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I have a C++ program that reads link_map obtained from dlinfo(handle, RTLD_DI_LINKMAP, &link_map); where handle was obtained by dlopen(NULL, RTLD_LAZY);

The program displays information about dynamic section for every library in link_map. I'm trying to use d_ptr value from DT_SYMTAB tag (dyn->d_un.d_ptr) to read symbols from the library.

One thing that I don't understand is why this address is small for linux-vdso.so.1. I found that this is an offset from loaded_link_map->l_addr.

I was looking information in many places. (eg. Use dlinfo to print all symbols in a library, http://www.sco.com/developers/gabi/latest/ch5.dynamic.html, https://flapenguin.me/2017/05/10/elf-lookup-dt-gnu-hash/, http://www.sco.com/developers/gabi/1998-04-29/ch4.sheader.html, http://s.eresi-project.org/inc/articles/elf-rtld.txt)

I cannot find answers to this questions :

  1. How to determinate if d_ptr is a "program virtual addresses" or "offset from loaded_link_map->l_addr"?

  2. If small value (offsett) in linux-vdso.so.1 is an exception or bug or this is ok and I don't understand why?

This is my simple program code: 

#include <link.h>
#include <cstdio>

static const char *tag = "dl_util";

const char *dyn_tag_to_name(ElfW(Sxword) d_tag) {
    switch (d_tag) {
        case DT_STRTAB: return("DT_STRTAB");
        case DT_SYMTAB: return("DT_SYMTAB");
        default: return("UNKNOWN");
    }
}

void print_dynamic_sections(const link_map *l) {
    const ElfW(Dyn) *const dyn_start = l->l_ld;
    const ElfW(Addr) load_addr = l->l_addr;

    printf("Dynamic Sections:\n");
    printf("|%-18s|%16s|\n", "Tag", "Ptr");
    for (const ElfW(Dyn) *dyn = dyn_start; dyn->d_tag != DT_NULL; ++dyn) {
        if(dyn->d_tag == DT_STRTAB || dyn->d_tag == DT_SYMTAB)
        printf("|%-18s|%16lx|\n",
              dyn_tag_to_name(dyn->d_tag),
              dyn->d_un.d_val,
              (const void *) (dyn->d_un.d_ptr));

    }
}

int main() {

    void *handle;

    handle = dlopen(NULL, RTLD_LAZY);
    if (!handle) {
        printf("Error[%s]: %s\n", tag, dlerror());
        return 1;
    }

    struct link_map *l;
    int ld_ret = dlinfo(handle, RTLD_DI_LINKMAP,  &l);
    if (ld_ret) {
        printf("Error[%s]: %s\n", tag, dlerror());
        return 1;
    }
    printf("Loaded libraries: handle:0x%lx, prev:0x%lx\n", handle, (void*)l->l_prev);
    do {
        printf("   0x%lx, %s\n", l->l_addr, l->l_name);
        print_dynamic_sections(l);

        l = l->l_next;
    } while (l != NULL);
    return 0;
}

And the code output is:

Loaded libraries: handle:0x7fb3f5bf7120, prev:0x0
   0x562410906000, 
Dynamic Sections:
|Tag               |             Ptr|
|DT_STRTAB         |    562410906448|
|DT_SYMTAB         |    562410906328|
   0x7fff5978b000, linux-vdso.so.1
Dynamic Sections:
|Tag               |             Ptr|
|DT_STRTAB         |             298|
|DT_SYMTAB         |             1a8|
   0x7fb3f5b55000, /usr/lib/libdl.so.2
Dynamic Sections:
|Tag               |             Ptr|
|DT_STRTAB         |    7fb3f5b557a8|
|DT_SYMTAB         |    7fb3f5b553a0|
   0x7fb3f59c6000, /usr/lib/libstdc++.so.6
Dynamic Sections:
|Tag               |             Ptr|
|DT_STRTAB         |    7fb3f59eef90|
|DT_SYMTAB         |    7fb3f59ce758|

... more libraries
mojes
  • 11
  • 3

0 Answers0