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    Subject[PATCH v3 6/7] riscv: Add KPROBES_ON_FTRACE supported
    Date
    From: Guo Ren <guoren@linux.alibaba.com>

    This patch adds support for kprobes on ftrace call sites to avoids
    much of the overhead with regular kprobes. Try it with simple
    steps:

    1. Get _do_fork ftrace call site.
    Dump of assembler code for function _do_fork:
    0xffffffe00020af64 <+0>: addi sp,sp,-128
    0xffffffe00020af66 <+2>: sd s0,112(sp)
    0xffffffe00020af68 <+4>: sd ra,120(sp)
    0xffffffe00020af6a <+6>: addi s0,sp,128
    0xffffffe00020af6c <+8>: sd s1,104(sp)
    0xffffffe00020af6e <+10>: sd s2,96(sp)
    0xffffffe00020af70 <+12>: sd s3,88(sp)
    0xffffffe00020af72 <+14>: sd s4,80(sp)
    0xffffffe00020af74 <+16>: sd s5,72(sp)
    0xffffffe00020af76 <+18>: sd s6,64(sp)
    0xffffffe00020af78 <+20>: sd s7,56(sp)
    0xffffffe00020af7a <+22>: mv s4,a0
    0xffffffe00020af7c <+24>: mv a0,ra
    0xffffffe00020af7e <+26>: nop <<<<<<<< here!
    0xffffffe00020af82 <+30>: nop
    0xffffffe00020af86 <+34>: ld s3,0(s4)

    2. Set _do_fork+26 as the kprobe.
    echo 'p:myprobe _do_fork+26 dfd=%a0 filename=%a1 flags=%a2 mode=+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
    echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
    cat /sys/kernel/debug/tracing/trace
    tracer: nop

    entries-in-buffer/entries-written: 3/3 #P:1

    _-----=> irqs-off
    / _----=> need-resched
    | / _---=> hardirq/softirq
    || / _--=> preempt-depth
    ||| / delay
    TASK-PID CPU# |||| TIMESTAMP FUNCTION
    | | | |||| | |
    sh-87 [000] .... 551.557031: myprobe: (_do_fork+0x1a/0x2e6) dfd=0xffffffe00020af7e filename=0xffffffe00020b34e flags=0xffffffe00101e7c0 mode=0x20af86ffffffe0

    cat /sys/kernel/debug/kprobes/list
    ffffffe00020af7e k _do_fork+0x1a [FTRACE]
    ^^^^^^

    Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
    Cc: Masami Hiramatsu <mhiramat@kernel.org>
    Cc: Palmer Dabbelt <palmerdabbelt@google.com>
    Cc: Paul Walmsley <paul.walmsley@sifive.com>
    Cc: Björn Töpel <bjorn.topel@gmail.com>
    Cc: Zong Li <zong.li@sifive.com>
    Cc: Pekka Enberg <penberg@kernel.org>
    ---
    arch/riscv/Kconfig | 1 +
    arch/riscv/kernel/probes/Makefile | 1 +
    arch/riscv/kernel/probes/ftrace.c | 52 +++++++++++++++++++++++++++++++++++++++
    3 files changed, 54 insertions(+)
    create mode 100644 arch/riscv/kernel/probes/ftrace.c

    diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
    index a41b785..0e9f5eb 100644
    --- a/arch/riscv/Kconfig
    +++ b/arch/riscv/Kconfig
    @@ -60,6 +60,7 @@ config RISCV
    select HAVE_FUTEX_CMPXCHG if FUTEX
    select HAVE_GENERIC_VDSO if MMU && 64BIT
    select HAVE_KPROBES
    + select HAVE_KPROBES_ON_FTRACE
    select HAVE_KRETPROBES
    select HAVE_PCI
    select HAVE_PERF_EVENTS
    diff --git a/arch/riscv/kernel/probes/Makefile b/arch/riscv/kernel/probes/Makefile
    index cb62991..7f0840d 100644
    --- a/arch/riscv/kernel/probes/Makefile
    +++ b/arch/riscv/kernel/probes/Makefile
    @@ -1,5 +1,6 @@
    # SPDX-License-Identifier: GPL-2.0
    obj-$(CONFIG_KPROBES) += kprobes.o decode-insn.o simulate-insn.o
    obj-$(CONFIG_KPROBES) += kprobes_trampoline.o
    +obj-$(CONFIG_KPROBES_ON_FTRACE) += ftrace.o
    obj-$(CONFIG_UPROBES) += uprobes.o decode-insn.o simulate-insn.o
    CFLAGS_REMOVE_simulate-insn.o = $(CC_FLAGS_FTRACE)
    diff --git a/arch/riscv/kernel/probes/ftrace.c b/arch/riscv/kernel/probes/ftrace.c
    new file mode 100644
    index 00000000..e0fe58a
    --- /dev/null
    +++ b/arch/riscv/kernel/probes/ftrace.c
    @@ -0,0 +1,52 @@
    +// SPDX-License-Identifier: GPL-2.0
    +
    +#include <linux/kprobes.h>
    +
    +/* Ftrace callback handler for kprobes -- called under preepmt disabed */
    +void kprobe_ftrace_handler(unsigned long ip, unsigned long parent_ip,
    + struct ftrace_ops *ops, struct pt_regs *regs)
    +{
    + struct kprobe *p;
    + struct kprobe_ctlblk *kcb;
    +
    + p = get_kprobe((kprobe_opcode_t *)ip);
    + if (unlikely(!p) || kprobe_disabled(p))
    + return;
    +
    + kcb = get_kprobe_ctlblk();
    + if (kprobe_running()) {
    + kprobes_inc_nmissed_count(p);
    + } else {
    + unsigned long orig_ip = instruction_pointer(regs);
    + instruction_pointer_set(regs, ip);
    +
    + __this_cpu_write(current_kprobe, p);
    + kcb->kprobe_status = KPROBE_HIT_ACTIVE;
    + if (!p->pre_handler || !p->pre_handler(p, regs)) {
    + /*
    + * Emulate singlestep (and also recover regs->pc)
    + * as if there is a nop
    + */
    + instruction_pointer_set(regs,
    + (unsigned long)p->addr + MCOUNT_INSN_SIZE);
    + if (unlikely(p->post_handler)) {
    + kcb->kprobe_status = KPROBE_HIT_SSDONE;
    + p->post_handler(p, regs, 0);
    + }
    + instruction_pointer_set(regs, orig_ip);
    + }
    +
    + /*
    + * If pre_handler returns !0, it changes regs->pc. We have to
    + * skip emulating post_handler.
    + */
    + __this_cpu_write(current_kprobe, NULL);
    + }
    +}
    +NOKPROBE_SYMBOL(kprobe_ftrace_handler);
    +
    +int arch_prepare_kprobe_ftrace(struct kprobe *p)
    +{
    + p->ainsn.api.insn = NULL;
    + return 0;
    +}
    --
    2.7.4
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