get function address from name [.debug_info ??]

Question

Hi,

I was trying to write a small debug utility and for this I need to get the function/global variable address given its name. This is built-in debug utility, which means that the debug utility will run from within the code to be debugged or in plain words I cannot parse the executable file.

Now is there a well-known way to do that ? The plan I have is to make the .debug_* sections to to be loaded into to memory [which I plan to do by a cheap trick like this in ld script]

.data { *(.data) __sym_start = .; (debug_); __sym_end = .; }

Now I have to parse the section to get the information I need, but I am not sure this is doable or is there issues with this - this is all just theory. But it also seems like too much of work :-) is there a simple way. Or if someone can tell upfront why my scheme will not work, it ill also be helpful.

Thanks in Advance, Alex.

Answer 1

If you are running under a system with dlopen(3) and dlsym(3) (like Linux) you should be able to:

char thing_string[] = "thing_you_want_to_look_up";
void * handle = dlopen(NULL, RTLD_LAZY | RTLD_NOLOAD);
  // you could do RTLD_NOW as well.  shouldn't matter
if (!handle) {
   fprintf(stderr, "Dynamic linking on main module : %s\n", dlerror() );
   exit(1);
}

void * addr = dlsym(handle, thing_string);
fprintf(stderr, "%s is at %p\n", thing_string, addr);

I don't know the best way to do this for other systems, and this probably won't work for static variables and functions. C++ symbol names will be mangled, if you are interested in working with them.

To expand this to work for shared libraries you could probably get the names of the currently loaded libraries from /proc/self/maps and then pass the library file names into dlopen, though this could fail if the library has been renamed or deleted.

There are probably several other much better ways to go about this.

edit without using dlopen

/* name_addr.h */
struct name_addr {
     const char * sym_name;
     const void * sym_addr;
};
typedef struct name_addr name_addr_t;
void * sym_lookup(cost char * name);
extern const name_addr_t name_addr_table;
extern const unsigned name_addr_table_size;

---

/* name_addr_table.c */
#include "name_addr.h"

#define PREMEMBER( X ) extern const void * X
#define REMEMBER( X ) { .sym_name = #X , .sym_addr = (void *) X }

PREMEMBER(strcmp);
PREMEMBER(printf);
PREMEMBER(main);
PREMEMBER(memcmp);
PREMEMBER(bsearch);
PREMEMBER(sym_lookup);
/* ... */

const name_addr_t name_addr_table[] =
{
       /* You could do a #include here that included the list, which would allow you
        * to have an empty list by default without regenerating the entire file, as
        * long as your compiler only warns about missing include targets.
        */
     REMEMBER(strcmp),
     REMEMBER(printf),
     REMEMBER(main),
     REMEMBER(memcmp),
     REMEMBER(bsearch),
     REMEMBER(sym_lookup);
     /* ... */
};
const unsigned name_addr_table_size = sizeof(name_addr_table)/sizeof(name_addr_t);

---

/* name_addr_code.c */
#include "name_addr.h"
#include <string.h>

void * sym_lookup(cost char * name) {
    unsigned to_go = name_addr_table_size;
    const name_addr_t *na = name_addr_table;
    while(to_to) {
       if ( !strcmp(name, na->sym_name) ) {
            return na->sym_addr;
       }
       na++;
       to_do--;
    }
    /* set errno here if you are using errno */
    return NULL;  /* Or some other illegal value */
}

If you do it this way the linker will take care of filling in the addresses for you after everything has been laid out. If you include header files for all of the symbols that you are listing in your table then you will not get warnings when you compile the table file, but it will be much easier just to have them all be extern void * and let the compiler warn you about all of them (which it probably will, but not necessarily).

You will also probably want to sort your symbols by name such that you can use a binary search of the list rather than iterate through it.

You should note that if you have members in the table which are not otherwise referenced by the program (like if you had an entry for sqrt in the table, but didn't call it) the linker will then want (need) to link those functions into your image. This can make it blow up.

Also, if you were taking advantage of global optimizations having this table will likely make those less effective since the compiler will think that all of the functions listed could be accessed via pointer from this list and that it cannot see all of the call points.

Putting static functions in this list is not straight forward. You could do this by changing the table to dynamic and doing it at run time from a function in each module, or possibly by generating a new section in your object file that the table lives in. If you are using gcc:

#define SECTION_REMEMBER(X) \
   static const name_addr_t _name_addr##X = \
     {.sym_name= #X , .sym_addr = (void *) X } \
     __attribute__(section("sym_lookup_table" ) )

And tack a list of these onto the end of each .c file with all of the symbols that you want to remember from that file. This will require linker work so that the linker will know what to do with these members, but then you can iterate over the list by looking at the begin and end of the section that it resides in (I don't know exactly how to do this, but I know it can be done and isn't TOO difficult). This will make having a sorted list more difficult, though. Also, I'm not entirely certain initializing the .sym_name to a string literal's address would not result in cramming the string into this section, but I don't think it would. If it did then this would break things.

You can still use objdump to get a list of the symbols that the object file (probably elf) contains, and then filter this for the symbols you are interested in, and then regenerate the table file the table's members listed.