U-boot And GOT

游行到此

Disassemble u-boot
mips_fp_be-objdump -D u-boot

Some segment
bfc00400 reset>:
……
bfc00440:    04110002     bal    bfc0044c reset+0x4c>
bfc00444:    00000000     nop
bfc00448:    bfc34500     cache    0x3,17664(s8)
bfc0044c:    03e0e021     move    gp,ra
bfc00450:    8fe90000     lw    t1,0(ra)
bfc00454:    0120e021     move    gp,t1
bfc00458:    8f9902b0     lw    t9,688(gp)
bfc0045c:    0320f809     jalr    t9
bfc00460:    00000000     nop
bfc00464:    8f9904ec     lw    t9,1260(gp)
bfc00468:    0320f809     jalr    t9
bfc0046c:    00000000     nop
……

bfc07a70 lowlevel_init>:
bfc07a70:    3c08a043     lui    t0,0xa043
……
bfc0fa90 mips_cache_reset>:
bfc0fa90:    240a4000     li    t2,16384
……
bfc34500 _GLOBAL_OFFSET_TABLE_>:
bfc34500:    00000000     nop
bfc34504:    80000000     lb    zero,0(zero)
bfc34508:    bfc00000     cache    0x0,0(s8)
bfc3450c:    bfc30000     cache    0x3,0(s8)
bfc34510:    bfc10000     cache    0x1,0(s8)
bfc34514:    bfc20000     cache    0x2,0(s8)
……
bfc347b0:    bfc07a70     cache    0x0,31344(s8)
bfc347b4:    bfc35154     cache    0x3,20820(s8)
bfc347b8:    bfc16fb0     cache    0x1,28592(s8)
……
bfc349e8:    bfc20144     cache    0x2,324(s8)
bfc349ec:    bfc0fa90     cache    0x0,-1392(s8)
bfc349f0:    bfc1336c     cache    0x1,13164(s8)
……
reset:
    ……
    /* Initialize GOT pointer.
    */
    bal 1f
    nop
    .word _GLOBAL_OFFSET_TABLE_
    1:
    move gp, ra
    lw t1, 0(ra)
    move    gp, t1
    /* Initialize any external memory.
     */
    la t9, lowlevel_init
    jalr t9
    nop
    /* Initialize caches...
     */
    la t9, mips_cache_reset
    jalr t9
    nop   

Analysis
Initialize the GOT pointer
“bal 1f” load the the address of _GLOBAL_OFFSET_TABLE_ into ra.
“lw t1, 0(ra)” load the destination register (ra) with the contents of the word that is at the memory location.
Finally, GOT header is saved into gp.

Example 1: Get the address of lowlevel_init
Use assemble code, the following sentence can get the address.

       la      t9, lowlevel_init

But in final code, to get the location-independent address, u-boot adds the gp and an offset to get the address.

bfc00458:       8f9902b0       lw    t9,688(gp)

Please refer to the GOT table, start at “bfc34500 ”.
The address of GOT is 0xbfc34500, the offset is 688 (0x2b0), the result is 0xbfc347b0. The content of address 0xbfc347b0 is 0xbfc07a70. And the address 0xbfc07a70 just is the address of lowlevel_init.
Example 2: Get the address of mips_cache_reset
The address of mips_cache_reset is:
0xbfc0fa90 = 0xbfc34500 + 0x4ec.

Question



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