Re: [PATCH] s390: speedup strn{cpy,len}_from_user.

[linux@horizon.com]

>>  static inline long
>>  strncpy_from_user(char *dst, const char *src, long count)
>>  {
>>          long res = -EFAULT;
>>          might_sleep();
>> -        if (access_ok(VERIFY_READ, src, 1))
>> -                res = __strncpy_from_user_asm(dst, src, count);
>> +        if (access_ok(VERIFY_READ, src, 1)) {
>> +                res = __strncpy_from_user_asm(count, dst, src);
>> +	}
>>          return res;
>>  }

> Shouldn't the access_ok() check be passed `count', rather than `1'?

This deserves a comment, but no.

"count" is the maximum length of the string, not the actual length.
If you passed it to access_ok(), you'd falsely EFAULT a short string
near the end of the address space.

Assuming that there's a guard page at the end of the user address space,
__strncpy_from_user_asm is guaranteed to hit it as part of normal
operations, and will get the fault.