linux-kernel - Re: [RFC PATCH] sys_read: add a compat_sys

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Date:	Tue, 14 Jun 2016 11:05:59 +0800
From:	Weidong Wang <wangweidong1@...wei.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"Zhangjian (Bamvor)" <bamvor.zhangjian@...wei.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Hanjun Guo <guohanjun@...wei.com>,
	Linux-Api <linux-api@...r.kernel.org>
Subject: Re: [RFC PATCH] sys_read: add a compat_sys_read for 64bit system

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

> .
>