Re: Cant debug on Gentoo 64

["Paul Pluzhnikov" <ppluzhnikov at google dot com>]

On Wed, Aug 6, 2008 at 1:48 AM, Eran Ifrah <eran.ifrah@gmail.com> wrote:

> warning: no loadable sections found in added symbol-file
> system-supplied DSO at <some address goes here>

I have just debugged this warning on our newer systems.
Are you using kernel 2.6.24 or above?

64-bit kernels that have VDSO, but are built with "old" binutils,
will trigger that warning.

You can verify whether your kernel is built correctly by compiling
and running this program:

--- cut ---
#include <elf.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <errno.h>
#include <unistd.h>

void dump_vdso(void *ptr)
{
  int fd = open("vdso64.so", O_WRONLY|O_CREAT, 755);
  if (-1 == fd) {
    perror("open2"); exit(1);
  }
  printf("vdso at %p\n", ptr);
  if (write(fd, ptr, 4096) != 4096) {
    perror("write"); exit(1);
  }
}

int main()
{
  int fd = open("/proc/self/auxv", O_RDONLY, 0);
  if (-1 == fd) {
    perror("open"); exit(1);
  }
  Elf64_auxv_t auxv;
  while (0 < read(fd, &auxv, sizeof(auxv))) {
    switch (auxv.a_type) {
      case AT_NULL: break;
      case AT_SYSINFO_EHDR:
        dump_vdso(auxv.a_un.a_ptr);
        return 0;
    }
  }
  printf("No AT_SYSINFO_EHDR found\n");
  return 0;
}
--- cut ---

If it finds a kernel VDSO, it will dump it to disk.
Then do 'readelf -l vdso64.so'.

If you see this:

Program Headers:
 Type           Offset             VirtAddr           PhysAddr
                FileSiz            MemSiz              Flags  Align
 LOAD           0x0000000000000000 0xffffffffff700000 0xffffffffff700000
                0x0000000000000a50 0x0000000000000a50  R E    100000

(alignment other than 1000), your kernel build is bad.

> placing a breakpoint at main does not work and program simply executes
> until it terminates (return 0).

I didn't see that problem (but I am using gdb-6.8).

Cheers,
-- 
Paul Pluzhnikov