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[Qemu-devel] Emulating MIPS self-examining code

From: Dmitry Antipov
Subject: [Qemu-devel] Emulating MIPS self-examining code
Date: Mon, 08 Feb 2010 17:26:33 +0300
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I'm trying to emulate the following MIPS code (taken from the bootloader of my 

        /* Initialize GOT pointer.
        ** Global symbols can't be resolved before this is done, and as such we 
        ** use any global symbols in this code.  We use the bal/ move xxx,ra 
combination to access
        ** data in a PC relative manner to avoid this.  This code will 
correctly set the
        ** gp regardless of whether the code has already been relocated or not.
        ** This code determines the current gp by computing the link time (gp - 
        ** and adding this to the current pc.
        ** runtime_gp = runtime_pc + (linktime_gp - linktime_pc)
        ** U-boot is running from the address it is linked at at this time, so 
        ** general case code is not strictly necessary here.

        /* Branch and link to get current PC in ra */
        bal     1f
        .extern _GLOBAL_OFFSET_TABLE_
        .word   _GLOBAL_OFFSET_TABLE_  /* This contains the linked address of 
the GOT */
        /* The ra register now contains the runtime address of the above memory 
location */

        .word   . - 4                  /* This contains the link time address 
of the previous word,
                                        which is also what the link time 
expected PC value is */
        move    gp, ra    /* Move current PC into gp register */
        lw      t1, 0(ra) /* Load linked address of the GOT into t1 */
        lw      t2, 4(ra) /* Load the link time address of the GOT storage 
location into t2 */
        sub     t1, t2    /* Subtract t2 from t1. */
        /* t1 now contains the difference between the link-time GOT table 
address and the link time expected PC */

        /* Add this difference to the current PC (copied into gp above) so that 
gp now has the current runtime
        ** GOT table address */
        add     gp, t1  # calculate current location of offset table

Corresponding objdump output is:

bfc306c4:       04110003        bal     bfc306d4 <func+0x28>
bfc306c8:       00000000        nop
bfc306cc:       bfc79d10        cache   0x7,-25328(s8)
bfc306d0:       bfc306cc        cache   0x3,1740(s8)
bfc306d4:       03e0e02d        move    gp,ra
bfc306d8:       8fe90000        lw      a5,0(ra)
bfc306dc:       8fea0004        lw      a6,4(ra)
bfc306e0:       012a4822        sub     a5,a5,a6
bfc306e4:       0389e020        add     gp,gp,a5
bfc79d10:       00000000        nop

This is a kind of self-examining code (bfc306cc..bfc306d0 is treated as data). 
The problem
is that QEMU translates this (master?)piece into two translation blocks 
and bfc306d4..bfc306e4, silently ignoring bfc306cc..bfc306d0 because there is 
no way
to execute in that area. Due to this, 0(ra) at bfc306d8 can't be evaluated 

Is there any ideas on how to get such code emulated?


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