[MachO] REGRESSION: Opening a universal Mach-O binary no longer allows me to select an architecture

[bdash]

Version and Platform (required):

• Binary Ninja Version: 5.3.9135-dev Ultimate, d7369f57
• OS: macos
• OS Version: 26.2.0
• CPU Architecture: arm64

Bug Description:
I have had files.universal.architecturePreference set to ["arm64e", "arm64"] for quite some time now. The behavior prior to 5.3.9135-dev when opening a universal binary is that if one of those slices is present, it would be preselected in the architecture selector pop-up in the Open with Options dialog. I could either click Open to open it, or change the architecture and open a different one instead.

With 5.3.9135-dev, having files.universal.architecturePreference set means that if the binary contains an architecture in that list, that slice is opened without any user prompt.

Prior to 5.3.9135-dev, the setting was described as:

The preferred Mach-O architecture to select from Universal archives. On its own, changes the default Mach-O selected at load to match the specified architecture. To automatically open that slice without an extra dialog appearing, remove 'Universal' from ui.files.detection.openWithOptions.

I cannot see any new setting that controls this behavior.

Steps To Reproduce:

1. Set files.universal.architecturePreference to ["arm64e", "arm64"].
2. Open /usr/libexec/syspolicyd on stable, dev prior to 5.3.9135, and 5.3.9135-dev.

Expected Behavior:
Changing the underlying mechanism that is used to load universal binaries should not have changed when the user is given an opportunity to select which architecture to load.

[bdash]

https://github.com/Vector35/binaryninja/pull/1365 was what I had proposed for making this more explicit. It adds a dedicated setting to control when to prompt for to select an architecture (never, only if no preferred slice, always).

[bpotchik]

The files.universal.architecturePreference setting has always been about automatic loading. For the past five years its description read:

"Specify an architecture preference for automatic loading of a Mach-O file from a Universal archive. By default, the first object file in the listing is loaded."

Over time, the setting was also used to preselect the architecture in the Open with Options dialog, but that was a secondary side-effect rather than its primary function.

Universal binaries are now handled through the new Container Transform system, which provides a consistent experience across all container formats (ZIP, TAR, IMG4, Universal, etc.). In this system, architecturePreference does exactly what it was always documented to do: when set and a match is found, it automatically loads that architecture slice.

If you want interactive architecture selection, you have a couple of options:

1. Don't set architecturePreference. When unset, the Container Browser will present all available architectures for you to choose from.
2. Right-click a file in the recent files list to force-open with the Container Browser, which lets you select any architecture regardless of the preference setting.

The Container Browser also remembers your last selection per container type, so if you pick arm64e once, it will be preselected the next time you open a different Universal binary. No setting configuration is required as this is automatically handled by the Container Browser.

The workflow you're describing (preselect a default but allow overriding) is well understood, and the Container Browser's last-selection memory effectively provides that without conflating two different concerns in a single setting.

[plafosse]

@bpotchik If we're changing someone's workflow even if it is to more closely align with historical intentions we should add a new setting (or accept @bdash's PR) to preserve people's preferred workflow.

[bdash]

Container browser preserving the last selection per type is not a replacement for an explicitly set list of preferred architectures.

I'm not even sure that preserving the last selection per type is desirable. Deciding to open a specific architecture slice once does not mean that I want to use that architecture going forwards. The fact that opening an architecture slice changes what will be selected by default in the future means I now have to pay more attention when opening binaries. I no longer know which architecture will be selected. I have to look and confirm every single time.

[psifertex]

@bdash My understanding was that the default selection in the container browser was also informed by the preferred list of architectures. Pulling now to test that.

My expectation is that the container browser has a generic "if you've opened this container before and opened this file, default to this one" feature but that in cases where it hasn't been opened yet it will default to the architecture in the preference list.

This seems like a convenient way to both honor the setting while also remembering what the user did last for any given file. Testing now to see if that is indeed how it behaves.

UPDATE: After some testing, what I expected isn't happening, need to have some internal discussion.