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    Subject[PATCH 06/12] hw-breakpoints: use the new wrapper routines to access debug registers in process/thread code
    Date
    From: K.Prasad <prasad@linux.vnet.ibm.com>

    This patch enables the use of abstract debug registers in
    process-handling routines, according to the new hardware breakpoint
    Api.

    [ Impact: adapt thread breakpoints handling code to the new breakpoint Api ]

    Original-patch-by: Alan Stern <stern@rowland.harvard.edu>
    Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
    Reviewed-by: Alan Stern <stern@rowland.harvard.edu>
    Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    ---
    arch/x86/kernel/process.c | 22 ++++++----------------
    arch/x86/kernel/process_32.c | 28 ++++++++++++++++++++++++++++
    arch/x86/kernel/process_64.c | 31 +++++++++++++++++++++++++++++++
    3 files changed, 65 insertions(+), 16 deletions(-)

    diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
    index 291527c..19a686c 100644
    --- a/arch/x86/kernel/process.c
    +++ b/arch/x86/kernel/process.c
    @@ -15,6 +15,8 @@
    #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);
    @@ -46,6 +48,8 @@ void free_thread_xstate(struct task_struct *tsk)
    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");
    }
    @@ -106,12 +110,8 @@ void flush_thread(void)

    clear_tsk_thread_flag(tsk, TIF_DEBUG);

    - tsk->thread.debugreg[0] = 0;
    - tsk->thread.debugreg[1] = 0;
    - tsk->thread.debugreg[2] = 0;
    - tsk->thread.debugreg[3] = 0;
    - tsk->thread.debugreg6 = 0;
    - tsk->thread.debugreg7 = 0;
    + if (unlikely(test_tsk_thread_flag(tsk, TIF_DEBUG)))
    + flush_thread_hw_breakpoint(tsk);
    memset(tsk->thread.tls_array, 0, sizeof(tsk->thread.tls_array));
    /*
    * Forget coprocessor state..
    @@ -193,16 +193,6 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
    else if (next->debugctlmsr != prev->debugctlmsr)
    update_debugctlmsr(next->debugctlmsr);

    - if (test_tsk_thread_flag(next_p, TIF_DEBUG)) {
    - set_debugreg(next->debugreg[0], 0);
    - set_debugreg(next->debugreg[1], 1);
    - set_debugreg(next->debugreg[2], 2);
    - set_debugreg(next->debugreg[3], 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 */
    diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
    index b5e4bfe..297ffff 100644
    --- a/arch/x86/kernel/process_32.c
    +++ b/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 +267,13 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,

    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 +293,13 @@ int copy_thread(unsigned long clone_flags, unsigned long sp,
    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 +438,23 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
    lazy_load_gs(next->gs);

    percpu_write(current_task, next_p);
    + /*
    + * There's a problem with moving the arch_install_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
    + * the 'current' task does not have TIF_DEBUG flag set and will think it
    + * 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
    + * physical debug registers 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 the exception is due
    + * to lazy debug register switching.
    + */
    + if (unlikely(test_tsk_thread_flag(next_p, TIF_DEBUG)))
    + arch_install_thread_hw_breakpoint(next_p);

    return prev_p;
    }
    diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
    index 5a1a1de..f7b276d 100644
    --- a/arch/x86/kernel/process_64.c
    +++ b/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 *dead_task)
    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_flags, unsigned long sp,

    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 +505,24 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p)
    */
    if (tsk_used_math(next_p) && next_p->fpu_counter > 5)
    math_state_restore();
    + /*
    + * There's a problem with moving the arch_install_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
    + * the 'current' task does not have TIF_DEBUG flag set and will think it
    + * 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
    + * physical debug registers 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 the exception is due
    + * to lazy debug register switching.
    + */
    + if (unlikely(test_tsk_thread_flag(next_p, TIF_DEBUG)))
    + arch_install_thread_hw_breakpoint(next_p);
    +
    return prev_p;
    }

    --
    1.6.2.3


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