[PATCH bpf-next v3 2/4] libbpf: Access first syscall argument with CO-RE direct read on arm64

[Xu Kuohai]

On 8/31/2024 3:26 PM, Xu Kuohai wrote:
> On 8/31/2024 12:19 PM, Pu Lehui wrote:
>> From: Pu Lehui <pulehui at huawei.com>
>>
>> Currently PT_REGS_PARM1 SYSCALL(x) is consistent with PT_REGS_PARM1_CORE
>> SYSCALL(x), which will introduce the overhead of BPF_CORE_READ(), taking
>> into account the read pt_regs comes directly from the context, let's use
>> CO-RE direct read to access the first system call argument.
>>
>> Suggested-by: Andrii Nakryiko <andrii at kernel.org>
>> Signed-off-by: Pu Lehui <pulehui at huawei.com>
>> ---
>>   tools/lib/bpf/bpf_tracing.h | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/tools/lib/bpf/bpf_tracing.h b/tools/lib/bpf/bpf_tracing.h
>> index e7d9382efeb3..051c408e6aed 100644
>> --- a/tools/lib/bpf/bpf_tracing.h
>> +++ b/tools/lib/bpf/bpf_tracing.h
>> @@ -222,7 +222,7 @@ struct pt_regs___s390 {
>>   struct pt_regs___arm64 {
>>       unsigned long orig_x0;
>> -};
>> +} __attribute__((preserve_access_index));
>>   /* arm64 provides struct user_pt_regs instead of struct pt_regs to userspace */
>>   #define __PT_REGS_CAST(x) ((const struct user_pt_regs *)(x))
>> @@ -241,7 +241,7 @@ struct pt_regs___arm64 {
>>   #define __PT_PARM4_SYSCALL_REG __PT_PARM4_REG
>>   #define __PT_PARM5_SYSCALL_REG __PT_PARM5_REG
>>   #define __PT_PARM6_SYSCALL_REG __PT_PARM6_REG
>> -#define PT_REGS_PARM1_SYSCALL(x) PT_REGS_PARM1_CORE_SYSCALL(x)
>> +#define PT_REGS_PARM1_SYSCALL(x) (((const struct pt_regs___arm64 *)(x))->orig_x0)
>>   #define PT_REGS_PARM1_CORE_SYSCALL(x) \
>>       BPF_CORE_READ((const struct pt_regs___arm64 *)(x), __PT_PARM1_SYSCALL_REG)
> 
> Cool!
> 
> Acked-by: Xu Kuohai <xukuohai at huawei.com>
> 
> 

Wait, it breaks the following test:

--- a/tools/testing/selftests/bpf/progs/bpf_syscall_macro.c
+++ b/tools/testing/selftests/bpf/progs/bpf_syscall_macro.c
@@ -43,7 +43,7 @@ int BPF_KPROBE(handle_sys_prctl)

         /* test for PT_REGS_PARM */

-       bpf_probe_read_kernel(&tmp, sizeof(tmp), &PT_REGS_PARM1_SYSCALL(real_regs));
+       tmp = PT_REGS_PARM1_SYSCALL(real_regs);
         arg1 = tmp;
         bpf_probe_read_kernel(&arg2, sizeof(arg2), &PT_REGS_PARM2_SYSCALL(real_regs));
         bpf_probe_read_kernel(&arg3, sizeof(arg3), &PT_REGS_PARM3_SYSCALL(real_regs));

Failed with verifier rejection:

0: R1=ctx() R10=fp0
; int BPF_KPROBE(handle_sys_prctl) @ bpf_syscall_macro.c:33
0: (bf) r6 = r1                       ; R1=ctx() R6_w=ctx()
; pid_t pid = bpf_get_current_pid_tgid() >> 32; @ bpf_syscall_macro.c:36
1: (85) call bpf_get_current_pid_tgid#14      ; R0_w=scalar()
; if (pid != filter_pid) @ bpf_syscall_macro.c:39
2: (18) r1 = 0xffff800082e0e000       ; R1_w=map_value(map=bpf_sysc.rodata,ks=4,vs=4)
4: (61) r1 = *(u32 *)(r1 +0)          ; R1_w=607
; pid_t pid = bpf_get_current_pid_tgid() >> 32; @ bpf_syscall_macro.c:36
5: (77) r0 >>= 32                     ; R0_w=scalar(smin=0,smax=umax=0xffffffff,var_off=(0x0; 0xffffffff))
; if (pid != filter_pid) @ bpf_syscall_macro.c:39
6: (5e) if w1 != w0 goto pc+98        ; R0_w=607 R1_w=607
; real_regs = PT_REGS_SYSCALL_REGS(ctx); @ bpf_syscall_macro.c:42
7: (79) r8 = *(u64 *)(r6 +0)          ; R6_w=ctx() R8_w=scalar()
; tmp = PT_REGS_PARM1_SYSCALL(real_regs); @ bpf_syscall_macro.c:46
8: (79) r1 = *(u64 *)(r8 +272)
R8 invalid mem access 'scalar'
processed 8 insns (limit 1000000) max_states_per_insn 0 total_states 0 peak_states 0 mark_read 0