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SubjectRe: [PATCH 7/7] powerpc/kprobes: Allow probing on any ftrace address
Masami Hiramatsu wrote:
> 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.

...

Joe Perches wrote:
> On Fri, 2019-06-21 at 23:50 +0900, Masami Hiramatsu wrote:
>> On Tue, 18 Jun 2019 20:17:06 +0530
>> "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com> wrote:
>
> trivia:
>
>> > diff --git a/arch/powerpc/kernel/kprobes-ftrace.c b/arch/powerpc/kernel/kprobes-ftrace.c
> []
>> > @@ -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");
>
> Please use the appropriate KERN_<LEVEL> or pr_<level>
>

All good points. Thanks for the review.


- Naveen


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