Re: [Patch 08/12] Modify Ptrace routines to access breakpoint registers

K.Prasad
2009-05-27T14:15:27Z

Modify Ptrace routines to access breakpoint registers

This patch modifies the ptrace code to use the new wrapper routines around the
debug/breakpoint registers. The unused debugreg<n> registers in 'struct
thread_struct' are also removed.

Original-patch-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
Signed-off-by: Maneesh Soni <maneesh@linux.vnet.ibm.com>
Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
---
arch/x86/include/asm/processor.h | 4
arch/x86/kernel/ptrace.c | 231 +++++++++++++++++++++++----------------
2 files changed, 141 insertions(+), 94 deletions(-)

Index: linux-2.6-tip.hbkpt/arch/x86/kernel/ptrace.c
===================================================================
--- linux-2.6-tip.hbkpt.orig/arch/x86/kernel/ptrace.c
+++ linux-2.6-tip.hbkpt/arch/x86/kernel/ptrace.c
@@ -34,6 +34,7 @@
#include <asm/prctl.h>
#include <asm/proto.h>
#include <asm/ds.h>
+#include <asm/hw_breakpoint.h>

#include <trace/syscall.h>

@@ -136,11 +137,6 @@ static int set_segment_reg(struct task_s
return 0;
}

-static unsigned long debugreg_addr_limit(struct task_struct *task)
-{
- return TASK_SIZE - 3;
-}
-
#else /* CONFIG_X86_64 */

#define FLAG_MASK (FLAG_MASK_32 | X86_EFLAGS_NT)
@@ -265,15 +261,6 @@ static int set_segment_reg(struct task_s
return 0;
}

-static unsigned long debugreg_addr_limit(struct task_struct *task)
-{
-#ifdef CONFIG_IA32_EMULATION
- if (test_tsk_thread_flag(task, TIF_IA32))
- return IA32_PAGE_OFFSET - 3;
-#endif
- return TASK_SIZE_MAX - 7;
-}
-
#endif /* CONFIG_X86_32 */

static unsigned long get_flags(struct task_struct *task)
@@ -464,95 +451,159 @@ static int genregs_set(struct task_struc
}

/*
- * This function is trivial and will be inlined by the compiler.
- * Having it separates the implementation details of debug
- * registers from the interface details of ptrace.
+ * Decode the length and type bits for a particular breakpoint as
+ * stored in debug register 7. Return the "enabled" status.
*/
-static unsigned long ptrace_get_debugreg(struct task_struct *child, int n)
+static int decode_dr7(unsigned long dr7, int bpnum, unsigned *len,
+ unsigned *type)
{
- switch (n) {
- case 0: return child->thread.debugreg0;
- case 1: return child->thread.debugreg1;
- case 2: return child->thread.debugreg2;
- case 3: return child->thread.debugreg3;
- case 6: return child->thread.debugreg6;
- case 7: return child->thread.debugreg7;
- }
- return 0;
+ int bp_info = dr7 >> (DR_CONTROL_SHIFT + bpnum * DR_CONTROL_SIZE);
+
+ *len = (bp_info & 0xc) | 0x40;
+ *type = (bp_info & 0x3) | 0x80;
+ return (dr7 >> (bpnum * DR_ENABLE_SIZE)) & 0x3;
}

-static int ptrace_set_debugreg(struct task_struct *child,
- int n, unsigned long data)
+static void ptrace_triggered(struct hw_breakpoint *bp, struct pt_regs *regs)
{
+ struct thread_struct *thread = &(current->thread);
int i;

- if (unlikely(n == 4 || n == 5))
- return -EIO;
+ /*
+ * Store in the virtual DR6 register the fact that the breakpoint
+ * was hit so the thread's debugger will see it.
+ */
+ for (i = 0; i < hbp_kernel_pos; i++)
+ /*
+ * We will check bp->info.address against the address stored in
+ * thread's hbp structure and not debugreg[i]. This is to ensure
+ * that the corresponding bit for 'i' in DR7 register is enabled
+ */
+ if (bp->info.address == thread->hbp[i]->info.address)
+ break;

- if (n < 4 && unlikely(data >= debugreg_addr_limit(child)))
- return -EIO;
+ thread->debugreg6 |= (DR_TRAP0 << i);
+}

- switch (n) {
- case 0: child->thread.debugreg0 = data; break;
- case 1: child->thread.debugreg1 = data; break;
- case 2: child->thread.debugreg2 = data; break;
- case 3: child->thread.debugreg3 = data; break;
+/*
+ * Handle ptrace writes to debug register 7.
+ */
+static int ptrace_write_dr7(struct task_struct *tsk, unsigned long data)
+{
+ struct thread_struct *thread = &(tsk->thread);
+ unsigned long old_dr7 = thread->debugreg7;
+ int i, orig_ret = 0, rc = 0;
+ int enabled, second_pass = 0;
+ unsigned len, type;
+ struct hw_breakpoint *bp;

- case 6:
- if ((data & ~0xffffffffUL) != 0)
- return -EIO;
- child->thread.debugreg6 = data;
- break;
+ data &= ~DR_CONTROL_RESERVED;
+restore:
+ /*
+ * Loop through all the hardware breakpoints, making the
+ * appropriate changes to each.
+ */
+ for (i = 0; i < HBP_NUM; i++) {
+ enabled = decode_dr7(data, i, &len, &type);
+ bp = thread->hbp[i];
+
+ if (!enabled) {
+ if (bp) {
+ /* Don't unregister the breakpoints right-away,
+ * unless all register_user_hw_breakpoint()
+ * requests have succeeded. This prevents
+ * any window of opportunity for debug
+ * register grabbing by other users.
+ */
+ if (!second_pass)
+ continue;
+ unregister_user_hw_breakpoint(tsk, bp);
+ kfree(bp);
+ }
+ continue;
+ }
+ if (!bp) {
+ rc = -ENOMEM;
+ bp = kzalloc(sizeof(struct hw_breakpoint), GFP_KERNEL);
+ if (bp) {
+ bp->info.address = thread->debugreg[i];
+ bp->triggered = ptrace_triggered;
+ bp->info.len = len;
+ bp->info.type = type;
+ rc = register_user_hw_breakpoint(tsk, bp);
+ if (rc)
+ kfree(bp);
+ }
+ } else
+ rc = modify_user_hw_breakpoint(tsk, bp);
+ if (rc)
+ break;
+ }
+ /*
+ * Make a second pass to free the remaining unused breakpoints
+ * or to restore the original breakpoints if an error occurred.
+ */
+ if (!second_pass) {
+ second_pass = 1;
+ if (rc < 0) {
+ orig_ret = rc;
+ data = old_dr7;
+ }
+ goto restore;
+ }
+ return ((orig_ret < 0) ? orig_ret : rc);
+}

- case 7:
- /*
- * Sanity-check data. Take one half-byte at once with
- * check = (val >> (16 + 4*i)) & 0xf. It contains the
- * R/Wi and LENi bits; bits 0 and 1 are R/Wi, and bits
- * 2 and 3 are LENi. Given a list of invalid values,
- * we do mask |= 1 << invalid_value, so that
- * (mask >> check) & 1 is a correct test for invalid
- * values.
- *
- * R/Wi contains the type of the breakpoint /
- * watchpoint, LENi contains the length of the watched
- * data in the watchpoint case.
- *
- * The invalid values are:
- * - LENi == 0x10 (undefined), so mask |= 0x0f00. [32-bit]
- * - R/Wi == 0x10 (break on I/O reads or writes), so
- * mask |= 0x4444.
- * - R/Wi == 0x00 && LENi != 0x00, so we have mask |=
- * 0x1110.
- *
- * Finally, mask = 0x0f00 | 0x4444 | 0x1110 == 0x5f54.
- *
- * See the Intel Manual "System Programming Guide",
- * 15.2.4
- *
- * Note that LENi == 0x10 is defined on x86_64 in long
- * mode (i.e. even for 32-bit userspace software, but
- * 64-bit kernel), so the x86_64 mask value is 0x5454.
- * See the AMD manual no. 24593 (AMD64 System Programming)
- */
-#ifdef CONFIG_X86_32
-#define DR7_MASK 0x5f54
-#else
-#define DR7_MASK 0x5554
-#endif
- data &= ~DR_CONTROL_RESERVED;
- for (i = 0; i < 4; i++)
- if ((DR7_MASK >> ((data >> (16 + 4*i)) & 0xf)) & 1)
- return -EIO;
- child->thread.debugreg7 = data;
- if (data)
- set_tsk_thread_flag(child, TIF_DEBUG);
- else
- clear_tsk_thread_flag(child, TIF_DEBUG);
- break;
+/*
+ * Handle PTRACE_PEEKUSR calls for the debug register area.
+ */
+unsigned long ptrace_get_debugreg(struct task_struct *tsk, int n)
+{
+ struct thread_struct *thread = &(tsk->thread);
+ unsigned long val = 0;
+
+ if (n < HBP_NUM)
+ val = thread->debugreg[n];
+ else if (n == 6)
+ val = thread->debugreg6;
+ else if (n == 7)
+ val = thread->debugreg7;
+ return val;
+}
+
+/*
+ * Handle PTRACE_POKEUSR calls for the debug register area.
+ */
+int ptrace_set_debugreg(struct task_struct *tsk, int n, unsigned long val)
+{
+ struct thread_struct *thread = &(tsk->thread);
+ int rc = 0;
+
+ /* There are no DR4 or DR5 registers */
+ if (n == 4 || n == 5)
+ return -EIO;
+
+ if (n == 6) {
+ tsk->thread.debugreg6 = val;
+ goto ret_path;
+ }
+ if (n < HBP_NUM) {
+ if (thread->hbp[n]) {
+ if (arch_check_va_in_userspace(val,
+ thread->hbp[n]->info.len) == 0) {
+ rc = -EIO;
+ goto ret_path;
+ }
+ thread->hbp[n]->info.address = val;
+ }
+ thread->debugreg[n] = val;
}
+ /* All that's left is DR7 */
+ if (n == 7)
+ rc = ptrace_write_dr7(tsk, val);

- return 0;
+ret_path:
+ return rc;
}

/*
Index: linux-2.6-tip.hbkpt/arch/x86/include/asm/processor.h
===================================================================
--- linux-2.6-tip.hbkpt.orig/arch/x86/include/asm/processor.h
+++ linux-2.6-tip.hbkpt/arch/x86/include/asm/processor.h
@@ -434,10 +434,6 @@ struct thread_struct {
#endif
unsigned long gs;
/* Hardware debugging registers: */
- unsigned long debugreg0;
- unsigned long debugreg1;
- unsigned long debugreg2;
- unsigned long debugreg3;
unsigned long debugreg[HBP_NUM];
unsigned long debugreg6;
unsigned long debugreg7;