linux-kernel - Re: X86_64 BUG: missing FS/GS LDT reload on fork()

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Message-ID: <4BD1E061.8030605@zytor.com>
Date:	Fri, 23 Apr 2010 11:01:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Samuel Thibault <samuel.thibault@...ri.fr>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, x86@...nel.org,
	olivier.aumage@...ia.fr, yannick.martin@...ia.fr
Subject: Re: X86_64 BUG: missing FS/GS LDT reload on fork()

On 04/23/2010 10:04 AM, Samuel Thibault wrote:
> Hello,
> 
> I have an issue with FS/GS LDT reload in the child of fork(). The
> attached testcase fails quite often.  It sets an LDT entry up, uses
> prctl to set gs's base to a 64bit value, then loads gs with the LDT
> entry. The LDT entry is now in effect. After a fork call, the LDT entry
> is not in effect any more, the 64bit base is back!
> 

Okay... I have to say that I'm more than a bit confused why you're doing
this, but the __switch_no code in process_64.c has the following:

                /*
                 * Check if the user used a selector != 0; if yes
                 *  clear 64bit base, since overloaded base is always
                 *  mapped to the Null selector
                 */
                if (fsindex)
                        prev->fs = 0;

[and the same for gs]

However, copy_thread() doesn't have the equivalent code, and __switch_to
clearly expects that to be maintained as an invariant -- it doesn't
check on entry, only on exit.

The following patch looks like it should address that.

	-hpa

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