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InitcallMechanism/Introduction

Date and Version

The original version of this paper was started on the 26th of August 2003; the kernel code quoted refered to version 2.4.22 and the binutils utility came from binutils-1.4. This document was rechecked on the 11th of October, 2006. At that time small modifications were made to the text, all code was re-checked, and all output re-generated from the new code. The kernel being used was 2.6.18, binutils 2.15.94.0.2.2, and gcc 4.0.0.

Notation

All kernel files that are referenced in this paper are specifed by a path name relative to the kernel's root directory. For example the setup.c file for the PowerPC architecture would be given as: arch/ppc/kernel/setup.c. A specific function (e.g. early_init) within a given file is expressed as arch/ppc/kernel/setup.c:early_init() regardless of what parameters it accepts (if any) and what it returns (if anything).

Architecture

As far as I know, this mechanism is not architecture-dependent. I actually found it while tracing through the boot process of the PowerPC architecture, but my code tests were performed on an x86-based machine. The code that makes this work is in the kernel's init/ directory, which (as is my understanding), contains initialization code which is used by all architectures. The ability to use this mechanism depends more on specific support given by the GNU tools and the ELF executable format rather than architecture-specific support.

Tools

Peering into the world of object files is made easier using the tools:

  • objdump

  • nm and

  • readelf

"objdump -t" output format

I often dump the symbols of a file using objdump -t. I can't seem to easily locate any documentation on the output format so I've included some quick notes here. A typical use of this tool would look something like the following:

[trevor]$ objdump -t add.o      

add.o:     file format elf32-i386

SYMBOL TABLE:
00000000 l    df *ABS*     00000000 add.c
00000000 l    d  .text     00000000 
00000000 l    d  .data     00000000 
00000000 l    d  .bss      00000000 
00000000 l    d  .comment  00000000 
00000000 g     F .text     0000000b add

Here is my understanding of the column descriptions. This information comes from browsing through bfd/syms.c:bfd_print_symbol_vandf() (where "vandf" stands for "value and flags").

The flags which are described above are part of a larger set of symbols and attributes which are defined in bfd/bfd.h. The entire set of flags (or attributes) and their meanings are given below. NOTE: objdump -t doesn't try to display the values of all the possible flags, just the ones mentioned above.

Definition

Symbol

Description

0x00000

BSF_NO_FLAGS

placeholder for no defined flags

0x00001

BSF_LOCAL

The symbol has local scope (i.e. a static in C). VALUE(1) is this symbol's offset into the data section.

0x00002

BSF_GLOBAL

The symbol has global scope (i.e. initialized data in C). VALUE(2) is this symbol's offset into the data section.

BSF_GLOBAL

BSF_EXPORT

This symbol has global scope and is exported. Same as BSF_GLOBAL.

0x00008

BSF_DEBUGGING

The symbol is a debugging record. The VALUEs are arbitrary, unless BSF_DEBUGGING_RELOC is set.

0x00010

ELF

BSF_FUNCTION

Function entry point.

0x00020

BSF_KEEP

used by the linker

0x00040

BSF_KEEP_G

used by the linker

0x00080

BSF_WEAK

Weak global symbol. This symbol is overridable (without warning) by a regular global symbol of the same name.

0x00100

ELF

BSF_SECTION_SYM

This symbol points to a section.

0x00200

BSF_OLD_COMMON

This symbol used to be *COM*, but is now allocated.

0x00400

COFF

BSF_NOT_AT_END

This symbol appears where it is declared and not at the end of a section.

0x00800

BSF_CONSTRUCTOR

This symbol indicates the start of the constructor section.

0x01000

BSF_WARNING

The presence of this symbol acts to indicate that there is a warning on the next symbol.

0x02000

BSF_INDIRECT

This symbol is an indirect pointer to the symbol with the same name as the next symbol.

0x04000

ELF

BSF_FILE

This symbol contains a filename.

0x08000

ELF

BSF_DYNAMIC

This symbol is associated with dynamic linking.

0x10000

ELF

BSF_OBJECT

This symbol denotes a data object.

0x20000

BSF_DEBUGGING_RELOC

This is a debugging symbol. VALUE(1) is the offset into the data section. BSF_DEBUGGING should be set too.

0x40000

ELF

BSF_THREAD_LOCAL

This symbol is used for thread local storage.

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last edited 2006-10-11 23:52:47 by TrevorWoerner