Re: [PATCH RESEND v2 2/2] riscv: kprobe: Allow coexistence of ftrace and kprobe

From: Li Huafei
Date: Mon Sep 26 2022 - 10:57:45 EST

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On 2022/9/25 12:56, Masami Hiramatsu (Google) wrote:
> On Fri, 23 Sep 2022 16:43:43 +0800
> Li Huafei <lihuafei1@xxxxxxxxxx> wrote:
>
>>>> The problem is that we think that only the first instruction address at
>>>> the function entry is needed for ftrace, but it is actually the first 4.
>>>> As Masami mentioned in [1], we can treat the first 4 instructions as a
>>>> 16-byte virtual instruction, and func+4, func+8 and func+12 will be
>>>> changed to func+0. This way, ftrace and kprobe do not bother each other
>>>> and can coexist.
>>>>
>>>> After this patch:
>>>>
>>>> # cd /sys/kernel/debug/tracing/
>>>> # echo cmdline_proc_show > set_ftrace_filter
>>>> # echo function > current_tracer
>>>> # echo 'p cmdline_proc_show' > kprobe_events
>>>> # echo 'p cmdline_proc_show+4' >> kprobe_events
>>>> # echo 'p cmdline_proc_show+8' >> kprobe_events
>>>> # echo 'p cmdline_proc_show+12' >> kprobe_events
>>>> # echo 'p cmdline_proc_show+16' >> kprobe_events
>>>
>>> Oh, no, I meant, if it is a 16 byte virtual insturiction, it should
>>> reject probing the address inside the instruction. It is rejected as
>>> same as the case you probe "function+1" address (that is not an
>>> instruction boundary, right?)
>>>
>>> So, it should reject it instead of modifying the probed address. (Return
>>> -EILSEQ) It is usual there is any instruction which can not be probed.
>
> I think it should return -EBUSY, because this is ftrace reserved area.
>

What you mean is:

sym+0 => OK
sym+offset(0<offset<16) => -EBUSY

If so, I think we can put the check that we can't probe the ftrace
reserved area into arch_prepare_kprobe(), something like this(just an
idea, not tested):

diff --git a/arch/riscv/kernel/probes/kprobes.c
b/arch/riscv/kernel/probes/kprobes.c
index e6e950b7cf32..0e38be896a73 100644
--- a/arch/riscv/kernel/probes/kprobes.c
+++ b/arch/riscv/kernel/probes/kprobes.c
@@ -48,6 +48,16 @@ static void __kprobes arch_simulate_insn(struct
kprobe *p, struct pt_regs *regs)
post_kprobe_handler(p, kcb, regs);
}

+static bool is_ftrace_reserved(unsigned long addr)
+{
+ unsigned long offset;
+
+ if(WARN_ON(!kallsyms_lookup_size_offset(addr, NULL, &offset)))
+ return false;
+
+ return offset > 0 && offset < 16;
+}
+
int __kprobes arch_prepare_kprobe(struct kprobe *p)
{
unsigned long probe_addr = (unsigned long)p->addr;
@@ -55,6 +65,9 @@ int __kprobes arch_prepare_kprobe(struct kprobe *p)
if (probe_addr & 0x1)
return -EILSEQ;

+ if (is_ftrace_reserved(probe_addr))
+ return -EBUSY;
+
/* copy instruction */
p->opcode = *p->addr;

Now riscv doesn't need to implement arch_adjust_kprobe_addr() for now.

>>>
>>> Or, is there any real reason you "have to" probe such intermediate
>>> address?
>>
>> No. In fact in my first version of the patch(see [1]),
>> arch_adjust_kprobe_addr() returned -EILSEQ when probing intermediate
>> addresses. but Guo pointed out that I changed the API and it did.
>>
>> Currently there are x86 & powerpc using their own arch_adjust_kprobe_addr().
>>
>> On x86, when X86_KERNEL_IBT is enabled, arch_adjust_kprobe_addr()
>> adjusts sym+0 (endbr) to sym+4 (ftrace location). If we probe through
>> kprobe_events, we get:
>>
>> # cd /sys/kernel/debug/tracing/
>> # echo 'p cmdline_proc_show' > kprobe_events
>> # echo 'p cmdline_proc_show+4' >> kprobe_events
>> # ls events/kprobes/
>> enable p_cmdline_proc_show_0
>> filter p_cmdline_proc_show_4
>> # cat /sys/kernel/debug/kprobes/list
>> ffffffff812d31e4 k cmdline_proc_show+0x4 [DISABLED][FTRACE]
>> ffffffff812d31e4 k cmdline_proc_show+0x4 [DISABLED][FTRACE]
>
> This is because
>
> - X86_KERNEL_IBT is an option.
> - endbr is considered as a nop. (No accessible register has been changed)
> - endbr is not involved in the ftrace itself.
>

Yes, I agree.

> I think I'd better to warn user that with X86_KERNEL_IBT the
> probed address seems different in the document, because this

I think it can be clarified, although there may not be many users who
are confused now. :)

> result seems something wrong. (see the event address is different
> from where user specified.)
>
> cat-152 [003] ..... 340.044650: p_cmdline_proc_show_4: (cmdline_proc_show+0x4/0x30)
> cat-152 [003] ..... 340.044652: p_cmdline_proc_show_0: (cmdline_proc_show+0x4/0x30)
>
> BPF is also working on this issue now;
>
> https://lore.kernel.org/all/20220922210320.1076658-1-jolsa@xxxxxxxxxx/T/#u
>
> Maybe ftrace also should change to show the event address as +0
> if the kprobe is on the function entry.
>
>
> Anyway, so What you are doing is something like below;
>
> # echo 'p cmdline_proc_show+5' > kprobe_events
> # echo 'p cmdline_proc_show+6' > kprobe_events
> # echo 'p cmdline_proc_show+7' > kprobe_events
> # echo 'p cmdline_proc_show+8' > kprobe_events
>
> All of those will fail because there is an ftrace replaced 5 byte nop
> and it should NOT be replaced by the kprobes.
>
>> '/sys/kernel/debug/tracing/events/kprobes/' shows that we generated two
>> probe_events, but '/sys/kernel/debug/kprobes/list' shows that it is the
>> same address with two probes (aggr_kprobe).
>
> Yes, this is because the endbr is a special instruction, not the
> ftrace instruction. When arm64 instroduces similar instruction,
> we have to do similar thing.
>
>>
>> The same is true for powerpc:
>>
>> # cd /sys/kernel/debug/tracing/
>> # echo 'p cmdline_proc_show' > kprobe_events
>> # echo 'p cmdline_proc_show+12' >> kprobe_events
>> # ls events/kprobes/
>> enable p_cmdline_proc_show_0
>> filter p_cmdline_proc_show_12
>> # cat /sys/kernel/debug/kprobes/list
>> c000000000515c0c k cmdline_proc_show+0xc [DISABLED][FTRACE]
>> c000000000515c0c k cmdline_proc_show+0xc [DISABLED][FTRACE]
>> c0000000000508c0 k __kretprobe_trampoline+0x0 [OPTIMIZED]
>
> OK, so what about following rule?
>
> kprobes on ftrace only accept the cases if the probe address is
> either
> - func + 0 (as function entry)
> - func + offset, where the offset is what the ftrace returns.
>

Yes, I agree. For riscv these two addresses are the same, i.e. func+0.

Thanks,
Huafei

> If ftrace shows the IP (PC) is func+0 (x86 without endbr) it allows
> only func+0. If it shows IP (PC) is func+4, it allows both func + 0
> and func+4. But not allows other offsets for kprobes on ftrace because
> such intermediate address doesn't have any means and no difference.
> (the register sets also same)
> This doesn't break even if you introduces something like endbr on
> RISC-V in the future too.
>
>>
>> So for kprobe, can we consider all instructions before the "real"
>> instruction of the function as a virtual instruction?
>
> Yes, as far as those are the "ftrace reserved" instructions.
>
>> And then refuse to
>> probe the intermediate address of this virtual instruction to avoid
>> ambiguity for the user here.
>
> Yes.
>
> Thank you,
>
>>
>> [1]
>> https://lore.kernel.org/lkml/20220426015751.88582-2-lihuafei1@xxxxxxxxxx/
>>
>> Thanks,
>> Huafei
>>
>>>
>>> Thank you,
>>>
>>>> # ls events/kprobes/
>>>> enable p_cmdline_proc_show_12 p_cmdline_proc_show_8
>>>> filter p_cmdline_proc_show_16
>>>> p_cmdline_proc_show_0 p_cmdline_proc_show_4
>>>> # echo 1 > events/kprobes/enable
>>>> # cat ../kprobes/list
>>>> ffffffff8025cba4 k cmdline_proc_show+0x0 [FTRACE]
>>>> ffffffff8025cba4 k cmdline_proc_show+0x0 [FTRACE]
>>>> ffffffff8025cba4 k cmdline_proc_show+0x0 [FTRACE]
>>>> ffffffff8025cba4 k cmdline_proc_show+0x0 [FTRACE]
>>>> ffffffff8025cbb4 k cmdline_proc_show+0x10
>>>> # cat /proc/cmdline
>>>> nokaslr rootwait root=/dev/vda rw
>>>> # cat trace
>>>> # tracer: function
>>>> #
>>>> # entries-in-buffer/entries-written: 6/6 #P:4
>>>> #
>>>> # _-----=> irqs-off/BH-disabled
>>>> # / _----=> need-resched
>>>> # | / _---=> hardirq/softirq
>>>> # || / _--=> preempt-depth
>>>> # ||| / _-=> migrate-disable
>>>> # |||| / delay
>>>> # TASK-PID CPU# ||||| TIMESTAMP FUNCTION
>>>> # | | | ||||| | |
>>>> cat-144 [000] ...2. 179.164908: p_cmdline_proc_show_12: (cmdline_proc_show+0x0/0x46)
>>>> cat-144 [000] ...2. 179.165617: p_cmdline_proc_show_8: (cmdline_proc_show+0x0/0x46)
>>>> cat-144 [000] ...2. 179.165653: p_cmdline_proc_show_4: (cmdline_proc_show+0x0/0x46)
>>>> cat-144 [000] ...2. 179.165655: p_cmdline_proc_show_0: (cmdline_proc_show+0x0/0x46)
>>>> cat-144 [000] ...2. 179.165837: cmdline_proc_show <-seq_read_iter
>>>> cat-144 [000] d.... 179.166099: p_cmdline_proc_show_16: (cmdline_proc_show+0x10/0x46)
>>>>
>>>> [1] https://patchwork.kernel.org/project/linux-arm-kernel/patch/20191218140622.57bbaca5@xhacker.debian/
>>>>
>>>> Fixes: c22b0bcb1dd0 ("riscv: Add kprobes supported")
>>>> Suggested-by: Guo Ren <guoren@xxxxxxxxxx>
>>>> Signed-off-by: Li Huafei <lihuafei1@xxxxxxxxxx>
>>>> ---
>>>> v1: https://lore.kernel.org/lkml/20220426015751.88582-1-lihuafei1@xxxxxxxxxx/
>>>> v2: https://lore.kernel.org/lkml/20220819025522.154189-2-lihuafei1@xxxxxxxxxx/
>>>>
>>>> v2 RESEND:
>>>> - Rebase onto v6.0-rc4.
>>>>
>>>> Changlog in v1 -> v2:
>>>> - Allows probing func+offs(<16) instead of returning -EILSEQ, which
>>>> does not change the arch_adjust_kprobe_addr() interface convention.
>>>> - ftrace_location(addr) is unnecessary, consistent with powerpc and
>>>> x86.
>>>>
>>>> arch/riscv/include/asm/ftrace.h | 26 ++++++++++++++++++++++++++
>>>> arch/riscv/kernel/ftrace.c | 26 --------------------------
>>>> arch/riscv/kernel/probes/kprobes.c | 20 ++++++++++++++++++++
>>>> 3 files changed, 46 insertions(+), 26 deletions(-)
>>>>
>>>> diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
>>>> index 04dad3380041..c59e4a63f1c1 100644
>>>> --- a/arch/riscv/include/asm/ftrace.h
>>>> +++ b/arch/riscv/include/asm/ftrace.h
>>>> @@ -77,6 +77,32 @@ do { \
>>>> */
>>>> #define MCOUNT_INSN_SIZE 8
>>>>
>>>> +/*
>>>> + * Put 16 bytes at the front of the function within the patchable function
>>>> + * entry nops' area.
>>>> + *
>>>> + * 0: REG_S ra, -SZREG(sp)
>>>> + * 1: auipc ra, 0x?
>>>> + * 2: jalr -?(ra)
>>>> + * 3: REG_L ra, -SZREG(sp)
>>>> + *
>>>> + * So the opcodes is:
>>>> + * 0: 0xfe113c23 (sd)/0xfe112e23 (sw)
>>>> + * 1: 0x???????? -> auipc
>>>> + * 2: 0x???????? -> jalr
>>>> + * 3: 0xff813083 (ld)/0xffc12083 (lw)
>>>> + */
>>>> +#if __riscv_xlen == 64
>>>> +#define INSN0 0xfe113c23
>>>> +#define INSN3 0xff813083
>>>> +#elif __riscv_xlen == 32
>>>> +#define INSN0 0xfe112e23
>>>> +#define INSN3 0xffc12083
>>>> +#endif
>>>> +
>>>> +#define FUNC_ENTRY_SIZE 16
>>>> +#define FUNC_ENTRY_JMP 4
>>>> +
>>>> #ifndef __ASSEMBLY__
>>>> struct dyn_ftrace;
>>>> int ftrace_init_nop(struct module *mod, struct dyn_ftrace *rec);
>>>> diff --git a/arch/riscv/kernel/ftrace.c b/arch/riscv/kernel/ftrace.c
>>>> index 552088e9acc4..9d951aab662a 100644
>>>> --- a/arch/riscv/kernel/ftrace.c
>>>> +++ b/arch/riscv/kernel/ftrace.c
>>>> @@ -70,32 +70,6 @@ static int __ftrace_modify_call(unsigned long hook_pos, unsigned long target,
>>>> return 0;
>>>> }
>>>>
>>>> -/*
>>>> - * Put 16 bytes at the front of the function within the patchable function
>>>> - * entry nops' area.
>>>> - *
>>>> - * 0: REG_S ra, -SZREG(sp)
>>>> - * 1: auipc ra, 0x?
>>>> - * 2: jalr -?(ra)
>>>> - * 3: REG_L ra, -SZREG(sp)
>>>> - *
>>>> - * So the opcodes is:
>>>> - * 0: 0xfe113c23 (sd)/0xfe112e23 (sw)
>>>> - * 1: 0x???????? -> auipc
>>>> - * 2: 0x???????? -> jalr
>>>> - * 3: 0xff813083 (ld)/0xffc12083 (lw)
>>>> - */
>>>> -#if __riscv_xlen == 64
>>>> -#define INSN0 0xfe113c23
>>>> -#define INSN3 0xff813083
>>>> -#elif __riscv_xlen == 32
>>>> -#define INSN0 0xfe112e23
>>>> -#define INSN3 0xffc12083
>>>> -#endif
>>>> -
>>>> -#define FUNC_ENTRY_SIZE 16
>>>> -#define FUNC_ENTRY_JMP 4
>>>> -
>>>> int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
>>>> {
>>>> unsigned int call[4] = {INSN0, 0, 0, INSN3};
>>>> diff --git a/arch/riscv/kernel/probes/kprobes.c b/arch/riscv/kernel/probes/kprobes.c
>>>> index e6e950b7cf32..ef7aabec9681 100644
>>>> --- a/arch/riscv/kernel/probes/kprobes.c
>>>> +++ b/arch/riscv/kernel/probes/kprobes.c
>>>> @@ -21,6 +21,26 @@ DEFINE_PER_CPU(struct kprobe_ctlblk, kprobe_ctlblk);
>>>> static void __kprobes
>>>> post_kprobe_handler(struct kprobe *, struct kprobe_ctlblk *, struct pt_regs *);
>>>>
>>>> +#ifdef CONFIG_DYNAMIC_FTRACE
>>>> +kprobe_opcode_t *arch_adjust_kprobe_addr(unsigned long addr, unsigned long offset,
>>>> + bool *on_func_entry)
>>>> +{
>>>> + /*
>>>> + * The first 4 instructions at the beginning of the ftraced function
>>>> + * are used as a jump to ftrace trampoline, which we can think of as
>>>> + * a 16-byte virtual instruction. func+4, func+8 and func+12 will be
>>>> + * changed to func+0. This allows the function entry to be probed with
>>>> + * the help of KPROBES_ON_FTRACE and does not break ftrace
>>>> + * functionality.
>>>> + */
>>>> + if (offset < FUNC_ENTRY_SIZE)
>>>> + offset = 0;
>>>> +
>>>> + *on_func_entry = !offset;
>>>> + return (kprobe_opcode_t *)(addr + offset);
>>>> +}
>>>> +#endif
>>>> +
>>>> static void __kprobes arch_prepare_ss_slot(struct kprobe *p)
>>>> {
>>>> unsigned long offset = GET_INSN_LENGTH(p->opcode);
>>>> --
>>>> 2.17.1
>>>>
>>>
>>>
>
>

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