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Message-ID: <2f32fff3-6b58-583f-8e85-06ec1553d3f4@huawei.com>
Date: Mon, 2 Aug 2021 11:52:00 +0800
From: "liuqi (BA)" <liuqi115@...wei.com>
To: "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com>,
Masami Hiramatsu <mhiramat@...nel.org>,
Linuxarm <linuxarm@...wei.com>
CC: "catalin.marinas@....com" <catalin.marinas@....com>,
"will@...nel.org" <will@...nel.org>,
"naveen.n.rao@...ux.ibm.com" <naveen.n.rao@...ux.ibm.com>,
"anil.s.keshavamurthy@...el.com" <anil.s.keshavamurthy@...el.com>,
"davem@...emloft.net" <davem@...emloft.net>,
"linux-arm-kernel@...ts.infradead.org"
<linux-arm-kernel@...ts.infradead.org>,
"Zengtao (B)" <prime.zeng@...ilicon.com>,
"robin.murphy@....com" <robin.murphy@....com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] arm64: kprobe: Enable OPTPROBE for arm64
On 2021/7/31 20:21, Song Bao Hua (Barry Song) wrote:
>
>
>> -----Original Message-----
>> From: Masami Hiramatsu [mailto:mhiramat@...nel.org]
>> Sent: Saturday, July 31, 2021 1:16 PM
>> To: Song Bao Hua (Barry Song) <song.bao.hua@...ilicon.com>
>> Cc: liuqi (BA) <liuqi115@...wei.com>; catalin.marinas@....com;
>> will@...nel.org; naveen.n.rao@...ux.ibm.com; anil.s.keshavamurthy@...el.com;
>> davem@...emloft.net; linux-arm-kernel@...ts.infradead.org; Zengtao (B)
>> <prime.zeng@...ilicon.com>; robin.murphy@....com; Linuxarm
>> <linuxarm@...wei.com>; linux-kernel@...r.kernel.org
>> Subject: Re: [PATCH] arm64: kprobe: Enable OPTPROBE for arm64
>>
>> On Fri, 30 Jul 2021 10:04:06 +0000
>> "Song Bao Hua (Barry Song)" <song.bao.hua@...ilicon.com> wrote:
>>
>>>>>>>
>>>>>>> Hi Qi,
>>>>>>>
>>>>>>> Thanks for your effort!
>>>>>>>
>>>>>>> On Mon, 19 Jul 2021 20:24:17 +0800
>>>>>>> Qi Liu <liuqi115@...wei.com> wrote:
>>>>>>>
>>>>>>>> This patch introduce optprobe for ARM64. In optprobe, probed
>>>>>>>> instruction is replaced by a branch instruction to detour
>>>>>>>> buffer. Detour buffer contains trampoline code and a call to
>>>>>>>> optimized_callback(). optimized_callback() calls opt_pre_handler()
>>>>>>>> to execute kprobe handler.
>>>>>>>
>>>>>>> OK so this will replace only one instruction.
>>>>>>>
>>>>>>>>
>>>>>>>> Limitations:
>>>>>>>> - We only support !CONFIG_RANDOMIZE_MODULE_REGION_FULL case to
>>>>>>>> guarantee the offset between probe point and kprobe pre_handler
>>>>>>>> is not larger than 128MiB.
>>>>>>>
>>>>>>> Hmm, shouldn't we depends on !CONFIG_ARM64_MODULE_PLTS? Or,
>>>>>>> allocate an intermediate trampoline area similar to arm optprobe
>>>>>>> does.
>>>>>>
>>>>>> Depending on !CONFIG_ARM64_MODULE_PLTS will totally disable
>>>>>> RANDOMIZE_BASE according to arch/arm64/Kconfig:
>>>>>> config RANDOMIZE_BASE
>>>>>> bool "Randomize the address of the kernel image"
>>>>>> select ARM64_MODULE_PLTS if MODULES
>>>>>> select RELOCATABLE
>>>>>
>>>>> Yes, but why it is required for "RANDOMIZE_BASE"?
>>>>> Does that imply the module call might need to use PLT in
>>>>> some cases?
>>>>>
>>>>>>
>>>>>> Depending on !RANDOMIZE_MODULE_REGION_FULL seems to be still
>>>>>> allowing RANDOMIZE_BASE via avoiding long jump according to:
>>>>>> arch/arm64/Kconfig:
>>>>>>
>>>>>> config RANDOMIZE_MODULE_REGION_FULL
>>>>>> bool "Randomize the module region over a 4 GB range"
>>>>>> depends on RANDOMIZE_BASE
>>>>>> default y
>>>>>> help
>>>>>> Randomizes the location of the module region inside a 4 GB window
>>>>>> covering the core kernel. This way, it is less likely for modules
>>>>>> to leak information about the location of core kernel data structures
>>>>>> but it does imply that function calls between modules and the core
>>>>>> kernel will need to be resolved via veneers in the module PLT.
>>>>>>
>>>>>> When this option is not set, the module region will be randomized
>> over
>>>>>> a limited range that contains the [_stext, _etext] interval of the
>>>>>> core kernel, so branch relocations are always in range.
>>>>>
>>>>> Hmm, this dependency looks strange. If it always in range, don't we need
>>>>> PLT for modules?
>>>>>
>>>>> Cataline, would you know why?
>>>>> Maybe it's a KASLR's Kconfig issue?
>>>>
>>>> I actually didn't see any problem after making this change:
>>>>
>>>> diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
>>>> index e07e7de9ac49..6440671b72e0 100644
>>>> --- a/arch/arm64/Kconfig
>>>> +++ b/arch/arm64/Kconfig
>>>> @@ -1781,7 +1781,6 @@ config RELOCATABLE
>>>>
>>>> config RANDOMIZE_BASE
>>>> bool "Randomize the address of the kernel image"
>>>> - select ARM64_MODULE_PLTS if MODULES
>>>> select RELOCATABLE
>>>> help
>>>> Randomizes the virtual address at which the kernel image is
>>>> @@ -1801,6 +1800,7 @@ config RANDOMIZE_BASE
>>>> config RANDOMIZE_MODULE_REGION_FULL
>>>> bool "Randomize the module region over a 4 GB range"
>>>> depends on RANDOMIZE_BASE
>>>> + select ARM64_MODULE_PLTS if MODULES
>>>> default y
>>>> help
>>>> Randomizes the location of the module region inside a 4 GB window
>>>>
>>>> and having this config:
>>>> # zcat /proc/config.gz | grep RANDOMIZE_BASE
>>>> CONFIG_RANDOMIZE_BASE=y
>>>>
>>>> # zcat /proc/config.gz | grep RANDOMIZE_MODULE_REGION_FULL
>>>> # CONFIG_RANDOMIZE_MODULE_REGION_FULL is not set
>>>>
>>>> # zcat /proc/config.gz | grep ARM64_MODULE_PLTS
>>>> # CONFIG_ARM64_MODULE_PLTS is not set
>>>>
>>>> Modules work all good:
>>>> # lsmod
>>>> Module Size Used by
>>>> btrfs 1355776 0
>>>> blake2b_generic 20480 0
>>>> libcrc32c 16384 1 btrfs
>>>> xor 20480 1 btrfs
>>>> xor_neon 16384 1 xor
>>>> zstd_compress 163840 1 btrfs
>>>> raid6_pq 110592 1 btrfs
>>>> ctr 16384 0
>>>> md5 16384 0
>>>> ip_tunnel 32768 0
>>>> ipv6 442368 28
>>>>
>>>>
>>>> I am not quite sure if there is a corner case. If no,
>>>> I would think the kconfig might be some improper.
>>>
>>> The corner case is that even CONFIG_RANDOMIZE_MODULE_REGION_FULL
>>> is not enabled, but if CONFIG_ARM64_MODULE_PLTS is enabled, when
>>> we can't get memory from the 128MB area in case the area is exhausted,
>>> we will fall back in module_alloc() to a 2GB area as long as either
>>> of the below two conditions is met:
>>>
>>> 1. KASAN is not enabled
>>> 2. KASAN is enabled and CONFIG_KASAN_VMALLOC is also enabled.
>>>
>>> void *module_alloc(unsigned long size)
>>> {
>>> u64 module_alloc_end = module_alloc_base + MODULES_VSIZE;
>>> gfp_t gfp_mask = GFP_KERNEL;
>>> void *p;
>>>
>>> /* Silence the initial allocation */
>>> if (IS_ENABLED(CONFIG_ARM64_MODULE_PLTS))
>>> gfp_mask |= __GFP_NOWARN;
>>>
>>> if (IS_ENABLED(CONFIG_KASAN_GENERIC) ||
>>> IS_ENABLED(CONFIG_KASAN_SW_TAGS))
>>> /* don't exceed the static module region - see below */
>>> module_alloc_end = MODULES_END;
>>>
>>> p = __vmalloc_node_range(size, MODULE_ALIGN, module_alloc_base,
>>> module_alloc_end, gfp_mask, PAGE_KERNEL, 0,
>>> NUMA_NO_NODE, __builtin_return_address(0));
>>>
>>> if (!p && IS_ENABLED(CONFIG_ARM64_MODULE_PLTS) &&
>>> (IS_ENABLED(CONFIG_KASAN_VMALLOC) ||
>>> (!IS_ENABLED(CONFIG_KASAN_GENERIC) &&
>>> !IS_ENABLED(CONFIG_KASAN_SW_TAGS))))
>>> /*
>>> * KASAN without KASAN_VMALLOC can only deal with module
>>> * allocations being served from the reserved module region,
>>> * since the remainder of the vmalloc region is already
>>> * backed by zero shadow pages, and punching holes into it
>>> * is non-trivial. Since the module region is not randomized
>>> * when KASAN is enabled without KASAN_VMALLOC, it is even
>>> * less likely that the module region gets exhausted, so we
>>> * can simply omit this fallback in that case.
>>> */
>>> p = __vmalloc_node_range(size, MODULE_ALIGN, module_alloc_base,
>>> module_alloc_base + SZ_2G, GFP_KERNEL,
>>> PAGE_KERNEL, 0, NUMA_NO_NODE,
>>> __builtin_return_address(0));
>>>
>>> if (p && (kasan_module_alloc(p, size) < 0)) {
>>> vfree(p);
>>> return NULL;
>>> }
>>>
>>> return p;
>>> }
>>>
>>> This should be happening quite rarely. But maybe arm64's document
>>> needs some minor fixup, otherwise, it is quite confusing.
>>
>> OK, so CONFIG_KASAN_VLALLOC=y and CONFIG_ARM64_MODULE_PLTS=y, the
>> module_alloc()
>> basically returns the memory in 128MB region, but can return the memory in 2GB
>> region. (This is OK because optprobe can filter it out)
>> But CONFIG_RANDOMIZE_MODULE_REGION_FULL=y, there is almost no chance to get
>> the memory in 128MB region.
>>
>> Hmm, for the optprobe in kernel text, maybe we can define 'optinsn_alloc_start'
>> by 'module_alloc_base - (SZ_2G - MODULES_VADDR)' and use __vmalloc_node_range()
>> to avoid this issue. But that is only for the kernel. For the modules, we may
>> always out of 128MB region.
>
> If we can have some separate PLT entries in each module for optprobe,
> we should be able to short-jump to the PLT entry and then PLT entry
> will further long-jump to detour out of the range. That is exactly
> the duty of PLT.
>
> Right now, arm64 has support on dynamic_ftrace by adding a
> section in module for ftrace PLT.
> arch/arm64/include/asm/module.lds.h:
> SECTIONS {
> #ifdef CONFIG_ARM64_MODULE_PLTS
> .plt 0 (NOLOAD) : { BYTE(0) }
> .init.plt 0 (NOLOAD) : { BYTE(0) }
> .text.ftrace_trampoline 0 (NOLOAD) : { BYTE(0) }
> #endif
> ...
> }
>
> arch/arm64/kernel/module.c will initialize some PLT entries
> for ftrace:
>
> static int module_init_ftrace_plt(const Elf_Ehdr *hdr,
> const Elf_Shdr *sechdrs,
> struct module *mod)
> {
> #if defined(CONFIG_ARM64_MODULE_PLTS) && defined(CONFIG_DYNAMIC_FTRACE)
> const Elf_Shdr *s;
> struct plt_entry *plts;
>
> s = find_section(hdr, sechdrs, ".text.ftrace_trampoline");
> if (!s)
> return -ENOEXEC;
>
> plts = (void *)s->sh_addr;
>
> __init_plt(&plts[FTRACE_PLT_IDX], FTRACE_ADDR);
>
> if (IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS))
> __init_plt(&plts[FTRACE_REGS_PLT_IDX], FTRACE_REGS_ADDR);
>
> mod->arch.ftrace_trampolines = plts;
> #endif
> return 0;
> }
>
> Ftrace will then use those PLT entries in arch/arm64/kernel/ftrace.c:
> static struct plt_entry *get_ftrace_plt(struct module *mod, unsigned long addr)
> {
> #ifdef CONFIG_ARM64_MODULE_PLTS
> struct plt_entry *plt = mod->arch.ftrace_trampolines;
>
> if (addr == FTRACE_ADDR)
> return &plt[FTRACE_PLT_IDX];
> if (addr == FTRACE_REGS_ADDR &&
> IS_ENABLED(CONFIG_DYNAMIC_FTRACE_WITH_REGS))
> return &plt[FTRACE_REGS_PLT_IDX];
> #endif
> return NULL;
> }
>
> /*
> * Turn on the call to ftrace_caller() in instrumented function
> */
> int ftrace_make_call(struct dyn_ftrace *rec, unsigned long addr)
> {
> unsigned long pc = rec->ip;
> u32 old, new;
> long offset = (long)pc - (long)addr;
>
> if (offset < -SZ_128M || offset >= SZ_128M) {
> struct module *mod;
> struct plt_entry *plt;
>
> if (!IS_ENABLED(CONFIG_ARM64_MODULE_PLTS))
> return -EINVAL;
>
> /*
> * On kernels that support module PLTs, the offset between the
> * branch instruction and its target may legally exceed the
> * range of an ordinary relative 'bl' opcode. In this case, we
> * need to branch via a trampoline in the module.
> *
> * NOTE: __module_text_address() must be called with preemption
> * disabled, but we can rely on ftrace_lock to ensure that 'mod'
> * retains its validity throughout the remainder of this code.
> */
> preempt_disable();
> mod = __module_text_address(pc);
> preempt_enable();
>
> if (WARN_ON(!mod))
> return -EINVAL;
>
> plt = get_ftrace_plt(mod, addr);
> if (!plt) {
> pr_err("ftrace: no module PLT for %ps\n", (void *)addr);
> return -EINVAL;
> }
>
> addr = (unsigned long)plt;
> }
>
> old = aarch64_insn_gen_nop();
> new = aarch64_insn_gen_branch_imm(pc, addr, AARCH64_INSN_BRANCH_LINK);
>
> return ftrace_modify_code(pc, old, new, true);
> }
>
> This might be the direction to go later. Anyway, "Rome wasn't built
> in a day", for this stage, we might focus on optprobe for the case
> of non-randomized module region :-).
>
> BTW, @liuqi, if users set "nokaslr" in bootargs, will your optprobe
> always work and not fall back to normal kprobe even we remove the
> dependency on RANDOMIZED_MODULE_REGION_FULL?
>
Hi Barry,
I do some tests on Hip08 platform, using nokaslr in booting cmdline and
remove dependency on RANDOMIZED_MODULE_REGION_FULL, optprobe seems work.
Here is the log:
estuary:/$ uname -a
Linux (none) 5.13.0-rc4+ #37 SMP PREEMPT Mon Aug 2 08:13:37 CST 2021
aarch64 GNU/Linux
estuary:/$ zcat /proc/config.gz | grep RANDOMIZE_MODULE_REGION
CONFIG_RANDOMIZE_MODULE_REGION_FULL=y
estuary:/$ zcat /proc/config.gz | grep OPTPROBE
CONFIG_OPTPROBES=y
CONFIG_HAVE_OPTPROBES=y
estuary:/$ cat /proc/cmdline
console=ttyAMA0,115200 earlycon=pl011,0x9000000 kpti=off nokaslr
estuary:/$ cat /sys/bus/platform/devices/hello_driver/kprobe_test
[ 61.304143] do_empty returned 0 and took 200 ns to execute
[ 61.304662] do_empty returned 0 and took 110 ns to execute
[ 61.305196] do_empty returned 0 and took 100 ns to execute
[ 61.305745] do_empty returned 0 and took 90 ns to execute
[ 61.306262] do_empty returned 0 and took 90 ns to execute
[ 61.306781] do_empty returned 0 and took 90 ns to execute
[ 61.307286] do_empty returned 0 and took 90 ns to execute
[ 61.307798] do_empty returned 0 and took 90 ns to execute
[ 61.308314] do_empty returned 0 and took 90 ns to execute
[ 61.308828] do_empty returned 0 and took 90 ns to execute
[ 61.309323] do_empty returned 0 and took 80 ns to execute
[ 61.309832] do_empty returned 0 and took 80 ns to execute
[ 61.310357] do_empty returned 0 and took 80 ns to execute
[ 61.310871] do_empty returned 0 and took 80 ns to execute
[ 61.311361] do_empty returned 0 and took 80 ns to execute
[ 61.311851] do_empty returned 0 and took 90 ns to execute
[ 61.312358] do_empty returned 0 and took 90 ns to execute
[ 61.312879] do_empty returned 0 and took 80 ns to execute
Thanks,
Qi
>>
>> Thank you,
>>
>> --
>> Masami Hiramatsu <mhiramat@...nel.org>
>
> Thanks
> Barry
> .
>
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