Chaosforge Forum

DoomRL => Bug Reports => Topic started by: Omegthemad on May 12, 2011, 01:40

Title: [0.9.9.3] Display goes crazy on wearing Bulk Gothic Armor
Post by: Omegthemad on May 12, 2011, 01:40

Stuck a bulk mod on an Ao100 run, thinking "surely they'll have blocked 300% armour, it sounds broken". Instead, it worked properly, the max armour durability went to 300%. Simultaneously, the text showing what armour I was wearing, and the background to that text only, started randomly changing colours.

The bug seems to stop when the armour is worn down a certain amount (221% at present, I didn't record the value at which it stopped). Not convinced screenshots would add much, it's reasonably replicable (or should be...)

Title: Re: [0.9.9.3] Display goes crazy on wearing Bulk Gothic Armor
Post by: thelaptop on May 12, 2011, 02:25

My educated guess is that this is related to a buffer overflow bug.

Kornel, you seriously need a default case in your switch statements to catch anything that falls off the cracks...

(Insert appropriate language structure that is equivalent to the C/C++ switch statement for FreePascal.)

Title: Re: [0.9.9.3] Display goes crazy on wearing Bulk Gothic Armor
Post by: ParaSait on May 12, 2011, 04:36

Kinda like pacman level 256. :P

Title: Re: [0.9.9.3] Display goes crazy on wearing Bulk Gothic Armor
Post by: Deathwind on May 13, 2011, 19:06

Time for a binary lesson,

Spoiler (click to show/hide)

Each place value in binary is double the one on it's right
0000 0001 = 1
0000 0010 = 2
0000 0100 = 4
0000 1000 = 8
0001 0000 = 16
0010 0000 = 32
0100 0000 = 64
1000 0000 = 128

counting to 16 in binary
0000 0001=1
0000 0010=2
0000 0011=3
0000 0100=4
0000 0101=5
0000 0110=6
0000 0111=7
0000 1000=8
0000 1001=9
0000 1010=10
0000 1011=11
0000 1100=12
0000 1101=13
0000 1110=14
0000 1111=15
0001 0000=16

any number can be represented by combining bits
5 = 4+1 = 0000 0101
7 = 4+2+1 = 0000 0111
43 = 32+8+2+1 = 0010 1011
255 = 128+64+32+16+8+4+2+1 = 1111 1111
300 = 256+32+8+4 = 0000 0001 0010 1100

Armor durability (and ammo for weapons) is stored in a one byte space of memory. A byte is 8 bits, as such one byte can only hold a max value of 255. When a larger value is stored in such a space the leading portion of the data will overwrite another section of memory. This behavior is called a buffer overflow and will generally cause a program to crash, but if it overwrites something noncritical (ie color data) the program will continue running with the altered data. Intentionally causing this to alter a running program is one of the most basic forms of hacking.

In your case the armor durability data was overwriting the code for the color of the text. The easy way to correct this bug store the durability for the gothic equipment in a 2 byte buffer but I'm not sure if that would work with the way the game is coded, otherwise you could make it cap at 255 or block bulk mods.