Re: [RFC PATCH] sys_read: add a compat_sys_read for 64bit system

[Andy Lutomirski]

On Tue, Jun 7, 2016 at 7:14 PM, Zhangjian (Bamvor)
<bamvor.zhangjian@xxxxxxxxxx> wrote:
> Hi,
>
> On 2016/6/8 9:33, Weidong Wang wrote:
>>
>> Test 32 progress and 64 progress on the 64bit system with
>> this progress:
>>
>> int main(int argc, char **argv)
>> {
>>          int fd = 0;
>>          int i, ret = 0;
>>          char buf[512];
>>          unsigned long count = -1;
>>
>>          fd = open("/tmp", O_RDONLY);
>>          if (fd < -1) {
>>                  printf("Pls check the directory is exist?\n");
>>                  return -1;
>>          }
>>          errno = 0;
>>          ret = read(fd, NULL, count);
>>          printf("Ret is %d errno %d\n", ret, errno);
>>          close(fd);
>>
>>          return 0;
>> }
>>
>> we get the different errno. The 64 progress we get errno is -14 while
>> the 32 progress is -21.

On 64-bit, you get -14 == -EFAULT.  Seems reasonable: you passed a bad pointer.

On 32-bit, you get -21 == -EISDIR.  Also seems reasonable: fd is a directory.

>>
>> The reason is that, the user progress would use a 32bit count, while
>> the sys_read size_t in kernel is 64bit.  When the uesrspace count is
>> -1(0xffffffff), it goes to the sys_read, it would be change to a positive
>> number.

That parameter is size_t, which is unsigned.  It's a positive number
in both cases.

I don't think there's a bug here.