linux-kernel - Re: Assignment of GDT entries

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Date:	Wed, 13 Sep 2006 22:44:26 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	"Albert Cahalan" <acahalan@...il.com>
Cc:	torvalds@...l.org, jeremy@...p.org, mingo@...e.hu, ak@...e.de,
	arjan@...radead.org, zach@...are.com, linux-kernel@...r.kernel.org
Subject: Re: Assignment of GDT entries

"Albert Cahalan" <acahalan@...il.com> writes:

> I think that would be a lower chance, not a greater chance.
> Reasons why an app might care:
>
> a. identify a 64-bit kernel
> b. far jumps between 32-bit and 64-bit code
> c. reload of ds/es after a string operation on thread-private data
>
> Perhaps i386 should change to match x86_64.

I agree that the difference is annoying.

However I just wrote a user space implementation of fork that
is capable of copying a process from an i386 only kernel to a x86_64
kernel, and executing there without having to detect the kernel type.

It didn't takes hacks to accomplish that.

The basic syscall is:
int set_thread_area (struct user_desc *u_info);
struct user_desc {
	unsigned int  entry_number;
	unsigned long base_addr;
	unsigned int  limit;
	unsigned int  seg_32bit:1;
	unsigned int  contents:2;
	unsigned int  read_exec_only:1;
	unsigned int  limit_in_pages:1;
	unsigned int  seg_not_present:1;
	unsigned int  useable:1;
};

If entry_number is -1 the kernel finds a free gdt entry and
sets up the segment and returns with entry_number set to the
segment number.

Eric
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