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I need to patch an arm7 program by replacing this fopen function by another function.

.text:00018D68 52 D7 FF EB                       BL              fopen

...

.plt:0000EAB8 ; FILE *fopen(const char *filename, const char *modes)

Do you know how to calculate the 24 bits that I need to write after EB? The documentation is not very clear. I tried to find a Branch offset calculator but not found.

Thx,

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you can use https://www.keystone-engine.org/showcase/ assembler

from keystone import *
target = 0xDEADBEEF
current = 0x18D68
ks = Ks(KS_ARCH_ARM, KS_MODE_ARM)
bytes(ks.asm("bl {}".format(hex(target)), current)[0]).hex()
| improve this answer | |
  • It's working!! Thank you – john Jul 7 at 12:54

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