Finding the rendezvous structure of tracee (program being debugged) - c++

Question

I need debugger I am writing to give me the name of shared lib that program being debugged is linking with, or loading dynamically. I get the rendezvous structure as described in link.h, and answers to other questions, using DT_DEBUG, in the loop over _DYNAMIC[].
First, debugger never hits the break point set at r_brk.
Then I put a break in the program being debugged, and use link_map to print all loaded libraries. It only prints libraries loaded by the debugger, not the program being debugged.
It seems that, the rendezvous structure I am getting belongs to the debugger itself. If so, could you please tell me how to get the rendezvous structure of the program I am debugging? If what I am doing must work, your confirmation will be helpful, perhaps with some hint as to what else might be needed.
Thank you.

Answer 1

// You need to include <link.h>. All structures are explained
// in elf(5) manual pages.
// Caller has opened "prog_name", the debugee, and fd is the
// file descriptor. You can send the name instead, and do open()
// here.
// Debugger is tracing the debugee, so we are using ptrace().
void getRandezvousStructure(int fd, pid_t pd, r_debug& rendezvous) {
Elf64_Ehdr elfHeader;
char* elfHdrPtr = (char*) &elfHeader;
read(fd, elfHdrPtr, sizeof(elfHeader));
Elf64_Addr debugeeEntry = elfHeader.e_entry; // entry point of debugee
// Here, set a break at debugeeEntry, and after "PTRACE_CONT",
// and waitpid(), remove the break, and set rip back to debugeeEntry.
// After that, here it goes.
lseek(fd, elfHeader.e_shoff, SEEK_SET); // offset of section header
Elf64_Shdr secHeader;
elfHdrPtr = (char*) &secHeader;
Elf64_Dyn* dynPtr;
// Keep reading until we get: secHeader.sh_addr.
// That is the address of _DYNAMIC.
for (int i = 0; i < elfHeader.e_shnum; i++) {
read(fd, elfHdrPtr, elfHeader.e_shentsize);
if (secHeader.sh_type == SHT_DYNAMIC) {
dynPtr = (Elf64_Dyn*) secHeader.sh_addr; // address of _DYNAMIC
break;
}
}
// Here, we get "dynPtr->d_un.d_ptr" which points to rendezvous
// structure, r_debug
uint64_t data;
for (;; dynPtr++) {
data = ptrace(PTRACE_PEEKDATA, pd, dynPtr, 0);
if (data == DT_NULL) break;
if (data == DT_DEBUG) {
data = ptrace(PTRACE_PEEKDATA, pd, (uint64_t) dynPtr + 8 , 0);
break;
}
}
// Using ptrace() we read sufficient chunk of memory of debugee
// to copy to rendezvous.
int ren_size = sizeof(rendezvous);
char* buffer = new char[2 * ren_size];
char* p = buffer;
int total = 0;
uint64_t value;
for (;;) {
value = ptrace(PTRACE_PEEKDATA, pd, data, 0);
memcpy(p, &value, sizeof(value));
total += sizeof(value);
if (total > ren_size + sizeof(value)) break;
data += sizeof(data);
p += sizeof(data);
}
// Finally, copy the memory to rendezvous, which was
// passed by reference.
memcpy(&rendezvous, buffer, ren_size);
delete [] buffer;
}