Allow read-only system properties to have arbitrary lengths

We need to be able to store build fingerprints that are over 92 characters
long, which is the current restriction for system property value
length.

Increasing the value maximum across the board has plenty of caveats,
particularly that an allocator would be required to handle
deallocation when replacing long property values with short values.
There is also no compelling reasons to do this.

But, increasing the length of simply read-only properties, such as the
build fingerprint, has less caveats as there will never be a
deallocation of these strings.

This change uses spare bits in the top of serial (only spare for
read-only properties) to indicate if a property is 'long' or not.  The
information required to access these 'long' properties is stored in a
union where the legacy property value is located.  An error message is
retained for legacy callers.

The new property is readable via __system_property_read_callback() and
most importantly android::base::GetProperty and higher level (Java,
`getprop`) callers.  All code should move to these higher level
functions as much as possible.

Bug: 23102347
Bug: 34954705
Test: bionic unit tests
Change-Id: Ia85e0d979b92afff601cc52b39114379617a0c64

0 files changed, 0 insertions, 0 deletions