[fasmarm] Syntax for far address loading

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Chris Leslie

Joined: 19 Oct 2024
Posts: 7
Location: Canberra

Chris Leslie 27 Nov 2025, 20:11

In fasmarm elf64 how can i load a register with a far label address? Adr x0, mylabel won't look far enough and ldr x0,=mylabel is not valid.

27 Nov 2025, 20:11

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20785
Location: In your JS exploiting you and your system

revolution 27 Nov 2025, 20:26

The address needs to be broken down into parts.

Code:

format elf64 executable at 0x123456789a0
processor cpu64_v8
my_label:
        movk    w1,(my_label shr 00) and 0xFFFF
        movk    w1,(my_label shr 16) and 0xFFFF,lsl 16
        movk    x1,(my_label shr 32) and 0xFFFF,lsl 32
        movk    x1,(my_label shr 48) and 0xFFFF,lsl 48

Code:

00000078: 72914301 V8     movk  w1,0x8a18
0000007c: 72a8ace1 V8     movk  w1,0x4567,lsl 16
00000080: f2c02461 V8     movk  x1,0x123,lsl 32
00000084: f2e00001 V8     movk  x1,0x0,lsl 48

27 Nov 2025, 20:26

Tomasz Grysztar

Joined: 16 Jun 2003
Posts: 8483
Location: Kraków, Poland

Tomasz Grysztar 27 Nov 2025, 20:28

Perhaps it's the ADRP that you're looking for? I have the technique demonstrated at the very end of chapter 1.6 of my formats tutorial.

27 Nov 2025, 20:28

Chris Leslie

Joined: 19 Oct 2024
Posts: 7
Location: Canberra

Chris Leslie 27 Nov 2025, 20:47

Yes, I need to load the page address with adrp and then add the offset of my label, but getting a syntax that passes is the problem. I will try to adapt the technique at the end of chapter 1.6.

27 Nov 2025, 20:47

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20785
Location: In your JS exploiting you and your system

revolution 27 Nov 2025, 22:33

For relative labels this can be used:

Code:

format elf64 executable at 0x123456789a0
processor cpu64_v8
my_label:
        adrp    x0,far_label
        add     x0,x0,far_label and 0xfff
rb 1 shl 25
far_label:

Code:

00000078: 90010000 V8     adrp  x0,0x002000000
0000007c: 91288000 V8     add   x0,x0,0xa20

27 Nov 2025, 22:33

Chris Leslie

Joined: 19 Oct 2024
Posts: 7
Location: Canberra

Chris Leslie 28 Nov 2025, 10:20

Thanks revolution. What is the purpose of at 0x123456789a0?
And, what does rb 1 shl 25 do in that position?

28 Nov 2025, 10:20

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20785
Location: In your JS exploiting you and your system

revolution 28 Nov 2025, 14:00

0x123456789a0 is just an example to demonstrate a "large" address that can't be encoded in one instruction.

rb 1 shl 25 is just an example to skip a large amount of space to create a "far" label that can't be reached in one instruction.

28 Nov 2025, 14:00

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