when I disassemble an OpenCV dynamic library file (libopencv_imgproc.so), I find that there are a lot of bl instructions which target address are in the .plt section. But, I could'n find its symbol info.

00110ee0 <cvResize>:
  110ee0:   b5f0        push    {r4, r5, r6, r7, lr}
  110ee2:   4603        mov r3, r0
  110ee4:   b0bf        sub sp, #252    ; 0xfc
  110ef4:   4ca8        ldr r4, [pc, #672]  ; (111198 <cvResize+0x2b8>)
  110ef6:   9500        str r5, [sp, #0]
  110ef8:   f70e ebda   blx 1f6b0 <_init+0x2b4>

I could not find the address 1f6b0 in the .text section. It actually is in the .plt section. However, I couldn't find it in the symbol table. How can I get these missed symbol info?

1 Answer 1


Doing it the hard way:

  1. disassemble the target PLT stub and figure out what pointer it's using. E.g.:

    359dc:  e28fc601    add ip, pc, #1048576    ; 0x100000
    359e0:  e28ccaa2    add ip, ip, #663552 ; 0xa2000
    359e4:  e5bcf900    ldr pc, [ip, #2304]!    ; 0x900

    Here, the ip will be: 0x359e4+0x100000+0xa2000 = 0x1D79E4, so the last LDR will dereference 1D79E4+0x900 = 0x1D82E4.

  2. look up the address in the list of relocs (objdump -dr):

    001d82e4  00005216 R_ARM_JUMP_SLOT   00000000   _ZN2cv10cvarrToMatEPKv

Doing it the easy way:

  1. Just use IDA :), or at least a newer objdump which knows about PLT stubs.

    000359dc <_ZN2cv10cvarrToMatEPKvbbi@plt>:

Please note that objdump relies on the fact that the order of PLT stubs usually matches the order of relocations in the PLT reloc list. In theory, one can patch a PLT stub after linking so it uses another symbol's pointer.

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