How does it actually work? Does it change just the firmware on the main filesystem? Does it touch the bootloader?

I'm trying to figure out what's probability of

  1. iPhone jailbreak bricking the phone
  2. Apple's update bricking the jailbroken iPhone (In the line with those questions, does Apple's update simply overwrites the flash partition with the new one and then applies the backed up data?)

Edit: should probably mention it's a 3GS with 4.1 firmware.

  • for your second part of question I must say yes if you update to next firmware it flush every thing and need to jailbreak again.
    – Am1rr3zA
    Sep 12 '10 at 23:53
  • I'm always puzzled by your second question. You have subverted the OS on your iPhone in some way, and have messed with its internals in a way Apple can't really predict. Now, why would you accept a software update? If I were to muck with the kernel and related software on one of my Linux boxes, which we can all agree I have a perfect right to do, would it be intelligent to just accept the next thing the update manager wanted to give me? Sep 24 '10 at 22:00
  • @David, of course, there is always non-zero risk. The question is how big, which depends on (1) whether jailbreak changes rarely- or frequently-updated part of the system, (2) whether it changes anything that's required to boot and restore the system. For example, if all Apple's firmware updater does is overwrite the main flash partition and jailbreak only touches the main partition, there is virtually no risk. On the other hand if Apple's updater depends on some bootloader specifics that jailbreak subtly changes, then I might be in trouble. Therefore I'm curious how exactly does it work.
    – Alex B
    Sep 27 '10 at 4:10
  • @Alex B: There have been reports of Apple updates being incompatible with jailbreaks, although I don't remember any lately. I haven't heard of problems that couldn't be fixed with a system restore and update, although of course that leaves you with an unjailbroken phone. You'd think somebody around here would know the details, but it isn't me. Sep 28 '10 at 21:23
  • 1
    So does jail breaking an iphone consist of changing file system and directory permissions or is it more complex? If so, does anyone know the low level technical details?
    – user3465
    Feb 8 '11 at 22:58

Jailbreaking utilizes an exploit in the OS (or in the latest cast, the boot ROM), and injects the payload onto the phone, which is usually a loader to install Cydia, or Cydia itself. Cydia is the source for applications not in the App Store, and once this is loaded, other software can be installed which could modify system files. One such case is Winterboard, which allows you to customize aspects of the UI.

With regards to question 1:

It depends on what happens during the jailbreak. If, for example, you're running the exploit to jailbreak the phone and the battery dies (or the software malfunctions), there's a chance you'll brick it. It's hard to pin a percentage on it.

With regards to question 2:

Since Apple is expecting their software to be running on the phone, it may do some things that could brick the phone. Again, it definitely varies on what has been changed, how the jailbreak was performed, etc. Personally if Apple comes out with an iOS update, I'm not going to just allow it to run. If I want it, first thing I'd do is restore the original firmware via iTunes.

  • So if I make sure that I always have full battery plugged in before messing anything, the only way for my phone to be permanently bricked is if the software malfunctions?
    – Pacerier
    Dec 29 '11 at 12:16

Jailbreak means replacing the content of the boot.

So, everything is changed. It's like a new harddrive with a new boot image on it, changing everything in that hard drive since how it start's, to host the actual Operating System that in Jailbreak devices is an altered OS so all methods, functions and properties are available without any restrictions.

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