Re: [PATCH v4 2/7] arm64: introduce interfaces to hotpatch kernel and module code

[Jiang Liu <liuj97@gmail.com>]

On 10/17/2013 07:38 PM, Will Deacon wrote:
> [adding Tixy for stop_machine() question below]
> 
> On Thu, Oct 17, 2013 at 07:19:35AM +0100, Jiang Liu wrote:
>> From: Jiang Liu <jiang.liu@huawei.com>
>>
>> Introduce three interfaces to patch kernel and module code:
>> aarch64_insn_patch_text_nosync():
>> 	patch code without synchronization, it's caller's responsibility
>> 	to synchronize all CPUs if needed.
>> aarch64_insn_patch_text_sync():
>> 	patch code and always synchronize with stop_machine()
>> aarch64_insn_patch_text():
>> 	patch code and synchronize with stop_machine() if needed
>>
>> Signed-off-by: Jiang Liu <jiang.liu@huawei.com>
>> Cc: Jiang Liu <liuj97@gmail.com>
>> ---
>>  arch/arm64/include/asm/insn.h | 24 +++++++++++++-
>>  arch/arm64/kernel/insn.c      | 77 +++++++++++++++++++++++++++++++++++++++++++
>>  2 files changed, 100 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/include/asm/insn.h b/arch/arm64/include/asm/insn.h
>> index 6190016..fc439b9 100644
>> --- a/arch/arm64/include/asm/insn.h
>> +++ b/arch/arm64/include/asm/insn.h
>> @@ -60,8 +60,30 @@ __AARCH64_INSN_FUNCS(nop,	0xFFFFFFFF, 0xD503201F)
>>  
>>  #undef	__AARCH64_INSN_FUNCS
>>  
>> -enum aarch64_insn_encoding_class aarch64_get_insn_class(u32 insn);
>> +/*
>> + * In ARMv8-A, A64 instructions have a fixed length of 32 bits and are always
>> + * little-endian. On the other hand, SCTLR_EL1.EE (bit 25, Exception Endianness)
>> + * flag controls endianness for EL1 explicit data accesses and stage 1
>> + * translation table walks as below:
>> + *	0: little-endian
>> + *	1: big-endian
>> + * So need to handle endianness when patching kernel code.
>> + */
> 
> You can delete this comment now that we're using the helpers...
> 
>> +static __always_inline u32 aarch64_insn_read(void *addr)
>> +{
>> +	return le32_to_cpu(*(u32 *)addr);
>> +}
>>  
>> +static __always_inline void aarch64_insn_write(void *addr, u32 insn)
>> +{
>> +	*(u32 *)addr = cpu_to_le32(insn);
>> +}
> 
> ... then just inline these calls directly.
> 
>> +enum aarch64_insn_encoding_class aarch64_get_insn_class(u32 insn);
>>  bool aarch64_insn_hotpatch_safe(u32 old_insn, u32 new_insn);
>>  
>> +int aarch64_insn_patch_text_nosync(void *addrs[], u32 insns[], int cnt);
>> +int aarch64_insn_patch_text_sync(void *addrs[], u32 insns[], int cnt);
>> +int aarch64_insn_patch_text(void *addrs[], u32 insns[], int cnt);
>> +
>>  #endif	/* _ASM_ARM64_INSN_H */
>> diff --git a/arch/arm64/kernel/insn.c b/arch/arm64/kernel/insn.c
>> index 1c501f3..8dd5fbe 100644
>> --- a/arch/arm64/kernel/insn.c
>> +++ b/arch/arm64/kernel/insn.c
>> @@ -16,6 +16,9 @@
>>   */
>>  #include <linux/compiler.h>
>>  #include <linux/kernel.h>
>> +#include <linux/smp.h>
>> +#include <linux/stop_machine.h>
>> +#include <asm/cacheflush.h>
>>  #include <asm/insn.h>
>>  
>>  static int aarch64_insn_encoding_cls[] = {
>> @@ -70,3 +73,77 @@ bool __kprobes aarch64_insn_hotpatch_safe(u32 old_insn, u32 new_insn)
>>  	return __aarch64_insn_hotpatch_safe(old_insn) &&
>>  	       __aarch64_insn_hotpatch_safe(new_insn);
>>  }
>> +
>> +int __kprobes aarch64_insn_patch_text_nosync(void *addrs[], u32 insns[],
>> +					     int cnt)
>> +{
>> +	int i;
>> +	u32 *tp;
>> +
>> +	if (cnt <= 0)
>> +		return -EINVAL;
> 
> Isn't cnt always 1 for the _nosync patching? Can you just drop the argument
> and simplify this code? Patching a sequence without syncing is always racy.
Will drop the third parameter and simplify the code.

> 
>> +	for (i = 0; i < cnt; i++) {
>> +		tp = addrs[i];
>> +		/* A64 instructions must be word aligned */
>> +		if ((uintptr_t)tp & 0x3)
>> +			return -EINVAL;
>> +		aarch64_insn_write(tp, insns[i]);
>> +		flush_icache_range((uintptr_t)tp, (uintptr_t)tp + sizeof(u32));
>> +	}
>> +
>> +	return 0;
>> +}
>> +
>> +struct aarch64_insn_patch {
>> +	void	**text_addrs;
>> +	u32	*new_insns;
>> +	int	insn_cnt;
>> +};
>> +
>> +static int __kprobes aarch64_insn_patch_text_cb(void *arg)
>> +{
>> +	struct aarch64_insn_patch *pp = arg;
>> +
>> +	return aarch64_insn_patch_text_nosync(pp->text_addrs, pp->new_insns,
>> +					      pp->insn_cnt);
>> +}
>> +
>> +int __kprobes aarch64_insn_patch_text_sync(void *addrs[], u32 insns[], int cnt)
>> +{
>> +	struct aarch64_insn_patch patch = {
>> +		.text_addrs = addrs,
>> +		.new_insns = insns,
>> +		.insn_cnt = cnt,
>> +	};
>> +
>> +	if (cnt <= 0)
>> +		return -EINVAL;
>> +
>> +	/*
>> +	 * Execute __aarch64_insn_patch_text() on every online CPU,
>> +	 * which ensure serialization among all online CPUs.
>> +	 */
>> +	return stop_machine(aarch64_insn_patch_text_cb, &patch, NULL);
>> +}
> 
> Whoa, whoa, whoa! The comment here is wrong -- we only run the patching on
> *one* CPU, which is the right thing to do. However, the arch/arm/ call to
> stop_machine in kprobes does actually run the patching code on *all* the
> online cores (including the cache flushing!). I think this is to work around
> cores without hardware cache maintenance broadcasting, but that could easily
> be called out specially (like we do in patch.c) and the flushing could be
> separated from the patching too.
> 
>> +int __kprobes aarch64_insn_patch_text(void *addrs[], u32 insns[], int cnt)
>> +{
>> +	int ret;
>> +
>> +	if (cnt == 1 && aarch64_insn_hotpatch_safe(aarch64_insn_read(addrs[0]),
>> +						   insns[0])) {
> 
> You could make aarch64_insn_hotpatch_safe take the cnt parameter and return
> false if cnt != 1.
> 
>> +		/*
>> +		 * It doesn't guarantee all CPUs see the new instruction
> 
> "It"? You mean the ARMv8 architecture?
Yes, I mean ARMv8 architecture.

> 
> Will
>