How To Access Memory Location Pointed By Register + Gdb

This is going to be a modest demonstration or 'tip' to analyze registers and retention via gdb when debugging a program. These commands are pretty much useful when debugging a program. It has its own use cases.

Examine registers:

$info registers is the control which tin can be used to see current register values at the moment from gdb prompt. Below control can be used equally a short cutting to view registers:

(gdb) i r rax 0x1 i 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 one 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

                                  
                  (gdb) i r $sp
                  sp: 0x7fff955a9dc0
                  (gdb) i r $rip
                  rip            0x37e78da373    0x37e78da373
                  (gdb)

Examining memory :

This is pretty much useful when debugging a plan:

"ten" is the command which tin can be used for the aforementioned purpose.. The full general format of 'x' command as shown hither.

                                  
                  (gdb) assist x                  Examine retentivity: ten/FMT ADDRESS.                  
                  Address is an expression for the memory address to examine.
                    FMT is a repeat count followed by a format letter of the alphabet and a size letter.
                    Format messages 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), due west(word), thousand(giant, 8 bytes).
                    The specified number of objects of the specified size are printed
                    co-ordinate to the format.
                  Defaults for format and size letters are those previously used.
                    The default count is one.  The default address is the following the final thing printed
                    with this control or "print".
                    (gdb)
                  In curt :
                  Formats:
                    o – octal
                    d – decimal
                    ten – hexadecimal
                    u – unsigned integer
                    s – string
                    t – binary
                  Units:
                    b – byte
                    h – half
                    due west – word
                    g – double discussion
                  Case use of 'x' command:
                  "3" words of memory 'above' stack pointer tin exist displayed by:                  
                                      But why I used "above" here? 'Ans': It is homework/assignment for y'all 🙂
                  (gdb) x/3xw $sp
                    0x7fff955a9dc0:    0x00000000    0x00000000    0x0041ecb1
                    (gdb)
                  "two" machine instructions from 0x37e78da373/eip
                  (gdb) x/2i 0x37e78da373
                    => 0x37e78da373 :    mov    (%rsp),%rdi
                    0x37e78da377 :    mov    %rax,%rdx
                    (gdb)
                  To display a string you tin can utilise: ' I selected a random address', so it may not give a human being-readable example string equally output.
                  (gdb) x/south 0x0041ecb1
                    0x41ecb1:     "A\211\307藟\001"
                    (gdb)