My program has a function to find the approximate location of the top (highest address) of the stack, by using a loop similar to what you would use for a backtrace:
uint64_t rbp = 0;
uint64_t raw_stack_start = 0;
asm volatile ("movq %%rbp, %0" : "=m"(rbp));
while (rbp) {
raw_stack_start = rbp;
rbp = *(uint64_t *)rbp;
}
GCC compiles the loop as follows (from objdump -D):
400700: 48 c7 45 f8 00 00 00 00 movq $0x0,-0x8(%rbp)
400708: 48 c7 45 f0 00 00 00 00 movq $0x0,-0x10(%rbp)
400710: 48 89 6d f8 mov %rbp,-0x8(%rbp)
400714: eb 13 jmp 400729 <find_stack_start+0x2d>
400716: 48 8b 45 f8 mov -0x8(%rbp),%rax
40071a: 48 89 45 f0 mov %rax,-0x10(%rbp)
40071e: 48 8b 45 f8 mov -0x8(%rbp),%rax
400722: 48 8b 00 mov (%rax),%rax
400725: 48 89 45 f8 mov %rax,-0x8(%rbp)
400729: 48 8b 45 f8 mov -0x8(%rbp),%rax
40072d: 48 85 c0 test %rax,%rax
400730: 75 e4 jne 400716 <find_stack_start+0x1a>
When I compile this code without the -static flag, it runs fine and gives me a plausible address. When I compile it with the -static flag, I get a segmentation fault, which GDB traces to instruction 0x400722 above, where I dereference %rax and store the value in %rax. Why would this cause a segmentation fault—with the -static flag but not without it?
Thanks!
Amittai
Amittai Aviram
PhD Student in Computer Science
Yale University
646 483 2639
amittai.aviram@xxxxxxxx
http://www.amittai.com
