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

Weidong Wang
2016-06-14T03:06:33Z

On 2016/6/10 1:08, Andy Lutomirski wrote:
> On Tue, Jun 7, 2016 at 7:14 PM, Zhangjian (Bamvor)
> <bamvor.zhangjian@huawei.com> 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.
>

Yep.
In the progress open the '/tmp' is a directory. If we do open a file '/tmp/files' (exist file),
the result would be different on x86-64bit machine.

On 64-bit, we get -14 == -EFAULT.
On 32-bit, we get the length of the file, the errno is 0.

Regards,
Weidong

> .
>