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

David Gibson
2009-05-28T06:53:31Z

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_!