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    SubjectRe: [PATCH 7/7] powerpc/kprobes: Allow probing on any ftrace address
    On Tue, 18 Jun 2019 20:17:06 +0530
    "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:

    > With KPROBES_ON_FTRACE, kprobe is allowed to be inserted on instructions
    > that branch to _mcount (referred to as ftrace location). With
    > -mprofile-kernel, we now include the preceding 'mflr r0' as being part
    > of the ftrace location.
    >
    > However, by default, probing on an instruction that is not actually the
    > branch to _mcount() is prohibited, as that is considered to not be at an
    > instruction boundary. This is not the case on powerpc, so allow the same
    > by overriding arch_check_ftrace_location()
    >
    > In addition, we update kprobe_ftrace_handler() to detect this scenarios
    > and to pass the proper nip to the pre and post probe handlers.
    >
    > Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
    > ---
    > arch/powerpc/kernel/kprobes-ftrace.c | 30 ++++++++++++++++++++++++++++
    > 1 file changed, 30 insertions(+)
    >
    > diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
    > index 972cb28174b2..6a0bd3c16cb6 100644
    > --- a/arch/powerpc/kernel/kprobes-ftrace.c
    > +++ b/arch/powerpc/kernel/kprobes-ftrace.c
    > @@ -12,14 +12,34 @@
    > #include <linux/preempt.h>
    > #include <linux/ftrace.h>
    >
    > +/*
    > + * With -mprofile-kernel, we patch two instructions -- the branch to _mcount
    > + * as well as the preceding 'mflr r0'. Both these instructions are claimed
    > + * by ftrace and we should allow probing on either instruction.
    > + */
    > +int arch_check_ftrace_location(struct kprobe *p)
    > +{
    > + if (ftrace_location((unsigned long)p->addr))
    > + p->flags |= KPROBE_FLAG_FTRACE;
    > + return 0;
    > +}
    > +
    > /* Ftrace callback handler for kprobes */
    > void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
    > struct ftrace_ops *ops, struct pt_regs *regs)
    > {
    > struct kprobe *p;
    > + int mflr_kprobe = 0;
    > struct kprobe_ctlblk *kcb;
    >
    > p = get_kprobe((kprobe_opcode_t *)nip);
    > + if (unlikely(!p)) {

    Hmm, is this really unlikely? If we put a kprobe on the second instruction address,
    we will see p == NULL always.

    > + p = get_kprobe((kprobe_opcode_t *)(nip - MCOUNT_INSN_SIZE));
    > + if (!p)

    Here will be unlikely, because we can not find kprobe at both of nip and
    nip - MCOUNT_INSN_SIZE.

    > + return;
    > + mflr_kprobe = 1;
    > + }
    > +
    > if (unlikely(!p) || kprobe_disabled(p))

    "unlikely(!p)" is not needed here.

    Thank you,

    > return;
    >
    > @@ -33,6 +53,9 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
    > */
    > regs->nip -= MCOUNT_INSN_SIZE;
    >
    > + if (mflr_kprobe)
    > + regs->nip -= MCOUNT_INSN_SIZE;
    > +
    > __this_cpu_write(current_kprobe, p);
    > kcb->kprobe_status = KPROBE_HIT_ACTIVE;
    > if (!p->pre_handler || !p->pre_handler(p, regs)) {
    > @@ -45,6 +68,8 @@ void kprobe_ftrace_handler(unsigned long nip, unsigned long parent_nip,
    > kcb->kprobe_status = KPROBE_HIT_SSDONE;
    > p->post_handler(p, regs, 0);
    > }
    > + if (mflr_kprobe)
    > + regs->nip += MCOUNT_INSN_SIZE;
    > }
    > /*
    > * If pre_handler returns !0, it changes regs->nip. We have to
    > @@ -57,6 +82,11 @@ NOKPROBE_SYMBOL(kprobe_ftrace_handler);
    >
    > int arch_prepare_kprobe_ftrace(struct kprobe *p)
    > {
    > + if ((unsigned long)p->addr & 0x03) {
    > + printk("Attempt to register kprobe at an unaligned address\n");
    > + return -EILSEQ;
    > + }
    > +
    > p->ainsn.insn = NULL;
    > p->ainsn.boostable = -1;
    > return 0;
    > --
    > 2.22.0
    >


    --
    Masami Hiramatsu <mhiramat@kernel.org>

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