klibc - Sep 2005 - klibc loader crash...

Hi all,

I've a simple program called empty.c :)

$ cat empty.c

int main(int argc, char* argv[])
{
    return 0;
}

$ cat empty410.s

        .file   "empty.c"
        .text
        .p2align 4,,15
.globl main
        .type   main, @function
main:
        xorl    %eax, %eax
        ret
        .size   main, .-main
        .ident  "GCC: (GNU) 4.1.0 20050922 (experimental)"
        .section        .note.GNU-stack,"",@progbits

binutils-2.15.94.0.2.2 produces for this code empty .data and .bss sections:

Program Header:
    PHDR off    0x00000034 vaddr 0x08048034 paddr 0x08048034 align 2**2
         filesz 0x000000c0 memsz 0x000000c0 flags r-x
  INTERP off    0x000000f4 vaddr 0x080480f4 paddr 0x080480f4 align 2**0
         filesz 0x0000002a memsz 0x0000002a flags r--
    LOAD off    0x00000000 vaddr 0x08048000 paddr 0x08048000 align 2**12
         filesz 0x00000123 memsz 0x00000123 flags r-x
    LOAD off    0x00000124 vaddr 0x08049124 paddr 0x08049124 align 2**12
         filesz 0x00000000 memsz 0x00000000 flags rw-
   STACK off    0x00000000 vaddr 0x00000000 paddr 0x00000000 align 2**2
         filesz 0x00000000 memsz 0x00000000 flags rwx
PAX_FLAGS off    0x00000000 vaddr 0x00000000 paddr 0x00000000 align 2**2
         filesz 0x00000000 memsz 0x00000000 flags --- 2800

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .interp       0000002a  080480f4  080480f4  000000f4  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  1 .text         00000003  08048120  08048120  00000120  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, CODE
  2 .data         00000000  08049124  08049124  00000124  2**2
                  CONTENTS, ALLOC, LOAD, DATA
  3 .bss          00000000  08049124  08049124  00000124  2**2
                  ALLOC

and /lib/klibc-*.so works fine.

binutils-2.16.91.0.3 doesn't produce useless empty sections:

Program Header:
    PHDR off    0x00000034 vaddr 0x08048034 paddr 0x08048034 align 2**2
         filesz 0x000000a0 memsz 0x000000a0 flags r-x
  INTERP off    0x000000f4 vaddr 0x080480f4 paddr 0x080480f4 align 2**0
         filesz 0x0000002a memsz 0x0000002a flags r--
    LOAD off    0x00000000 vaddr 0x08048000 paddr 0x08048000 align 2**12
         filesz 0x00000123 memsz 0x00000123 flags r-x
   STACK off    0x00000000 vaddr 0x00000000 paddr 0x00000000 align 2**2
         filesz 0x00000000 memsz 0x00000000 flags rwx
PAX_FLAGS off    0x00000000 vaddr 0x00000000 paddr 0x00000000 align 2**2
         filesz 0x00000000 memsz 0x00000000 flags --- 2800

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .interp       0000002a  080480f4  080480f4  000000f4  2**0
                  CONTENTS, ALLOC, LOAD, READONLY, DATA
  1 .text         00000003  08048120  08048120  00000120  2**4
                  CONTENTS, ALLOC, LOAD, READONLY, CODE

and klibc loader crashes (sigsegv) before it reaches main().

tested on klibc 1.0.14 / 1.1.1.

-- 
The only thing necessary for the triumph of evil
  is for good men to do nothing.
                                           - Edmund Burke

Hi,

This is not a klibc bug. It is a kernel issue.

Fixed in 2.6.14rc4-git1: [PATCH] binfmt_elf bss padding fix
 
 Nir Tzachar <tzachar@cs.bgu.ac.il> points out that if an ELF file
specifies a
 zero-length bss at a whacky address, we cannot load that binary because
 padzero() tries to zero out the end of the page at the whacky address, and
 that may not be writeable.
 
 See also http://bugzilla.kernel.org/show_bug.cgi?id=5411
 
 So teach load_elf_binary() to skip the bss settng altogether if the elf file
 has a zero-length bss segment.

-- 
The only thing necessary for the triumph of evil
  is for good men to do nothing.
                                           - Edmund Burke

Pawe? Sikora wrote:
> 
> This is true only for statically linked klibc.
> With shared version we get:
> 
> Sections:
> Idx Name          Size      VMA       LMA       File off  Algn
>   0 .interp       0000000e  080480f4  080480f4  000000f4  2**0
>                   CONTENTS, ALLOC, LOAD, READONLY, DATA
>   1 .text         00000003  08048110  08048110  00000110  2**4
>                   CONTENTS, ALLOC, LOAD, READONLY, CODE
> 
Right, of course, because those are all in shared bss.  Yup, need to fix 
the kernel.

	-hpa