This is going to be a small demonstration or ‘tip’ to analyze registers and memory via gdb when debugging a program. These commands are pretty much useful when debugging a program. It has its own use cases.
Examine registers:
[terminal]
$info registers is the command which can be used to see current register values at the moment from gdb prompt. Below command can be used as a short cut to view registers:
(gdb) i r
rax 0x1 1
rbx 0x7fff955a9df0 140735699131888
rcx 0xffffffffffffffff -1
rdx 0x7fff955a9e70 140735699132016
rsi 0x7fff955a9df0 140735699131888
rdi 0x16 22
rbp 0x7fff955a9e70 0x7fff955a9e70
rsp 0x7fff955a9dc0 0x7fff955a9dc0
r8 0x7fff955a9dd0 140735699131856
r9 0x1 1
r10 0x7fff955a9ef0 140735699132144
r11 0x293 659
r12 0x7fff955a9ef0 140735699132144
r13 0x0 0
r14 0x1 1
r15 0x0 0
rip 0x37e78da373 0x37e78da373
eflags 0x293 [ CF AF SF IF ]
cs 0x33 51
ss 0x2b 43
ds 0x0 0
es 0x0 0
fs 0x0 0
gs 0x0 0
[/terminal]
Register information can be fetched individually . For ex: “Stack pointer” and “Instruction pointer” information can be fetched by:
[terminal]
(gdb) i r $sp
sp: 0x7fff955a9dc0
(gdb) i r $rip
rip 0x37e78da373 0x37e78da373
(gdb)
[/terminal]
Examining memory :
This is pretty much useful when debugging a program:
“x” is the command which can be used for the same purpose.. The general format of ‘x’ command as shown here.
[terminal]
(gdb) help x
Examine memory: x/FMT ADDRESS.
ADDRESS is an expression for the memory address to examine.
FMT is a repeat count followed by a format letter and a size letter.
Format letters are o(octal), x(hex), d(decimal), u(unsigned decimal),
t(binary), f(float), a(address), i(instruction), c(char) and s(string).
Size letters are b(byte), h(halfword), w(word), g(giant, 8 bytes).
The specified number of objects of the specified size are printed
according to the format.
Defaults for format and size letters are those previously used.
The default count is 1. The default address is the following the last thing printed
with this command or “print”.
(gdb)
In short :
Formats:
o – octal
d – decimal
x – hexadecimal
u – unsigned integer
s – string
t – binary
Units:
b – byte
h – half
w – word
g – double word
Example use of ‘x’ command:
“3” words of memory ‘above’ stack pointer can be displayed by:
But why I used “above” here? ‘Ans’: It is homework/assignment for you 🙂
(gdb) x/3xw $sp
0x7fff955a9dc0: 0x00000000 0x00000000 0x0041ecb1
(gdb)
“2” machine instructions from 0x37e78da373/eip
(gdb) x/2i 0x37e78da373
=> 0x37e78da373 : mov (%rsp),%rdi
0x37e78da377 : mov %rax,%rdx
(gdb)
To display a string you can use: ‘ I selected a random address’, so it may not give a human-readable example string as output.
(gdb) x/s 0x0041ecb1
0x41ecb1: “A\211\307藟\001”
(gdb)
[/terminal]
I hope this helps.
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