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MODULE 000-3

GCC, G++, GDB AND FRIENDS 3

 

 

 

 

 

 

My Training Period: xx hours   

 

 

 

 

 

The as - A GNU Assembler

 

as

as is a portable GNU assembler (Gas).  GNU as is really a family of assemblers.  If you use (or have used) the GNU assembler on one architecture, you should find a fairly similar environment when you use it on another architecture.  Each version has much in common with the others, including object file formats, most assembler directives (often called pseudo-ops) and assembler syntax.

as is primarily intended to assemble the output of the GNU C compiler "gcc" for use by the linker "ld".  Nevertheless, we’ve tried to make as assemble correctly everything that other assemblers for the same machine would assemble.  Any exceptions are documented explicitly.  This doesn’t mean as always uses the same syntax as another assembler for the same architecture; for example, we know of several incompatible versions of 680x0 assembly language syntax.

Each time you run as, it assembles exactly one source program.   The source program is made up of one or more files.  (The standard input is also a file.)

You give as a command line that has zero or more input file names.  The input files are read (from left file name to right).  A command line argument (in any position) that has no special meaning is taken to be an input file name.

If you give as no file names it attempts to read one input file from the as standard input, which is normally your terminal.  You may have to type ctl-D to tell as there are no more programs to assemble.

Use -- if you need to explicitly name the standard input file in your command line.

If the source is empty, as produces a small, empty object file.  as may write warnings and error messages to the standard error file (usually your terminal).  This should not happen when a compiler runs as automatically.   Warnings report an assumption made so that as could keep assembling a flawed program; errors report a grave problem that stops the assembly.

If you are invoking as via the GNU C compiler, you can use the -Wa option to pass arguments through to the assembler.  The assembler arguments must be separated from each other (and the -Wa) by commas.  For example:

 

               gcc -c -g -O -Wa,-alh,-L file.c

 

This passes two options to the assembler: -alh (emit a listing to standard output with high-level and assembly source) and -L (retain local symbols in the symbol table). Usually you do not need to use this -Wa mechanism, since many compilers command-line options are automatically passed to the assembler by the compiler.  (You can call the GNU compiler driver with the -v option to see precisely what options it passes to each compilation pass, including the assembler.)

Usage: as [option...] [asmfile...]

Options:

-a[sub-option...]       turn on listings

Sub-options [default hls]:

c

omit false conditionals

d

omit debugging directives

h

include high-level source

l

include assembly

m

include macro expansions

n

omit forms processing

s

include symbols

=FILE

list to FILE (must be last sub-option)

--alternate

initially turn on alternate macro syntax

-D

produce assembler debugging messages

--defsym SYM=VAL

define symbol SYM to given value

--execstack

require executable stack for this object

--noexecstack

don't require executable stack for this object

-f

skip whitespace and comment preprocessing

-g --gen-debug

generate debugging information

--gstabs

generate STABS debugging information

--gstabs+

generate STABS debug info with GNU extensions

--gdwarf-2

generate DWARF2 debugging information

--help

show this message and exit

--target-help

show target specific options

-I DIR

add DIR to search list for .include directives

-J

don't warn about signed overflow

-K

warn when differences altered for long displacements

-L,--keep-locals

keep local symbols (e.g. starting with `L')

-M,--mri

assemble in MRI compatibility mode

--MD FILE

write dependency information in FILE (default none)

-nocpp

ignored

-o OBJFILE

name the object-file output OBJFILE (default a.out)

-R

fold data section into text section

--statistics

print various measured statistics from execution

--strip-local-absolute

strip local absolute symbols

  --traditional-format

Use same format as native assembler when possible

--version

print assembler version number and exit

-W  --no-warn

suppress warnings

--warn

don't suppress warnings

--fatal-warnings

treat warnings as errors

--itbl INSTTBL

extend instruction set to include instructions matching the specifications defined in file INSTTBL

-w

ignored

-X

ignored

-Z

generate object file even after errors

--listing-lhs-width

set the width in words of the output data column of the listing

--listing-lhs-width2

set the width in words of the continuation lines of the output data column; ignored if smaller than the width of the first line

--listing-rhs-width

set the max width in characters of the lines from the source file

--listing-cont-lines

set the maximum number of continuation lines used for the output data column of the listing

-Q

ignored

-V

print assembler version number

-k

ignored

-n

do not optimize code alignment

-q

quieting some warnings

-s

ignored

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The ld - A GNU Linker

 

ld

ld is a GNU linker.  ld combines a number of objects and archive files, relocates their data and ties up symbol references. Usually the last step in compiling a program is to run ld ld accepts Linker Command Language files written in a superset of AT&T’s Link Editor Command Language syntax, to provide explicit and total control over the linking process.

This version of ld uses the general purpose BFD libraries to operate on object files.  This allows ld to read, combine, and write object files in many different format for example, COFF or "a.out".  Different formats may be linked together to produce any available kind of object file.

Aside from its flexibility, the GNU linker is more helpful than other linkers in providing diagnostic information.  Many linkers abandon execution immediately upon encountering an error; whenever possible, ld continues executing, allowing you to identify other errors (or, in some cases, to get an output file in spite of the error).

The GNU linker ld is meant to cover a broad range of situations, and to be as compatible as possible with other linkers.   As a result, you have many choices to control its behavior.

Usage: ld [options] file...

Options:

-a KEYWORD

Shared library control for HP/UX compatibility

-A ARCH, --architecture ARCH

Set architecture

-b TARGET, --format TARGET

Specify target for following input files

-c FILE, --mri-script FILE

Read MRI format linker script

-d, -dc, -dp

Force common symbols to be defined

-e ADDRESS, --entry ADDRESS

  Set start address

-E, --export-dynamic

Export all dynamic symbols

-EB

Link big-endian objects

-EL

Link little-endian objects

-f SHLIB, --auxiliary SHLIB

Auxiliary filter for shared object symbol table

-F SHLIB, --filter SHLIB

Filter for shared object symbol table

-g

Ignored

-G SIZE, --gpsize SIZE

Small data size (if no size, same as --shared)

-h FILENAME, -soname FILENAME

Set internal name of shared library

-I PROGRAM, --dynamic-linker PROGRAM

Set PROGRAM as the dynamic linker to use

-l LIBNAME, --library LIBNAME

Search for library LIBNAME

-L DIRECTORY, --library-path DIRECTORY

Add DIRECTORY to library search path

-m EMULATION

Set emulation

-M, --print-map

Print map file on standard output

-n, --nmagic

Do not page align data

-N, --omagic

Do not page align data, do not make text readonly

--no-omagic

Page align data, make text readonly

-o FILE, --output FILE

Set output file name

-O

Optimize output file

-Qy

Ignored for SVR4 compatibility

-q, --emit-relocs

Generate relocations in final output

-r, -i, --relocatable

Generate relocatable output

-R FILE, --just-symbols FILE

Just link symbols (if directory, same as --rpath)

-s, --strip-all

Strip all symbols

-S, --strip-debug

Strip debugging symbols

--strip-discarded

Strip symbols in discarded sections

--no-strip-discarded

Do not strip symbols in discarded sections

-t, --trace

Trace file opens

-T FILE, --script FILE

Read linker script

-u SYMBOL, --undefined SYMBOL

Start with undefined reference to SYMBOL

  --unique [=SECTION]

Don't merge input [SECTION | orphan] sections

-Ur

Build global constructor/destructor tables

-v, --version

Print version information

-V

Print version and emulation information

-x, --discard-all

Discard all local symbols

-X, --discard-locals

Discard temporary local symbols (default)

--discard-none

Don't discard any local symbols

-y SYMBOL, --trace-symbol SYMBOL

Trace mentions of SYMBOL

-Y PATH

Default search path for Solaris compatibility

-(, --start-group

Start a group

-), --end-group

End a group

--accept-unknown-input-arch

Accept input files whose architecture cannot be determined

--no-accept-unknown-input-arch

Reject input files whose architecture is unknown

-assert KEYWORD

Ignored for SunOS compatibility

-Bdynamic, -dy, -call_shared

Link against shared libraries

-Bstatic, -dn, -non_shared, -static

Do not link against shared libraries

-Bsymbolic

Bind global references locally

--check-sections

Check section addresses for overlaps (default)

--no-check-sections

Do not check section addresses for overlaps

--cref

Output cross reference table

--defsym SYMBOL=EXPRESSION

Define a symbol

--demangle [=STYLE]

Demangle symbol names [using STYLE]

--demangler DSO:FUNCTION

Set DSO and demangler function

--embedded-relocs

Generate embedded relocs

-fini SYMBOL

Call SYMBOL at unload-time

--force-exe-suffix

Force generation of file with .exe suffix

--gc-sections

Remove unused sections (on some targets)

--no-gc-sections

Don't remove unused sections (default)

--hash-size=<NUMBER>

Set default hash table size close to <NUMBER>

--help

Print option help

-init SYMBOL

Call SYMBOL at load-time

-Map FILE

Write a map file

--no-define-common

Do not define Common storage

--no-demangle

Do not demangle symbol names

--no-keep-memory

Use less memory and more disk I/O

--no-undefined

Do not allow unresolved references in object files

--allow-shlib-undefined

Allow unresolved references in shared libraries

--no-allow-shlib-undefined

Do not allow unresolved references in shared libs

--allow-multiple-definition

Allow multiple definitions

--no-undefined-version

Disallow undefined version

--no-warn-mismatch

Don't warn about mismatched input files

--no-whole-archive

Turn off --whole-archive

--noinhibit-exec

Create an output file even if errors occur

-nostdlib

Only use library directories specified on the command line

--oformat TARGET

Specify target of output file

-qmagic

Ignored for Linux compatibility

--reduce-memory-overheads

Reduce memory overheads, possibly taking much longer

--relax

Relax branches on certain targets

--retain-symbols-file FILE

Keep only symbols listed in FILE

-rpath PATH

Set runtime shared library search path

-rpath-link PATH

Set link time shared library search path

-shared, -Bshareable

Create a shared library

-pie, --pic-executable

Create a position independent executable

--sort-common

Sort common symbols by size

--spare-dynamic-tags COUNT

How many tags to reserve in .dynamic section

--split-by-file [=SIZE]

Split output sections every SIZE octets

--split-by-reloc [=COUNT]

Split output sections every COUNT relocs

--stats

Print memory usage statistics

--target-help

Display target specific options

--task-link SYMBOL

Do task level linking

--traditional-format

Use same format as native linker

--section-start SECTION=ADDRESS

Set address of named section

-Tbss ADDRESS

Set address of .bss section

-Tdata ADDRESS

Set address of .data section

-Ttext ADDRESS

Set address of .text section

--unresolved-symbols=<method>

How to handle unresolved symbols.

<method> is:

ignore-all, report-all, ignore-in-object-files, ignore-in-shared-libs

--verbose

Output lots of information during link

--version-script FILE

Read version information script

--version-exports-section SYMBOL

Take export symbols list from .exports, using SYMBOL as the version.

--warn-common

Warn about duplicate common symbols

--warn-constructors

Warn if global constructors/destructors are seen

--warn-multiple-gp

Warn if the multiple GP values are used

--warn-once

Warn only once per undefined symbol

--warn-section-align

Warn if start of section changes due to alignment

--warn-unresolved-symbols

Report unresolved symbols as warnings

--error-unresolved-symbols

Report unresolved symbols as errors

--fatal-warnings

Treat warnings as errors

--whole-archive

Include all objects from following archives

--add-needed

Set DT_NEEDED tags for DT_NEEDED entries in following dynamic libs

--no-add-needed

Do not set DT_NEEDED tags for DT_NEEDED entries in following dynamic libs

--as-needed

Only set DT_NEEDED for following dynamic libs if used

--no-as-needed

Always set DT_NEEDED for following dynamic libs

--wrap SYMBOL

Use wrapper functions for SYMBOL

 

 

ld: supported targets: elf32-i386 a.out-i386-linux efi-app-ia32 elf32-little elf32-big srec symbolsrec tekhex binary ihex trad-core

ld: supported emulations: elf_i386 i386linux

ld: emulation specific options:

elf_i386:

-Bgroup

Selects group name lookup rules for DSO

--disable-new-dtags

Disable new dynamic tags

--enable-new-dtags

Enable new dynamic tags

--eh-frame-hdr

Create .eh_frame_hdr section

-z combreloc

Merge dynamic relocs into one section and sort

-z defs

Report unresolved symbols in object files.

-z execstack

Mark executable as requiring executable stack

-z initfirst

Mark DSO to be initialized first at runtime

-z interpose

Mark object to interpose all DSOs but executable

-z loadfltr

Mark object requiring immediate process

-z muldefs

Allow multiple definitions

-z nocombreloc

Don't merge dynamic relocs into one section

-z nocopyreloc

Don't create copy relocs

-z nodefaultlib

Mark object not to use default search paths

-z nodelete

Mark DSO non-deletable at runtime

-z nodlopen

Mark DSO not available to dlopen

-z nodump

Mark DSO not available to dldump

-z noexecstack

Mark executable as not requiring executable stack

-z norelro

Don't create RELRO program header

-z now

Mark object non-lazy runtime binding

-z origin

Mark object requiring immediate $ORIGIN processing at runtime

-z relro

Create RELRO program header

-z KEYWORD

Ignored for Solaris compatibility

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Compile and link the assembly language using as and ld from GNU assembly (Gas) respectively.  This is AT&T assembly language.  There are others as well such as Intel assembly, High Level Assembly (HLA), Motorola etc.  Their syntaxes are different.  For Intel, available compilers that can be used are MASM (Microsoft) and NASM.

 

[bodo@bakawali testbed9]$ cat testshell.s

#another example creating the shell code

.section .data

.section .text

 

.globl _start

 

_start:

 

        jmp ender

 

        starter:

 

        xor %eax, %eax  #clean up the registers

        xor %ebx, %ebx

        xor %edx, %edx

        xor %ecx, %ecx

 

        mov $4, %al     #syscall write

        mov $1, %bl     #stdout is 1

        pop %ecx        #get the address of the string from the stack

        mov $0xd, %dl   #length of the string

        int $0x80

 

        xor %eax, %eax

        mov $1, %al     #exit the shellcode

        xor %ebx, %ebx

        int $0x80

 

        ender:

        call starter    #put the address of the string on the stack

        .string "TESTING HELLO"

 

[bodo@bakawali testbed9]$ as testshell.s -o testshell.o

[bodo@bakawali testbed9]$ ld testshell.o -o testshell

[bodo@bakawali testbed9]$ ls -F -l

total 44

-rwxrwxr-x  1 bodo bodo  742 Apr 10 18:24 testshell*

-rw-rw-r--  1 bodo bodo  558 Apr 10 18:24 testshell.o

-rw-rw-r--  1 bodo bodo  661 Feb 23 15:20 testshell.s

 

To run, it is same as the C program.

 

[bodo@bakawali testbed9]$ ./testshell

TESTING HELLO

 

 

 

 

 

 

 

 

 

 

 

Further related reading:

 

  1. Check the best selling C / C++, Networking, Linux and Open Source books at Amazon.com.

  2. Linux Socket programming tutorial.

  3. C, Buffer overflow and stack frame (construction and destruction).

  4. C, Compiler, Assembler and Linker.

 

 

 

 

 

 

 

 

|< Linux gnu gdb 2 | Main | Linux gnu nm and objdump 4 >| Site Index | Download |


C & Linux GNU Related Utilities:  Part 1 | Part 2 | Part 3 | Part 4 | Part 5 | Part 6