Re: [PATCH bpf-next v5 2/3] arm64: patching: Add aarch64_insn_set()

[Xu Kuohai <xukuohai@huawei.com>]

On 9/21/2023 10:50 PM, Puranjay Mohan wrote:
> Xu Kuohai <xukuohai@huaweicloud.com> writes:
> 
>> On 9/8/2023 10:43 PM, Puranjay Mohan wrote:
>>> The BPF JIT needs to write invalid instructions to RX regions of memory
>>> to invalidate removed BPF programs. This needs a function like memset()
>>> that can work with RX memory.
>>>
>>> Implement aarch64_insn_set() which is similar to text_poke_set() of x86.
>>>
>>> Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
>>> ---
>>>    arch/arm64/include/asm/patching.h |  1 +
>>>    arch/arm64/kernel/patching.c      | 40 +++++++++++++++++++++++++++++++
>>>    2 files changed, 41 insertions(+)
>>>
>>> diff --git a/arch/arm64/include/asm/patching.h b/arch/arm64/include/asm/patching.h
>>> index f78a0409cbdb..551933338739 100644
>>> --- a/arch/arm64/include/asm/patching.h
>>> +++ b/arch/arm64/include/asm/patching.h
>>> @@ -8,6 +8,7 @@ int aarch64_insn_read(void *addr, u32 *insnp);
>>>    int aarch64_insn_write(void *addr, u32 insn);
>>>    
>>>    int aarch64_insn_write_literal_u64(void *addr, u64 val);
>>> +int aarch64_insn_set(void *dst, const u32 insn, size_t len);
>>>    void *aarch64_insn_copy(void *dst, const void *src, size_t len);
>>>    
>>>    int aarch64_insn_patch_text_nosync(void *addr, u32 insn);
>>> diff --git a/arch/arm64/kernel/patching.c b/arch/arm64/kernel/patching.c
>>> index 243d6ae8d2d8..63d9e0e77806 100644
>>> --- a/arch/arm64/kernel/patching.c
>>> +++ b/arch/arm64/kernel/patching.c
>>> @@ -146,6 +146,46 @@ noinstr void *aarch64_insn_copy(void *dst, const void *src, size_t len)
>>>    	return dst;
>>>    }
>>>    
>>> +/**
>>> + * aarch64_insn_set - memset for RX memory regions.
>>> + * @dst: address to modify
>>> + * @c: value to set
>>
>> insn
> 
> Thanks for catching.
> 
>>> + * @len: length of memory region.
>>> + *
>>> + * Useful for JITs to fill regions of RX memory with illegal instructions.
>>> + */
>>> +noinstr int aarch64_insn_set(void *dst, const u32 insn, size_t len)
>>
>> const is unnecessary
>>
> 
> Will remove in next version.
> 
>>> +{
>>> +	unsigned long flags;
>>> +	size_t patched = 0;
>>> +	size_t size;
>>> +	void *waddr;
>>> +	void *ptr;
>>> +
>>> +	/* A64 instructions must be word aligned */
>>> +	if ((uintptr_t)dst & 0x3)
>>> +		return -EINVAL;
>>> +
>>> +	raw_spin_lock_irqsave(&patch_lock, flags);
>>> +
>>> +	while (patched < len) {
>>> +		ptr = dst + patched;
>>> +		size = min_t(size_t, PAGE_SIZE - offset_in_page(ptr),
>>> +			     len - patched);
>>> +
>>> +		waddr = patch_map(ptr, FIX_TEXT_POKE0);
>>> +		memset32(waddr, insn, size / 4);
>>> +		patch_unmap(FIX_TEXT_POKE0);
>>> +
>>> +		patched += size;
>>> +	}
>>> +	raw_spin_unlock_irqrestore(&patch_lock, flags);
>>> +
>>> +	caches_clean_inval_pou((uintptr_t)dst, (uintptr_t)dst + len);
>>> +
>>> +	return 0;
>>> +}
>>> +
>>
>> this function shares most of the code with aarch64_insn_copy(), how about
>> extracting the shared code to a separate function?
> 
> I was thinking of writing it like the text_poke api of x86. Where you
> can provide a function as an argument to work on a memory area.
> Essentially, it will look like:
> 
> typedef int text_poke_f(void *dst, const void *src, size_t len);
> 
> static void *aarch64_insn_poke(text_poke_f func, void *addr, const void *src, size_t len)
> 
> We can call this function with a wrapper of `copy_to_kernel_nofault` for copy
> and with a wrapper of memset32 for setting.
> 
> Do you think this is a good approach?
>
Sounds great, thanks.

>>
>>>    int __kprobes aarch64_insn_patch_text_nosync(void *addr, u32 insn)
>>>    {
>>>    	u32 *tp = addr;
> 
> Thanks,
> Puranjay