Disable file protection for BSGRunContext

[nickdowell]

Goal

Fix a potential segfault when accessing bsg_runContext after device is locked.

If an app uses the Data Protection Entitlement to set the default protection to NSFileProtectionComplete, the file mapping becomes invalidated once the underlying file gets encrypted - i.e. after locking the device. Dereferencing bsg_runContext then causes a segfault and crashes the app.

Changeset

Disables content / file protection of the memory mapped file, using the NSURL / NSURLFileProtection APIs.

Considered using lower-level APIs - fcntl with F_GETPROTECTIONCLASS and F_SETPROTECTIONCLASS is available but the protection class definitions are private to xnu and their mappings to corresponding NSURLFileProtectionType undocumented.

/*
 * Protection classes vary in their restrictions on read/writability.
 * A is generally the strictest, and D is effectively no restriction.
 */

#define PROTECTION_CLASS_A 1 // -> NSFileProtectionComplete
#define PROTECTION_CLASS_B 2 // -> NSFileProtectionCompleteUnlessOpen
#define PROTECTION_CLASS_C 3 // -> NSFileProtectionCompleteUntilFirstUserAuthentication
#define PROTECTION_CLASS_D 4 // -> NSFileProtectionNone
#define PROTECTION_CLASS_E 5
#define PROTECTION_CLASS_F 6

Testing

Reproduced the crash locally by adding the Data Protection Entitlement and a UIBackgroundTask to a sample app. After locking the device, the app would crash upon completion of a 10s background task (it appears that content gets protected after a 10 second grace period.)

Was unable to reproduce the crash with this fix in place.

[github-actions]

Bugsnag.framework binary size increased by 128 bytes from 814,440 to 814,568

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