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Subject[Patch 06/12] Use the new wrapper routines to access debug registers in process/thread code
From: Alan Stern <stern@rowland.harvard.edu>

This patch enables the use of abstract debug registers in
process-handling routines.

[K.Prasad: Split-out from the bigger patch and minor changes following
re-basing]

Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
---
arch/x86/include/asm/processor.h | 4 ----
arch/x86/kernel/process.c | 23 ++++++-----------------
arch/x86/kernel/process_32.c | 31 +++++++++++++++++++++++++++++++
arch/x86/kernel/process_64.c | 33 +++++++++++++++++++++++++++++++++
4 files changed, 70 insertions(+), 21 deletions(-)

Index: arch/x86/kernel/process.c
===================================================================
--- arch/x86/kernel/process.c.orig
+++ arch/x86/kernel/process.c
@@ -16,6 +16,8 @@
#include <asm/uaccess.h>
#include <asm/i387.h>
#include <asm/ds.h>
+#include <asm/debugreg.h>
+#include <asm/hw_breakpoint.h>

unsigned long idle_halt;
EXPORT_SYMBOL(idle_halt);
@@ -47,6 +49,8 @@ void free_thread_xstate(struct task_stru
kmem_cache_free(task_xstate_cachep, tsk->thread.xstate);
tsk->thread.xstate = NULL;
}
+ if (unlikely(test_tsk_thread_flag(tsk, TIF_DEBUG)))
+ flush_thread_hw_breakpoint(tsk);

WARN(tsk->thread.ds_ctx, "leaking DS context\n");
}
@@ -105,14 +109,9 @@ void flush_thread(void)
}
#endif

- clear_tsk_thread_flag(tsk, TIF_DEBUG);
+ if (unlikely(test_tsk_thread_flag(tsk, TIF_DEBUG)))
+ flush_thread_hw_breakpoint(tsk);

- tsk->thread.debugreg0 = 0;
- tsk->thread.debugreg1 = 0;
- tsk->thread.debugreg2 = 0;
- tsk->thread.debugreg3 = 0;
- tsk->thread.debugreg6 = 0;
- tsk->thread.debugreg7 = 0;
memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));
/*
* Forget coprocessor state..
@@ -194,16 +193,6 @@ void __switch_to_xtra(struct task_struct
else if (next->debugctlmsr != prev->debugctlmsr)
update_debugctlmsr(next->debugctlmsr);

- if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
- set_debugreg(next->debugreg0, 0);
- set_debugreg(next->debugreg1, 1);
- set_debugreg(next->debugreg2, 2);
- set_debugreg(next->debugreg3, 3);
- /* no 4 and 5 */
- set_debugreg(next->debugreg6, 6);
- set_debugreg(next->debugreg7, 7);
- }
-
if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
test_tsk_thread_flag(next_p, TIF_NOTSC)) {
/* prev and next are different */
Index: arch/x86/kernel/process_32.c
===================================================================
--- arch/x86/kernel/process_32.c.orig
+++ arch/x86/kernel/process_32.c
@@ -61,6 +61,8 @@
#include <asm/idle.h>
#include <asm/syscalls.h>
#include <asm/ds.h>
+#include <asm/debugreg.h>
+#include <asm/hw_breakpoint.h>

asmlinkage void ret_from_fork(void) __asm__("ret_from_fork");

@@ -265,7 +267,14 @@ int copy_thread(unsigned long clone_flag

task_user_gs(p) = get_user_gs(regs);

+ p->thread.io_bitmap_ptr = NULL;
+
tsk = current;
+ err = -ENOMEM;
+ if (unlikely(test_tsk_thread_flag(tsk, TIF_DEBUG)))
+ if (copy_thread_hw_breakpoint(tsk, p, clone_flags))
+ goto out;
+
if (unlikely(test_tsk_thread_flag(tsk, TIF_IO_BITMAP))) {
p->thread.io_bitmap_ptr = kmemdup(tsk->thread.io_bitmap_ptr,
IO_BITMAP_BYTES, GFP_KERNEL);
@@ -285,10 +294,13 @@ int copy_thread(unsigned long clone_flag
err = do_set_thread_area(p, -1,
(struct user_desc __user *)childregs->si, 0);

+out:
if (err && p->thread.io_bitmap_ptr) {
kfree(p->thread.io_bitmap_ptr);
p->thread.io_bitmap_max = 0;
}
+ if (err)
+ flush_thread_hw_breakpoint(p);

clear_tsk_thread_flag(p, TIF_DS_AREA_MSR);
p->thread.ds_ctx = NULL;
@@ -427,6 +439,25 @@ __switch_to(struct task_struct *prev_p,
lazy_load_gs(next->gs);

percpu_write(current_task, next_p);
+ /*
+ * There's a problem with moving the switch_to_thread_hw_breakpoint()
+ * call before current is updated. Suppose a kernel breakpoint is
+ * triggered in between the two. The hw-breakpoint handler will see
+ * that current is different from the task pointer stored in
+ * last_debugged_task, so it will think the task pointer is leftover
+ * from an old task (lazy switching) and will erase it. Then until the
+ * next context switch, no user-breakpoints will be installed.
+ *
+ * The real problem is that it's impossible to update both current and
+ * last_debugged_task at the same instant, so there will always be a
+ * window in which they disagree and a breakpoint might get triggered.
+ * Since we use lazy switching, we are forced to assume that a
+ * disagreement means that current is correct and last_debugged_task is
+ * old. But if you move the code above then you'll create a window in
+ * which current is old and last_debugged_task is correct.
+ */
+ if (unlikely(test_tsk_thread_flag(next_p, TIF_DEBUG)))
+ switch_to_thread_hw_breakpoint(next_p);

return prev_p;
}
Index: arch/x86/kernel/process_64.c
===================================================================
--- arch/x86/kernel/process_64.c.orig
+++ arch/x86/kernel/process_64.c
@@ -55,6 +55,8 @@
#include <asm/idle.h>
#include <asm/syscalls.h>
#include <asm/ds.h>
+#include <asm/debugreg.h>
+#include <asm/hw_breakpoint.h>

asmlinkage extern void ret_from_fork(void);

@@ -248,6 +250,8 @@ void release_thread(struct task_struct *
BUG();
}
}
+ if (unlikely(dead_task->thread.debugreg7))
+ flush_thread_hw_breakpoint(dead_task);
}

static inline void set_32bit_tls(struct task_struct *t, int tls, u32 addr)
@@ -303,12 +307,18 @@ int copy_thread(unsigned long clone_flag

p->thread.fs = me->thread.fs;
p->thread.gs = me->thread.gs;
+ p->thread.io_bitmap_ptr = NULL;

savesegment(gs, p->thread.gsindex);
savesegment(fs, p->thread.fsindex);
savesegment(es, p->thread.es);
savesegment(ds, p->thread.ds);

+ err = -ENOMEM;
+ if (unlikely(test_tsk_thread_flag(me, TIF_DEBUG)))
+ if (copy_thread_hw_breakpoint(me, p, clone_flags))
+ goto out;
+
if (unlikely(test_tsk_thread_flag(me, TIF_IO_BITMAP))) {
p->thread.io_bitmap_ptr = kmalloc(IO_BITMAP_BYTES, GFP_KERNEL);
if (!p->thread.io_bitmap_ptr) {
@@ -347,6 +357,9 @@ out:
kfree(p->thread.io_bitmap_ptr);
p->thread.io_bitmap_max = 0;
}
+ if (err)
+ flush_thread_hw_breakpoint(p);
+
return err;
}

@@ -492,6 +505,26 @@ __switch_to(struct task_struct *prev_p,
*/
if (tsk_used_math(next_p) && next_p->fpu_counter > 5)
math_state_restore();
+ /*
+ * There's a problem with moving the switch_to_thread_hw_breakpoint()
+ * call before current is updated. Suppose a kernel breakpoint is
+ * triggered in between the two. The hw-breakpoint handler will see
+ * that current is different from the task pointer stored in
+ * last_debugged_task, so it will think the task pointer is leftover
+ * from an old task (lazy switching) and will erase it. Then until the
+ * next context switch, no user-breakpoints will be installed.
+ *
+ * The real problem is that it's impossible to update both current and
+ * last_debugged_task at the same instant, so there will always be a
+ * window in which they disagree and a breakpoint might get triggered.
+ * Since we use lazy switching, we are forced to assume that a
+ * disagreement means that current is correct and last_debugged_task is
+ * old. But if you move the code above then you'll create a window in
+ * which current is old and last_debugged_task is correct.
+ */
+ if (unlikely(test_tsk_thread_flag(next_p, TIF_DEBUG)))
+ switch_to_thread_hw_breakpoint(next_p);
+
return prev_p;
}

Index: arch/x86/include/asm/processor.h
===================================================================
--- arch/x86/include/asm/processor.h.orig
+++ arch/x86/include/asm/processor.h
@@ -432,10 +432,6 @@ struct thread_struct {
unsigned long fs;
unsigned long gs;
/* Hardware debugging registers: */
- unsigned long debugreg0;
- unsigned long debugreg1;
- unsigned long debugreg2;
- unsigned long debugreg3;
unsigned long debugreg[HB_NUM];
unsigned long debugreg6;
unsigned long debugreg7;


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