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Date: Mon, 26 Sep 2022 21:40:38 +0800
From: Li Huafei <lihuafei1@...wei.com>
To: Guo Ren <guoren@...nel.org>,
"Masami Hiramatsu (Google)" <mhiramat@...nel.org>
CC: <palmer@...belt.com>, <rostedt@...dmis.org>, <mingo@...hat.com>,
<paul.walmsley@...ive.com>, <aou@...s.berkeley.edu>,
<naveen.n.rao@...ux.ibm.com>, <anil.s.keshavamurthy@...el.com>,
<davem@...emloft.net>, <jszhang@...nel.org>,
<peterz@...radead.org>, <liaochang1@...wei.com>, <me@...ki.ch>,
<penberg@...nel.org>, <linux-riscv@...ts.infradead.org>,
<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RESEND v2 2/2] riscv: kprobe: Allow coexistence of ftrace
and kprobe
On 2022/9/25 20:45, Guo Ren wrote:
> Hi Li,
>
> Current riscv ftrace detour instructions could be reduced to half [1].
>
> Before optimization (16 byte):
> <func_prolog>:
> 0: REG_S ra, -SZREG(sp)
> 4: auipc ra, ?
> 8: jalr ?(ra)
> 12: REG_L ra, -SZREG(sp)
> (func_boddy)
>
> The previous implementation is from afc76b8b8011 ("riscv: Using
> PATCHABLE_FUNCTION_ENTRY instead of MCOUNT")
>
> After optimization (8 byte):
> <func_prolog>:
> 0: auipc t0, ?
> 4: jalr t0, ?(t0)
> (func_boddy)
>
> [1]: https://lore.kernel.org/linux-riscv/20220921034910.3142465-4-guoren@kernel.org/
Nice patch! Thank you for the information. I will update my patch based
on it.
Thanks,
Huafei
>
> I'm appreciate you could base it on that patch.
>
> On Fri, Sep 23, 2022 at 4:43 PM Li Huafei <lihuafei1@...wei.com> wrote:
>>
>> Hi Masami,
>>
>> Thanks for your comment and sorry for the late reply.
>>
>> On 2022/9/10 11:25, Masami Hiramatsu (Google) wrote:
>>> Hi Li,
>>>
>>> On Fri, 9 Sep 2022 10:36:25 +0800
>>> Li Huafei <lihuafei1@...wei.com> wrote:
>>>
>>>> When using ftrace and kprobe at the same time, it was found that one
>>>> might cause the other to be unavailable. This can be reproduced by the
>>>> following steps.
>>>
>>> Yes, this must be solved by CONFIG_HAVE_KPROBES_ON_FTRACE.
>>>
>>>>
>>>> # 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
>>>> # ls events/kprobes/
>>>> enable p_cmdline_proc_show_0
>>>> filter p_cmdline_proc_show_4
>>>> # echo 1 > events/kprobes/p_cmdline_proc_show_4/enable
>>>> # echo 1 > events/kprobes/p_cmdline_proc_show_0/enable
>>>> [ 129.830108] 00000000ebed457d: expected (ffdb0097 4f0080e7) but got (00100073 4f0080e7)
>>>> [ 129.835990] ------------[ ftrace bug ]------------
>>>> [ 129.839510] ftrace failed to modify
>>>> [ 129.839536] [<ffffffff80258910>] cmdline_proc_show+0x0/0x46
>>>> [ 129.845831] actual: 23:3c:11:fe:73:00:10:00
>>>> [ 129.849299] Updating ftrace call site to call a different ftrace function
>>>> [ 129.853998] ftrace record flags: e0000002
>>>> [ 129.856771] (2) R
>>>> [ 129.856771] expected tramp: ffffffff80008e60
>>>> [ 129.861688] ------------[ cut here ]------------
>>>> [ 129.865092] WARNING: CPU: 0 PID: 14 at kernel/trace/ftrace.c:2085 ftrace_bug+0x21a/0x24c
>>>> [ 129.870949] Modules linked in:
>>>> [ 129.873301] CPU: 0 PID: 14 Comm: migration/0 Not tainted 5.18.0-rc3-00002-gd8bfcd250f58 #6
>>>> [ 129.879217] Hardware name: riscv-virtio,qemu (DT)
>>>> [ 129.882666] Stopper: multi_cpu_stop+0x0/0x168 <- stop_machine_cpuslocked+0xfa/0x12e
>>>> [ 129.888430] epc : ftrace_bug+0x21a/0x24c
>>>> [ 129.891254] ra : ftrace_bug+0x21a/0x24c
>>>> [ 129.894057] epc : ffffffff807c3bee ra : ffffffff807c3bee sp : ff20000000283c80
>>>> [ 129.899144] gp : ffffffff813a83b8 tp : ff60000080021600 t0 : ffffffff804155c0
>>>> [ 129.904257] t1 : 0720072007200720 t2 : 7420646574636570 s0 : ff20000000283cb0
>>>> [ 129.909402] s1 : ff6000007fe622a0 a0 : 0000000000000022 a1 : c0000000ffffefff
>>>> [ 129.914472] a2 : 0000000000000001 a3 : 0000000000000000 a4 : 341adec112294700
>>>> [ 129.919487] a5 : 341adec112294700 a6 : 0000000000000730 a7 : 0000000000000030
>>>> [ 129.924595] s2 : ffffffff80258910 s3 : ffffffffffffffea s4 : 0000000000000000
>>>> [ 129.929776] s5 : 0000000000000a35 s6 : ffffffff80d667c8 s7 : ff6000007fe04000
>>>> [ 129.934892] s8 : 0000000000000004 s9 : 0000000000000002 s10: 0000000000000001
>>>> [ 129.939992] s11: 0000000000000003 t3 : ff6000007ff20f00 t4 : ff6000007ff20f00
>>>> [ 129.945134] t5 : ff6000007ff20000 t6 : ff200000002839c8
>>>> [ 129.948838] status: 0000000000000100 badaddr: 0000000000000000 cause: 0000000000000003
>>>> [ 129.954580] [<ffffffff800bf110>] ftrace_replace_code+0xce/0xd0
>>>> [ 129.958771] [<ffffffff800bf280>] ftrace_modify_all_code+0xb4/0x12c
>>>> [ 129.964420] [<ffffffff800bf30a>] __ftrace_modify_code+0x12/0x1c
>>>> [ 129.969163] [<ffffffff800b4ea4>] multi_cpu_stop+0x9a/0x168
>>>> [ 129.973035] [<ffffffff800b4b1c>] cpu_stopper_thread+0xb4/0x15e
>>>> [ 129.977205] [<ffffffff8003457a>] smpboot_thread_fn+0x106/0x1e4
>>>> [ 129.981277] [<ffffffff80030ca6>] kthread+0xee/0x108
>>>> [ 129.984763] [<ffffffff800039ba>] ret_from_exception+0x0/0xc
>>>> [ 129.988809] ---[ end trace 0000000000000000 ]---
>>>> # cat trace
>>>> # tracer: function
>>>> #
>>>> # WARNING: FUNCTION TRACING IS CORRUPTED
>>>> # MAY BE MISSING FUNCTION EVENTS
>>>> # entries-in-buffer/entries-written: 0/0 #P:4
>>>> #
>>>> # _-----=> irqs-off/BH-disabled
>>>> # / _----=> need-resched
>>>> # | / _---=> hardirq/softirq
>>>> # || / _--=> preempt-depth
>>>> # ||| / _-=> migrate-disable
>>>> # |||| / delay
>>>> # TASK-PID CPU# ||||| TIMESTAMP FUNCTION
>>>> # | | | ||||| | |
>>>>
>>>> As you can see, the ftrace functionality is broken. This is because
>>>> DYNAMIC_FTRACE uses 4 instructions to make the jump from the function
>>>> entry to the ftrace trampoline.
>>>>
>>>> After 'echo function > current_tracer':
>>>>
>>>> <cmdline_proc_show>:
>>>> 0xffffffff80258910 <+0>: sd ra,-8(sp)
>>>> 0xffffffff80258914 <+4>: auipc ra,0xffdb0
>>>> 0xffffffff80258918 <+8>: jalr 1264(ra) # 0xffffffff80008e04 <ftrace_caller>
>>>> 0xffffffff8025891c <+12>: ld ra,-8(sp)
>>>>
>>>> After 'echo 1 > events/kprobes/p_cmdline_proc_show_4/enable':
>>>>
>>>> <cmdline_proc_show>:
>>>> 0xffffffff80258910 <+0>: sd ra,-8(sp)
>>>> 0xffffffff80258914 <+4>: ebreak
>>>> 0xffffffff80258918 <+8>: jalr 1264(ra)
>>>> 0xffffffff8025891c <+12>: ld ra,-8(sp)
>>>>
>>>> This time the second instruction is replaced with a breakpoint
>>>> instruction and the ftrace-related instructions are broken. Then, when
>>>> enabling 'p_cmdline_proc_show_0', a regs ftrace handler is registered
>>>> based on KPROBES_ON_FTRACE, ftrace_modify_call() changes 'ftrace_caller'
>>>> to 'ftrace_regs_caller', and checks for ftrace-related instructions is
>>>> modified or not. Here it has been modified, causing ftrace to report a
>>>> warning and disable itself.
>>>>
>>>> In turn, the breakpoints inserted by kprobe may be overwritten by
>>>> ftrace.
>>>
>>> In this case, we should NOT probe such address by kprobes. The reason
>>> why I introduce the CONFIG_KPROBES_ON_FTRACE is to avoid this issue.
>>>
>>
>> Yes, I think CONFIG_KPROBES_ON_FTRACE is mainly intended to solve the
>> problem of coexistence with ftrace and not just as an optimization.
>>
>>>
>>>>
>>>> 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.
>>>
>>> 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]
>>
>> '/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).
>>
>> 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]
>>
>> So for kprobe, can we consider all instructions before the "real"
>> instruction of the function as a virtual instruction? And then refuse to
>> probe the intermediate address of this virtual instruction to avoid
>> ambiguity for the user here.
>>
>> [1]
>> https://lore.kernel.org/lkml/20220426015751.88582-2-lihuafei1@huawei.com/
>>
>> 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@...nel.org>
>>>> Signed-off-by: Li Huafei <lihuafei1@...wei.com>
>>>> ---
>>>> v1: https://lore.kernel.org/lkml/20220426015751.88582-1-lihuafei1@huawei.com/
>>>> v2: https://lore.kernel.org/lkml/20220819025522.154189-2-lihuafei1@huawei.com/
>>>>
>>>> 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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