Re: [PATCH bpf-next 1/3] s390/bpf: Add orig_gpr2 to user_pt_regs

["Naveen N. Rao" <naveen.n.rao@linux.ibm.com>]

Hi Heiko,

Heiko Carstens wrote:
> On Wed, Feb 02, 2022 at 12:41:58AM +0100, Ilya Leoshkevich wrote:
>> user_pt_regs is used by eBPF in order to access userspace registers -
>> see commit 466698e654e8 ("s390/bpf: correct broken uapi for
>> BPF_PROG_TYPE_PERF_EVENT program type"). In order to access the first
>> syscall argument from eBPF programs, we need to export orig_gpr2.
>> 
>> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
>> ---
>>  arch/s390/include/asm/ptrace.h      | 2 +-
>>  arch/s390/include/uapi/asm/ptrace.h | 1 +
>>  2 files changed, 2 insertions(+), 1 deletion(-)
>> 
>> diff --git a/arch/s390/include/asm/ptrace.h b/arch/s390/include/asm/ptrace.h
>> index 4ffa8e7f0ed3..c8698e643904 100644
>> --- a/arch/s390/include/asm/ptrace.h
>> +++ b/arch/s390/include/asm/ptrace.h
>> @@ -83,9 +83,9 @@ struct pt_regs {
>>  			unsigned long args[1];
>>  			psw_t psw;
>>  			unsigned long gprs[NUM_GPRS];
>> +			unsigned long orig_gpr2;
>>  		};
>>  	};
>> -	unsigned long orig_gpr2;
>>  	union {
>>  		struct {
>>  			unsigned int int_code;
>> diff --git a/arch/s390/include/uapi/asm/ptrace.h b/arch/s390/include/uapi/asm/ptrace.h
>> index ad64d673b5e6..b3dec603f507 100644
>> --- a/arch/s390/include/uapi/asm/ptrace.h
>> +++ b/arch/s390/include/uapi/asm/ptrace.h
>> @@ -295,6 +295,7 @@ typedef struct {
>>  	unsigned long args[1];
>>  	psw_t psw;
>>  	unsigned long gprs[NUM_GPRS];
>> +	unsigned long orig_gpr2;
>>  } user_pt_regs;
> 
> Isn't this broken on nearly all architectures? I just checked powerpc,
> arm64, and riscv. While powerpc seems to mirror pt_regs as user_pt_regs,
> and therefore exports orig_gpr3, the bpf macros still seem to access the
> wrong location to access the first syscall parameter(?).

On powerpc, gpr[3] continues to be valid on syscall entry (so this test 
passes on powerpc), though orig_gpr3 will remain valid throughout.

I will submit a patch to use orig_gpr3 on powerpc.


Thanks!
- Naveen