Can functions/symbols in the same assembly file be re-ordered to different virtual addresses when linked?

Question

I have some code written in assembly which I need to copy at runtime (code that copies is written in C), hence I need to get the size of the function. I've written the function this way in assembly:

.globl function_foo
function_foo:
        ...
        ...
.globl function_foo_end
function_foo_end:

extern void function_foo(void);
extern void function_foo_end(void);

memcpy(dest, function_foo, (uintptr_t)function_foo_end - (uintptr_t)function_foo);

The code works as expected, but I'm concerned that if the symbols are re-ordered by the linking process, say if function_foo_end is moved before function_foo, or if function_foo and function_foo_end are not contiguous in memory, then memcpy may copy the wrong size.

Are these symbols guaranteed to be in order and contiguously placed in memory?

Answer 1

Linker works at granularity of sections so as long as your labels are within the same .text block linker will not split them apart.

There are certain post-link optimization tools (like BOLT) which can do more aggressive transformations of binary code, including moving labels around.

One alternative may be to compile your program two times:

• first test build is just to extract the function size (via readelf or like)
• you can then pass the extracted size to second compilation via macro (-DFUNC_SIZE=...)