An important UNIX® concept is the environment, which is defined by environment variables. Some are set by the system, others by you, yet others by the shell, or any program that loads another program.
I said earlier that when a program starts executing, the stack contains argc followed by the NULL-terminated argv array, followed by something else. The "something else" is
the environment, or, to be more
precise, a NULL-terminated array of pointers to environment variables. This is often referred to as env.
The structure of env is the same as that of argv, a list of memory addresses followed by a NULL (0). In this case, there is no "envc"—we figure out where the array ends by searching for
the final NULL.
The variables usually come in the name=value format,
but sometimes the =value part may be missing. We need to
account for that possibility.
I could just show you some code that prints the environment the same way the UNIX env command does. But I thought it would be more interesting to write a simple assembly language CGI utility.
I have a detailed CGI tutorial on my web site, but here is a very quick overview of CGI:
The web server communicates with the CGI program by setting environment variables.
The CGI program sends its output to stdout. The web server reads it from there.
It must start with an HTTP header followed by two blank lines.
It then prints the HTML code, or whatever other type of data it is producing.
Note: While certain environment variables use standard names, others vary, depending on the web server. That makes webvars quite a useful diagnostic tool.
Our webvars program, then, must send out the HTTP header followed by some HTML mark-up. It then must read the environment variables one by one and send them out as part of the HTML page.
The code follows. I placed comments and explanations right inside the code:
;;;;;;; webvars.asm ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; ; Copyright (c) 2000 G. Adam Stanislav ; All rights reserved. ; ; Redistribution and use in source and binary forms, with or without ; modification, are permitted provided that the following conditions ; are met: ; 1. Redistributions of source code must retain the above copyright ; notice, this list of conditions and the following disclaimer. ; 2. Redistributions in binary form must reproduce the above copyright ; notice, this list of conditions and the following disclaimer in the ; documentation and/or other materials provided with the distribution. ; ; THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND ; ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE ; IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ; ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE ; FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL ; DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS ; OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) ; HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT ; LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY ; OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF ; SUCH DAMAGE. ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; ; ; Version 1.0 ; ; Started: 8-Dec-2000 ; Updated: 8-Dec-2000 ; ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;; %include 'system.inc' section .data http db 'Content-type: text/html', 0Ah, 0Ah db '<?xml version="1.0" encoding="utf-8"?>', 0Ah db '<!DOCTYPE html PUBLIC "-//W3C/DTD XHTML Strict//EN" ' db '"DTD/xhtml1-strict.dtd">', 0Ah db '<html xmlns="http://www.w3.org/1999/xhtml" ' db 'xml.lang="en" lang="en">', 0Ah db '<head>', 0Ah db '<title>Web Environment</title>', 0Ah db '<meta name="author" content="G. Adam Stanislav" />', 0Ah db '</head>', 0Ah, 0Ah db '<body bgcolor="#ffffff" text="#000000" link="#0000ff" ' db 'vlink="#840084" alink="#0000ff">', 0Ah db '<div class="webvars">', 0Ah db '<h1>Web Environment</h1>', 0Ah db '<p>The following <b>environment variables</b> are defined ' db 'on this web server:</p>', 0Ah, 0Ah db '<table align="center" width="80" border="0" cellpadding="10" ' db 'cellspacing="0" class="webvars">', 0Ah httplen equ $-http left db '<tr>', 0Ah db '<td class="name"><tt>' leftlen equ $-left middle db '</tt></td>', 0Ah db '<td class="value"><tt><b>' midlen equ $-middle undef db '<i>(undefined)</i>' undeflen equ $-undef right db '</b></tt></td>', 0Ah db '</tr>', 0Ah rightlen equ $-right wrap db '</table>', 0Ah db '</div>', 0Ah db '</body>', 0Ah db '</html>', 0Ah, 0Ah wraplen equ $-wrap section .text global _start _start: ; First, send out all the http and xhtml stuff that is ; needed before we start showing the environment push dword httplen push dword http push dword stdout sys.write ; Now find how far on the stack the environment pointers ; are. We have 12 bytes we have pushed before "argc" mov eax, [esp+12] ; We need to remove the following from the stack: ; ; The 12 bytes we pushed for sys.write ; The 4 bytes of argc ; The EAX*4 bytes of argv ; The 4 bytes of the NULL after argv ; ; Total: ; 20 + eax * 4 ; ; Because stack grows down, we need to ADD that many bytes ; to ESP. lea esp, [esp+20+eax*4] cld ; This should already be the case, but let's be sure. ; Loop through the environment, printing it out .loop: pop edi or edi, edi ; Done yet? je near .wrap ; Print the left part of HTML push dword leftlen push dword left push dword stdout sys.write ; It may be tempting to search for the '=' in the env string next. ; But it is possible there is no '=', so we search for the ; terminating NUL first. mov esi, edi ; Save start of string sub ecx, ecx not ecx ; ECX = FFFFFFFF sub eax, eax repne scasb not ecx ; ECX = string length + 1 mov ebx, ecx ; Save it in EBX ; Now is the time to find '=' mov edi, esi ; Start of string mov al, '=' repne scasb not ecx add ecx, ebx ; Length of name push ecx push esi push dword stdout sys.write ; Print the middle part of HTML table code push dword midlen push dword middle push dword stdout sys.write ; Find the length of the value not ecx lea ebx, [ebx+ecx-1] ; Print "undefined" if 0 or ebx, ebx jne .value mov ebx, undeflen mov edi, undef .value: push ebx push edi push dword stdout sys.write ; Print the right part of the table row push dword rightlen push dword right push dword stdout sys.write ; Get rid of the 60 bytes we have pushed add esp, byte 60 ; Get the next variable jmp .loop .wrap: ; Print the rest of HTML push dword wraplen push dword wrap push dword stdout sys.write ; Return success push dword 0 sys.exit
This code produces a 1,396-byte executable. Most of it is data, i.e., the HTML mark-up we need to send out.
Assemble and link it as usual:
% nasm -f elf webvars.asm % ld -s -o webvars webvars.o
To use it, you need to upload webvars to your web server. Depending on how your web server is set up, you may have to store it in a special cgi-bin directory, or perhaps rename it with a .cgi extension.
Then you need to use your browser to view its output. To see its output on my web server, please go to http://www.int80h.org/webvars/. If curious about the additional environment variables present in a password protected web directory, go to http://www.int80h.org/private/, using the name asm and password programmer.