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    Subject[RFC Patch 4/9] Modify kprobe exception handler to recognise single-stepping by HW Breakpoint handler
    This patch modifies the kprobe handler to help it recognise single-stepping by
    the HW Breakpoint exception code. A per-cpu variable called 'sstep_reason' to
    distinguish the source of single-step exceptions.

    Signed-off-by: K.Prasad <prasad@linux.vnet.ibm.com>
    Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
    ---
    arch/x86/kernel/kprobes.c | 18 ++++++++++++++++--
    include/asm-x86/kprobes.h | 8 ++++++++
    2 files changed, 24 insertions(+), 2 deletions(-)

    Index: linux-bkpt-lkml-27-rc9/arch/x86/kernel/kprobes.c
    ===================================================================
    --- linux-bkpt-lkml-27-rc9.orig/arch/x86/kernel/kprobes.c
    +++ linux-bkpt-lkml-27-rc9/arch/x86/kernel/kprobes.c
    @@ -54,6 +54,7 @@
    #include <asm/pgtable.h>
    #include <asm/uaccess.h>
    #include <asm/alternative.h>
    +#include <asm/debugreg.h>

    void jprobe_return_end(void);

    @@ -517,6 +518,7 @@ static int __kprobes kprobe_handler(stru
    kprobe_opcode_t *addr;
    struct kprobe *p;
    struct kprobe_ctlblk *kcb;
    + unsigned int *ssr;

    addr = (kprobe_opcode_t *)(regs->ip - sizeof(kprobe_opcode_t));
    if (*addr != BREAKPOINT_INSTRUCTION) {
    @@ -541,6 +543,7 @@ static int __kprobes kprobe_handler(stru
    */
    preempt_disable();

    + ssr = &(__get_cpu_var(sstep_reason));
    kcb = get_kprobe_ctlblk();
    p = get_kprobe(addr);

    @@ -560,14 +563,17 @@ static int __kprobes kprobe_handler(stru
    * for jprobe processing, so get out doing nothing
    * more here.
    */
    - if (!p->pre_handler || !p->pre_handler(p, regs))
    + if (!p->pre_handler || !p->pre_handler(p, regs)) {
    setup_singlestep(p, regs, kcb);
    + (*ssr) |= SSTEP_KPROBE;
    + }
    return 1;
    }
    } else if (kprobe_running()) {
    p = __get_cpu_var(current_kprobe);
    if (p->break_handler && p->break_handler(p, regs)) {
    setup_singlestep(p, regs, kcb);
    + (*ssr) |= SSTEP_KPROBE;
    return 1;
    }
    } /* else: not a kprobe fault; let the kernel handle it */
    @@ -952,6 +958,7 @@ int __kprobes kprobe_exceptions_notify(s
    {
    struct die_args *args = data;
    int ret = NOTIFY_DONE;
    + unsigned int *ssr = &(__get_cpu_var(sstep_reason));

    if (args->regs && user_mode_vm(args->regs))
    return ret;
    @@ -962,8 +969,15 @@ int __kprobes kprobe_exceptions_notify(s
    ret = NOTIFY_STOP;
    break;
    case DIE_DEBUG:
    - if (post_kprobe_handler(args->regs))
    + /* We could be here due to single-stepping after a pre-handler
    + * execution of HW Breakpoint or kprobes. We determine the cause
    + * using the bitmask flag 'sstep_reason'.
    + */
    + if (((*ssr) & SSTEP_KPROBE) &&
    + post_kprobe_handler(args->regs)) {
    + current->thread.vdr6 &= ~DR_STEP;
    ret = NOTIFY_STOP;
    + }
    break;
    case DIE_GPF:
    /*
    Index: linux-bkpt-lkml-27-rc9/include/asm-x86/kprobes.h
    ===================================================================
    --- linux-bkpt-lkml-27-rc9.orig/include/asm-x86/kprobes.h
    +++ linux-bkpt-lkml-27-rc9/include/asm-x86/kprobes.h
    @@ -30,6 +30,14 @@
    struct pt_regs;
    struct kprobe;

    +/* Single stepping can be initiated for kprobes post handler or following HW
    + * Breakpoint exception. The bitmask below is used to identify the cause.
    + */
    +#define SSTEP_KPROBE 1
    +#define SSTEP_HWBKPT 2
    +
    +DECLARE_PER_CPU(unsigned int, sstep_reason);
    +
    typedef u8 kprobe_opcode_t;
    #define BREAKPOINT_INSTRUCTION 0xcc
    #define RELATIVEJUMP_INSTRUCTION 0xe9

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