Cirrus Logic CS485 Manual do Utilizador descarregar pdf (Página 34)

CS485G Spring 2015 34

gcc, use -fstack-protector (adds code to evidently suspi-

cious routines) or -fstack-protector-all (adds code to all

routines)

8048654: 65 a1 14 00 00 00 mov %gs:0x14,%eax

804865a: 89 45 f8 mov %eax,0xfffffff8(%ebp)

804865d: 31 c0 xor %eax,%eax

...

8048672: 8b 45 f8 mov 0xfffffff8(%ebp),%eax

8048675: 65 33 05 14 00 00 00 xor %gs:0x14,%eax

804867c: 74 05 je 8048683 <echo+0x36>

804867e: e8 a9 fd ff ff call 804842c <FAIL>

8. Malware

(a) Worm: a program that can run by itself, propagates a fully work-

ing version to other computers.

(b) Virus: code that adds itself to other programs, but cannot run

independently.

37 Linking

1. Lecture 20, 3/13/2015

2. Basic idea: combine results of one or more independent compilations

with libraries.

3. The individual compiled results are called relocatable object ﬁles;

the Unix convention is that their names end “.o”.

4. Beneﬁts

(a) The programmer can decompose work into small ﬁles, promot-

ing modularity.

(b) Experts (hah!) can program commonly used functions and place

them in libraries (C library, math library, ...).

of ﬁles.

(d) The linker can pick up only those functions that are used from

a library, so the entire library need not be part of the executable.

5. Symbol resolution

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Cirrus Logic CS485 Manual do Utilizador Página 34