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;
>> +}
>
> .
