Re: [PATCH bpf-next v2 4/6] bpf, arm64: Impelment bpf_arch_text_poke() for arm64

[Xu Kuohai]

On 4/15/2022 10:34 AM, Hou Tao wrote:
> Hi,
> 
> On 4/15/2022 12:22 AM, Xu Kuohai wrote:
>> Impelment bpf_arch_text_poke() for arm64, so bpf trampoline code can use
>> it to replace nop with jump, or replace jump with nop.
>>
>> Signed-off-by: Xu Kuohai <xukuohai@xxxxxxxxxx>
>> Acked-by: Song Liu <songliubraving@xxxxxx>
>> ---
>>  arch/arm64/net/bpf_jit_comp.c | 52 +++++++++++++++++++++++++++++++++++
>>  1 file changed, 52 insertions(+)
>>
>> diff --git a/arch/arm64/net/bpf_jit_comp.c b/arch/arm64/net/bpf_jit_comp.c
>> index 8ab4035dea27..1a1c3ea75ee2 100644
>> --- a/arch/arm64/net/bpf_jit_comp.c
>> +++ b/arch/arm64/net/bpf_jit_comp.c
>> @@ -9,6 +9,7 @@
>>  
>>  #include <linux/bitfield.h>
>>  #include <linux/bpf.h>
>> +#include <linux/memory.h>
>>  #include <linux/filter.h>
>>  #include <linux/printk.h>
>>  #include <linux/slab.h>
>> @@ -18,6 +19,7 @@
>>  #include <asm/cacheflush.h>
>>  #include <asm/debug-monitors.h>
>>  #include <asm/insn.h>
>> +#include <asm/patching.h>
>>  #include <asm/set_memory.h>
>>  
>>  #include "bpf_jit.h"
>> @@ -1529,3 +1531,53 @@ void bpf_jit_free_exec(void *addr)
>>  {
>>  	return vfree(addr);
>>  }
>> +
>> +static int gen_branch_or_nop(enum aarch64_insn_branch_type type, void *ip,
>> +			     void *addr, u32 *insn)
>> +{
>> +	if (!addr)
>> +		*insn = aarch64_insn_gen_nop();
>> +	else
>> +		*insn = aarch64_insn_gen_branch_imm((unsigned long)ip,
>> +						    (unsigned long)addr,
>> +						    type);
>> +
>> +	return *insn != AARCH64_BREAK_FAULT ? 0 : -EFAULT;
>> +}
>> +
>> +int bpf_arch_text_poke(void *ip, enum bpf_text_poke_type poke_type,
>> +		       void *old_addr, void *new_addr)
>> +{
>> +	int ret;
>> +	u32 old_insn;
>> +	u32 new_insn;
>> +	u32 replaced;
>> +	enum aarch64_insn_branch_type branch_type;
>> +
> In bpf_arch_text_poke() of x86, it disables the poking of kernel module, can you
> explain why it is OK to do so in arm64 ? Because there is no test cases for
> fentry on linux kernel module, could you please add some tests for it ?

Oops, I forget to check this condition. It's not safe to patch a ko
without ko unloading disabled.

For arm64, the fentry is only patched by ftrace since the nop
instruciton to be instrumented is not the first instruction, so
bpf_text_poke() fails when comparing the old instruction (pointed to by
the "old_addr") with the nop. Since the nop in fentry is reserved by
ftrace, I dont think it's reasonable to patch the nop by another
interface not provided by ftrace.

Besides, for long jumps outside the range of 128MB, a single branch
instruction is not sufficient, perhaps we could use ftrace trampoline or
some other method to support long jumps.

>> +	if (poke_type == BPF_MOD_CALL)
>> +		branch_type = AARCH64_INSN_BRANCH_LINK;
>> +	else
>> +		branch_type = AARCH64_INSN_BRANCH_NOLINK;
>> +
>> +	if (gen_branch_or_nop(branch_type, ip, old_addr, &old_insn) < 0)
>> +		return -EFAULT;
>> +
>> +	if (gen_branch_or_nop(branch_type, ip, new_addr, &new_insn) < 0)
>> +		return -EFAULT;
>> +
>> +	mutex_lock(&text_mutex);
>> +	if (aarch64_insn_read(ip, &replaced)) {
>> +		ret = -EFAULT;
>> +		goto out;
>> +	}
>> +
>> +	if (replaced != old_insn) {
>> +		ret = -EFAULT;
>> +		goto out;
>> +	}
>> +
>> +	ret =  aarch64_insn_patch_text_nosync((void *)ip, new_insn);
>> +out:
>> +	mutex_unlock(&text_mutex);
>> +	return ret;
>> +}
> 
> .