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clinuxsymbol-tablesymbol-tables

Is it possible to determine if a symbol is a variable or function in C?

I am implementing some limited remote debugging functionality for an application written in C running on a Linux box. The goal is to communicate with the application and lookup the value of an arbitrary variable or run an arbitrary function.

I am able to lookup symbols through dlsym() calls, but I am unable to determine if the address returned refers to a function or a variable. Is there a way to determine typing information via this symbol table?

Solution

  • You can read the file /proc/self/maps and parse the first three fields of each line:

    <begin-addr>-<end-addr> rwxp ...

    Then you search the line that contains the address you are looking for and check the permissions:

    • r-x: it is code;
    • rw-: it is writable data;
    • r--: it is read-only data;
    • any other combination: something weird (rwxp: generated code, ...).

    For example the following program:

    #include <stdio.h>

void foo() {}
int x;

int main()
{
    int y;
    printf("%p\n%p\n%p\n", foo, &x, &y);
    scanf("%*s");
    return 0;
}

    ...in my system gives this output:

    0x400570
0x6009e4
0x7fff4c9b4e2c

    ...and these are the relevant lines from /proc/<pid>/maps:

    00400000-00401000 r-xp 00000000 00:1d 641656       /tmp/a.out
00600000-00601000 rw-p 00000000 00:1d 641656       /tmp/a.out
....
7fff4c996000-7fff4c9b7000 rw-p 00000000 00:00 0    [stack]
....

    So the addresses are: code, data and data.