Re: [PATCH 1/3] arm64: ptrace: Add is_syscall_success to handle compat

[He Zhe]

On 4/16/21 8:33 PM, Catalin Marinas wrote:
> On Fri, Apr 16, 2021 at 03:55:31PM +0800, He Zhe wrote:
>> The general version of is_syscall_success does not handle 32-bit
>> compatible case, which would cause 32-bit negative return code to be
>> recoganized as a positive number later and seen as a "success".
>>
>> Since is_compat_thread is defined in compat.h, implementing
>> is_syscall_success in ptrace.h would introduce build failure due to
>> recursive inclusion of some basic headers like mutex.h. We put the
>> implementation to ptrace.c
>>
>> Signed-off-by: He Zhe <zhe.he@xxxxxxxxxxxxx>
>> ---
>>  arch/arm64/include/asm/ptrace.h |  3 +++
>>  arch/arm64/kernel/ptrace.c      | 10 ++++++++++
>>  2 files changed, 13 insertions(+)
>>
>> diff --git a/arch/arm64/include/asm/ptrace.h b/arch/arm64/include/asm/ptrace.h
>> index e58bca832dff..3c415e9e5d85 100644
>> --- a/arch/arm64/include/asm/ptrace.h
>> +++ b/arch/arm64/include/asm/ptrace.h
>> @@ -328,6 +328,9 @@ static inline void regs_set_return_value(struct pt_regs *regs, unsigned long rc)
>>  	regs->regs[0] = rc;
>>  }
>>  
>> +extern inline int is_syscall_success(struct pt_regs *regs);
>> +#define is_syscall_success(regs) is_syscall_success(regs)
>> +
>>  /**
>>   * regs_get_kernel_argument() - get Nth function argument in kernel
>>   * @regs:	pt_regs of that context
>> diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
>> index 170f42fd6101..3266201f8c60 100644
>> --- a/arch/arm64/kernel/ptrace.c
>> +++ b/arch/arm64/kernel/ptrace.c
>> @@ -1909,3 +1909,13 @@ int valid_user_regs(struct user_pt_regs *regs, struct task_struct *task)
>>  	else
>>  		return valid_native_regs(regs);
>>  }
>> +
>> +inline int is_syscall_success(struct pt_regs *regs)
>> +{
>> +	unsigned long val = regs->regs[0];
>> +
>> +	if (is_compat_thread(task_thread_info(current)))
>> +		val = sign_extend64(val, 31);
>> +
>> +	return !IS_ERR_VALUE(val);
>> +}
> It's better to use compat_user_mode(regs) here instead of
> is_compat_thread(). It saves us from worrying whether regs are for the
> current context.

Thanks. I'll use this for v2.

>
> I think we should change regs_return_value() instead. This function
> seems to be called from several other places and it has the same
> potential problems if called on compat pt_regs.

IMHO, now that we have had specific function, syscall_get_return_value, to get
syscall return code, we might as well use it. regs_return_value may be left for
where we want internal return code. I found such places below and haven't found
other places that syscall sign extension is concerned about.

kernel/test_kprobes.c
kernel/trace/trace_kprobe.c
samples/kprobes/kretprobe_example.c


Regards,
Zhe

>