linux-kernel - Re: question about head

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Date:   Tue, 22 Jan 2019 16:08:00 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Cao jin <caoj.fnst@...fujitsu.com>
Cc:     kirill.shutemov@...ux.intel.com,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, bp@...en8.de,
        "H. Peter Anvin" <hpa@...or.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: question about head_64.S

On Tue, Jan 22, 2019 at 03:31:25PM +0800, Cao jin wrote:
> Hi, Kirll,
> 
> On 1/15/19 7:45 PM, Cao jin wrote:
> > Hi,
> >   I have been digging into this file for a while, and I still have 2
> > questions unclear, hope to get your help.
> > 
> 
> > 
> > 2.
> > Why gdt64 has following definition?:
> > 
> > gdt64:
> > 	.word	gdt_end - gdt
> > 	.long	0
> > 	.word	0
> > 	.quad   0
> > 
> > obviously, gdt64 stores the GDTR content under x86_64, which is 10 bytes
> > long, so why not just:
> > 
> > gdt64:
> > 	.word	gdt_end - gdt
> > 	.quad   0
> > 
> > With above modification, it can boot.
> > 
> 
> Seems you introduced gdt64 code in commit beebaccd50, could you help
> with this question?

Looks like you are right. I've got confused at some point.

Could you prepare a patch?

> And it also remind me of another question about adjust_got which is also
> introduced by you. Because I failed to construct a test environment with
> ld version less than 2.24 until now, so I wanna do a quick ask here:
> does it make sense to adjust GOT from the 4th entry of it? Because as I
> know, the first 3 entries are special one, which (I guess) will be not used.

No.

These 3 entries are reserved for a special symbols (like entry 0 for
_DYNAMIC). It means linker should not use these entries for normal
symbols, but it doesn't mean that they don't need to be adjusted during
the load.

-- 
 Kirill A. Shutemov