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Command dereference

The dereference command (also aliased telescope for PEDA former users) aims to simplify the dereferencing of an address in GDB to determine the content it actually points to.

It is a useful convienence function to spare to process of manually tracking values with successive x/x in GDB.

dereference takes three optional arguments, a start address (or symbol or register, etc) to dereference (by default, $sp), the number of consecutive addresses to dereference (by default, 10) and the base location for offset calculation (by default the same as the start address):

gef➤  dereference
0x00007fffffffdec0│+0x0000: 0x00007ffff7ffe190  →  0x0000555555554000  →   jg 0x555555554047   ← $rsp, $r13
0x00007fffffffdec8│+0x0008: 0x00007ffff7ffe730  →  0x00007ffff7fd3000  →  0x00010102464c457f
0x00007fffffffded0│+0x0010: 0x00007ffff7faa000  →  0x00007ffff7de9000  →  0x03010102464c457f
0x00007fffffffded8│+0x0018: 0x00007ffff7ffd9f0  →  0x00007ffff7fd5000  →  0x00010102464c457f
0x00007fffffffdee0│+0x0020: 0x00007fffffffdee0  →  [loop detected]
0x00007fffffffdee8│+0x0028: 0x00007fffffffdee0  →  0x00007fffffffdee0  →  [loop detected]
0x00007fffffffdef0│+0x0030: 0x00000000f7fa57e3
0x00007fffffffdef8│+0x0038: 0x0000555555755d60  →  0x0000555555554a40  →   cmp BYTE PTR [rip+0x201601], 0x0        # 0x555555756048
0x00007fffffffdf00│+0x0040: 0x0000000000000004
0x00007fffffffdf08│+0x0048: 0x0000000000000001

Here is an example with arguments:

gef➤  telescope $rbp+0x10 -l 8
0x00007fffffffdf40│+0x0000: 0x00007ffff7fa5760  →  0x00000000fbad2887
0x00007fffffffdf48│+0x0008: 0x00000001f7e65b63
0x00007fffffffdf50│+0x0010: 0x0000000000000004
0x00007fffffffdf58│+0x0018: 0x0000000000000000
0x00007fffffffdf60│+0x0020: 0x00007fffffffdfa0  →  0x0000555555554fd0  →   push r15
0x00007fffffffdf68│+0x0028: 0x0000555555554980  →   xor ebp, ebp
0x00007fffffffdf70│+0x0030: 0x00007fffffffe080  →  0x0000000000000001
0x00007fffffffdf78│+0x0038: 0x0000000000000000

It also optionally accepts a second argument, the number of consecutive addresses to dereference (by default, 10).

For example, if you want to dereference all the stack entries inside a function context (on a 64bit architecture):

gef➤  p ($rbp - $rsp)/8
$3 = 4
gef➤  dereference -l 5
0x00007fffffffe170│+0x0000: 0x0000000000400690  →  push r15        ← $rsp
0x00007fffffffe178│+0x0008: 0x0000000000400460  →  xor ebp, ebp
0x00007fffffffe180│+0x0010: 0x00007fffffffe270  →  0x1
0x00007fffffffe188│+0x0018: 0x1
0x00007fffffffe190│+0x0020: 0x0000000000400690  →  push r15        ← $rbp

It is possible to change the offset calculation to use a different address than the start address:

gef➤  dereference $sp -l 7 -r $rbp
0x00007ffe6ddaa3e0│-0x0030: 0x0000000000000000    ← $rsp
0x00007ffe6ddaa3e8│-0x0028: 0x0000000000400970  →  <__libc_csu_init+0> push r15
0x00007ffe6ddaa3f0│-0x0020: 0x0000000000000000
0x00007ffe6ddaa3f8│-0x0018: 0x00000000004006e0  →  <_start+0> xor ebp, ebp
0x00007ffe6ddaa400│-0x0010: 0x00007ffe6ddaa500  →  0x0000000000000001
0x00007ffe6ddaa408│-0x0008: 0xa42456b3ee465800
0x00007ffe6ddaa410│+0x0000: 0x0000000000000000    ← $rbp

Just like with x, you can pass a negative number of addresses to dereference, to examine memory backwards from the start address:

gef➤  dereference $sp -l 3
0x00007fffffffcf90│+0x0010: 0x00007ffff7f5aaa0  →  0x0000000000000000
0x00007fffffffcf88│+0x0008: 0x00000000000204a0
0x00007fffffffcf80│+0x0000: 0x00005555555a6b60  →  0x0000000000000000    ← $rsp
gef➤  dereference $sp -l -3
0x00007fffffffcf80│+0x0000: 0x00005555555a6b60  →  0x0000000000000000    ← $rsp
0x00007fffffffcf78│-0x0008: 0x0000000000000020 (" "?)
0x00007fffffffcf70│-0x0010: 0x000000000000000a ("\n"?)