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From: Eric Lacombe <goretux@gmail.com>
To: Ingo Molnar <mingo@elte.hu>
Subject: [x86] do_arch_prctl - bug?
Date: Tue, 18 Nov 2008 15:33:32 +0100
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Hello,

I would like to know why the ARCH_SET_GS action of sys_arch_prctl, write the 
MSR MSR_KERNEL_GS_BASE and not the MSR MSR_GS_BASE when the variable "doit" 
equals 1? Is that a bug?

In other words, why the following code :
...
	if (doit) {
        	load_gs_index(0);
                ret = checking_wrmsrl(MSR_KERNEL_GS_BASE, addr)
	} 
...
is not the following one :
...
	if (doit) {
		ret = checking_wrmsrl(MSR_GS_BASE, addr)
        	load_gs_index(0);
	} 
...

I copy for clarity the beginning of the function "do_arch_prctl" :

long do_arch_prctl(struct task_struct *task, int code, unsigned long addr)
{ 
        int ret = 0; 
        int doit = task == current;
        int cpu;

        switch (code) { 
        case ARCH_SET_GS:
                if (addr >= TASK_SIZE_OF(task))
                        return -EPERM; 
                cpu = get_cpu();
                /* handle small bases via the GDT because that's faster to 
                   switch. */
                if (addr <= 0xffffffff) {  
                        set_32bit_tls(task, GS_TLS, addr); 
                        if (doit) { 
                                load_TLS(&task->thread, cpu);
                                load_gs_index(GS_TLS_SEL); 
                        }
                        task->thread.gsindex = GS_TLS_SEL; 
                        task->thread.gs = 0;
                } else { 
                        task->thread.gsindex = 0;
                        task->thread.gs = addr;
                        if (doit) {
                              load_gs_index(0);
                              ret = checking_wrmsrl(MSR_KERNEL_GS_BASE, addr);
                        } 
                }
                put_cpu();
                break;
[...]

Regards,

	Eric
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