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Date: Mon, 25 Jun 2007 23:42:25 +0200
From: Clemens Kolbitsch <clemens.kol@gmx.at>
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Subject: [Qemu-devel] sidt problem
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To: qemu-devel@nongnu.org

hi everyone!
i have a strange problem:

i use the following code on my linux 2.6.20 (kubuntu debian, i386) to 
dynamically get the location of the system-call table (as can also be 
found in /proc/kallsyms --> "sys_call_table") as it is quite interesting 
for new exploits ( :-)  )

on a real cpu this works fine, however crashes in qemu... obviously 
there is a bug somewhere. i have not found my way that deep into the 
qemu source, so i cannot really help to find the bug.

well, here is the code:

    struct
    {
        unsigned short limit;
        unsigned int base;
    } __attribute__ ((packed)) idtr;

    struct
    {
        unsigned short off1;
        unsigned short sel;
        unsigned char none, flags;
        unsigned short off2;
    } __attribute__ ((packed)) *igd;

    unsigned long *sys_call;
    unsigned char *pc;

    // find idt_table
    __asm__("sidt %0" : :"m"(idtr));

    // find system_call
    igd = idtr.base + 8 * 0x80;

    // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    // the next line crashes
    // !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
    sys_call = (igd->off2 << 16) | igd->off1;

    // find sys_call_table
    // ff 14 85 XX XX XX XX     call <sys_call_table>(,%eax,4)

    sys_call_table = 0x0;
    pc = (char*)sys_call;

    // check the first 100 bytes in system_call
    for (i = 0; i < 100; ++i)
    {
        if ((*(long*)++pc << 8) == 0x8514ff00)
        {
            sys_call_table = *(long*)(pc+3);
            break;
        }
    }


maybe, someone has time to look at this problem (by the way, i use the 
same system inside qemu as on my laptop)

greets!!