RE: Regarding "Inconsistency detected by ld.so" 32 Bit Elf

[Karthikeyan Shanmugam <karthikeyan24s at hotmail dot com>]

Thanks Carlos.

> It is a problem. Less people will be able to help you if you can't
> duplicate the problem on a common architecture. There is nothing
> target dependent about this problem. You should equally be able to
> produce an example program that shows the problem on x86-64.

I'll give a try once again.

>>> I know that QEMU has to do something similar. There are probably so
>>> few applications that need to do this that there is no good accepted
>>> standard way to accomplish this.

Yes. I've checked QEMU code and linker script. They are modifying only the executable start address and Text segment in the custom linker script but no new/custom section which reserves VMA space .

> Let me be clear here on two points.
> 
> I don't know what you mean by "detect-frce relocate."

For example below is the elf segments output captured for libc library.

  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  PHDR           
  INTERP         
      [Requesting program interpreter: /lib/ld.so.1]
  LOAD           0x000000 0x0fe40000 0x0fe40000 0x158608 0x158608 R E 0x10000
  LOAD           0x15930c 0x0ffa930c 0x0ffa930c 0x0460c 0x071f0 RW  0x10000
  DYNAMIC       
  NOTE           
  TLS            
  GNU_EH_FRAME  
  GNU_STACK      
  GNU_RELRO      

Here, the first load segment base address is non-zero, so just curious to know whether prelink has some mechanism to detect and relocate only these libraries? 

> 
> I also don't know what you mean by "non-zero PT_LOAD segment libraries."

For. Example in above case libc holds the non-zero first load segment.


> The size is always known. Look at the size of the loadable segments,
> and round up to page sizes and appropriate alignment.
> 

I've captured one sample output in which I noticed few discrepancies. Could you please clarify them? 

LIBC:
=====

/proc/<pid>/maps Output:
-------------------------
0fe40000-0ff99000 r-xp 00000000 00:0f 2392980    /lib/libc-2.11.1.so
0ff99000-0ffa9000 ---p 00159000 00:0f 2392980    /lib/libc-2.11.1.so
0ffa9000-0ffad000 r--p 00159000 00:0f 2392980    /lib/libc-2.11.1.so
0ffad000-0ffae000 rw-p 0015d000 00:0f 2392980    /lib/libc-2.11.1.so
0ffae000-0ffb1000 rw-p 00000000 00:00 0

Map Segments Difference:
------------------------
0ffb1000 - 0fe40000 = 1476 KB

readelf Output:
---------------

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  PHDR           0x000034 0x0fe40034 0x0fe40034 0x00140 0x00140 R E 0x4
  INTERP         0x14a7f0 0x0ff8a7f0 0x0ff8a7f0 0x0000d 0x0000d R   0x4
      [Requesting program interpreter: /lib/ld.so.1]
  LOAD           0x000000 0x0fe40000 0x0fe40000 0x158608 0x158608 R E 0x10000
  LOAD           0x15930c 0x0ffa930c 0x0ffa930c 0x0460c 0x071f0 RW  0x10000
  DYNAMIC        0x15ac60 0x0ffaac60 0x0ffaac60 0x000f8 0x000f8 RW  0x4
  NOTE           0x000174 0x0fe40174 0x0fe40174 0x00020 0x00020 R   0x4
  TLS            0x15930c 0x0ffa930c 0x0ffa930c 0x00008 0x00044 R   0x4
  GNU_EH_FRAME   0x14a800 0x0ff8a800 0x0ff8a800 0x01cdc 0x01cdc R   0x4
  GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x4
  GNU_RELRO      0x15930c 0x0ffa930c 0x0ffa930c 0x03cf4 0x03cf4 R   0x1


Load Segments:

  Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
  LOAD           0x000000 0x0fe40000 0x0fe40000 0x158608 0x158608 R E 0x10000
  LOAD           0x15930c 0x0ffa930c 0x0ffa930c 0x0460c 0x071f0 RW  0x10000

Memsize : 0x158608 - 160000 (Alignment)
        : 0x071f0  - 010000  (Alignment)
             -------
             170000 = ~1472 KB = ~1476 (Assuming Page size of Linux 4 KB)

Size (1476) is matching with the memory allocation (map pointed above).
====

LIBNSL:
======

/proc/<pid>/maps Output:
-------------------------
0f880000-0f895000 r-xp 00000000 00:0f 2392922    /lib/libnsl-2.11.1.so
0f895000-0f8a5000 ---p 00015000 00:0f 2392922    /lib/libnsl-2.11.1.so
0f8a5000-0f8a6000 r--p 00015000 00:0f 2392922    /lib/libnsl-2.11.1.so
0f8a6000-0f8a7000 rw-p 00016000 00:0f 2392922    /lib/libnsl-2.11.1.so
0f8a7000-0f8a9000 rw-p 00000000 00:00 0

Map Segments Difference:
------------------------
0f8a9000 - 0f880000 = 164 KB

readelf Output:
---------------

Program Headers:
  Type           Offset   VirtAddr   PhysAddr   FileSiz  MemSiz   Flg Align
  PHDR           0x000034 0x0f880034 0x0f880034 0x000100 0x000100 R E 0x4
  INTERP         0x0143c4 0x0f8943c4 0x0f8943c4 0x00000d 0x00000d R   0x4
        [Requesting program interpreter: /lib/ld.so.1]
  LOAD           0x000000 0x0f880000 0x0f880000 0x014ed0 0x014ed0 R E 0x10000
  LOAD           0x015abc 0x0f8a5abc 0x0f8a5abc 0x0006f4 0x002c00 RW  0x10000
  DYNAMIC        0x015af4 0x0f8a5af4 0x0f8a5af4 0x0000e8 0x0000e8 RW  0x4
  NOTE           0x000134 0x0f880134 0x0f880134 0x000020 0x000020 R   0x4
  GNU_STACK      0x000000 0x00000000 0x00000000 0x000000 0x000000 RW  0x4
  GNU_RELRO      0x015abc 0x0f8a5abc 0x0f8a5abc 0x000544 0x000544 R   0x1

Load Segments:
---------------

  LOAD           0x000000 0x0f880000 0x0f880000 0x014ed0 0x014ed0 R E 0x10000
  LOAD           0x015abc 0x0f8a5abc 0x0f8a5abc 0x0006f4 0x002c00 RW  0x10000

Memsize : 0x014ed0 - 020000 (Alignment)
        : 0x002c00 - 010000 (Alignment)
             -------
             030000 = ~192 KB

Here the load segments mem size is not matching with the above memory allocation. Please correct me if the above calculation is wrong.

I'm doing this to make sure maximum memory segment is allocated to shared libraries so that reposition can be avoided as I'm planning for relocation.


> The location is not known, the kernel decides that.

Can you please provide me some pointers or docs related to this ? More interested to understand how Kernel decides on ELF placement from code perspective.

Thanks..