In Secure programming cookbook for C and C++ from John Viega I met the following statement
asm("value_stored: \n"
".long 0xFFFFFFFF \n"
);
I do not really understand the use of .long
directive in assembly, but here it is used to embed a precalculated value in the executable. Can I somehow force the position of these bytes in the executable? I have tried to put it at the end of main
(thinking that this way will be at the end of .text
section), but I got segmentation fault. Putting it outside the main works.
Even at the end of main
the inline assembler sequence will generate code to be executed. In my environment objdump -d foo.o
shows:
00000000004004b4 <main>:
4004b4: 55 push %rbp
4004b5: 48 89 e5 mov %rsp,%rbp
00000000004004b8 <value>:
4004b8: ff (bad)
4004b9: ff (bad)
4004ba: ff (bad)
4004bb: ff (bad)
4004bc: b8 01 00 00 00 mov $0x1,%eax
4004c1: 5d pop %rbp
4004c2: c3 retq
This can be mitigated by jumping over it
asm("jmp 1f"
"value: .long 0xffffffff"
"1:");
Keywords Nf or Nb create local temporary labels to jump forward or backwards.
Another option will be to place the variable to a named segment, which can be sorted in the linker file as the last segment in either .text or .data.
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