	 */
> +	if (bp->info.name)
> +		bp->info.address = (unsigned long)
> +					kallsyms_lookup_name(bp->info.name);
> +	if (bp->info.address)
> +		return 0;
> +	return -EINVAL;
> +}
> +
> +/*
> + * Validate the arch-specific HW Breakpoint register settings
> + */
> +int arch_validate_hwbkpt_settings(struct hw_breakpoint *bp,
> +						struct task_struct *tsk)
> +{
> +	unsigned int align;
> +	int ret = -EINVAL;
> +
> +	switch (bp->info.type) {
> +	/*
> +	 * Ptrace-refactoring code
> +	 * For now, we'll allow instruction breakpoint only for user-space
> +	 * addresses
> +	 */
> +	case HW_BREAKPOINT_EXECUTE:
> +		if ((!arch_check_va_in_userspace(bp->info.address,
> +							bp->info.len)) &&
> +			bp->info.len != HW_BREAKPOINT_LEN_EXECUTE)
> +			return ret;
> +		break;
> +	case HW_BREAKPOINT_WRITE:
> +		break;
> +	case HW_BREAKPOINT_RW:
> +		break;
> +	default:
> +		return ret;
> +	}
> +
> +	switch (bp->info.len) {
> +	case HW_BREAKPOINT_LEN_1:
> +		align = 0;
> +		break;
> +	case HW_BREAKPOINT_LEN_2:
> +		align = 1;
> +		break;
> +	case HW_BREAKPOINT_LEN_4:
> +		align = 3;
> +		break;
> +#ifdef CONFIG_X86_64
> +	case HW_BREAKPOINT_LEN_8:
> +		align = 7;
> +		break;
> +#endif
> +	default:
> +		return ret;
> +	}
> +
> +	if (bp->triggered)
> +		ret = arch_store_info(bp);
> +
> +	if (ret < 0)
> +		return ret;
> +	/*
> +	 * Check that the low-order bits of the address are appropriate
> +	 * for the alignment implied by len.
> +	 */
> +	if (bp->info.address & align)
> +		return -EINVAL;
> +
> +	/* Check that the virtual address is in the proper range */
> +	if (tsk) {
> +		if (!arch_check_va_in_userspace(bp->info.address, bp->info.len))
> +			return -EFAULT;
> +	} else {
> +		if (!arch_check_va_in_kernelspace(bp->info.address,
> +								bp->info.len))
> +			return -EFAULT;
> +	}
> +	return 0;
> +}
> +
> +void arch_update_user_hw_breakpoint(int pos, struct task_struct *tsk)
> +{
> +	struct thread_struct *thread = &(tsk->thread);
> +	struct hw_breakpoint *bp = thread->hbp[pos];
> +
> +	thread->debugreg7 &= ~dr7_masks[pos];
> +	if (bp) {
> +		thread->debugreg[pos] = bp->info.address;
> +		thread->debugreg7 |= encode_dr7(pos, bp->info.len,
> +							bp->info.type);
> +	} else
> +		thread->debugreg[pos] = 0;
> +}
> +
> +void arch_flush_thread_hw_breakpoint(struct task_struct *tsk)
> +{
> +	int i;
> +	struct thread_struct *thread = &(tsk->thread);
> +
> +	thread->debugreg7 = 0;
> +	for (i = 0; i < HB_NUM; i++)
> +		thread->debugreg[i] = 0;
> +}
> +
> +/*
> + * Handle debug exception notifications.
> + */
> +int __kprobes hw_breakpoint_handler(struct die_args *args)
> +{
> +	int i, cpu, rc = NOTIFY_STOP;
> +	struct hw_breakpoint *bp;
> +	/* The DR6 value is stored in args->err */
> +	unsigned long dr7, dr6 = args->err;
> +
> +	/* Do an early return if no trap bits are set in DR6 */
> +	if ((dr6 & DR_TRAP_BITS) == 0)
> +		return NOTIFY_DONE;
> +
> +	/* Lazy debug register switching */
> +	if (!test_tsk_thread_flag(current, TIF_DEBUG))
> +		switch_to_none_hw_breakpoint();

Shouldn't you also drop out of the handler here, rather than having to
again check for a false alarm below.

It's also not immediately clear to me how lazy switching buys you
anything here, since it's only lazy switching off, not lazy switching
on.

> +	get_debugreg(dr7, 7);
> +	/* Disable breakpoints during exception handling */
> +	set_debugreg(0UL, 7);
> +	/*
> +	 * Assert that local interrupts are disabled
> +	 * Reset the DRn bits in the virtualized register value.
> +	 * The ptrace trigger routine will add in whatever is needed.
> +	 */
> +	current->thread.debugreg6 &= ~DR_TRAP_BITS;
> +	cpu = get_cpu();
> +
> +	/* Handle all the breakpoints that were triggered */
> +	for (i = 0; i < HB_NUM; ++i) {
> +		if (likely(!(dr6 & (DR_TRAP0 << i))))
> +			continue;
> +		/*
> +		 * Find the corresponding hw_breakpoint structure and
> +		 * invoke its triggered callback.
> +		 */
> +		if (i >= hbp_kernel_pos)
> +			bp = per_cpu(this_hbp_kernel[i], cpu);
> +		else {
> +			bp = current->thread.hbp[i];
> +			/* False alarm due to lazy DR switching */
> +			if (!bp)
> +				continue;
> +			rc = NOTIFY_DONE;
> +		}
> +		(bp->triggered)(bp, args->regs);
> +	}
> +	if (dr6 & (~DR_TRAP_BITS))
> +		rc = NOTIFY_DONE;
> +
> +	set_debugreg(dr7, 7);
> +	put_cpu_no_resched();
> +	return rc;
> +}
> +
> +/*
> + * Handle debug exception notifications.
> + */
> +int __kprobes hw_breakpoint_exceptions_notify(
> +		struct notifier_block *unused, unsigned long val, void *data)
> +{
> +	if (val != DIE_DEBUG)
> +		return NOTIFY_DONE;
> +
> +	return hw_breakpoint_handler(data);
> +}
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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