Fixes#22236
If reading a file and not stopping on block bounds returned by `size()`, we could
allow reading from (_file_size+1-15) (block boundary) and try to decrypt this
buffer (last one).
Check on last block in `transform` would wrap around size due to us being >=
file size (l).
Simplest example:
Actual data size: 4095
Physical file size: 4095 + key block size (typically 16)
Read from 4096: -> 15 bytes (padding) -> transform return _file_size - read offset
-> wraparound -> rather larger number than we expected
(not to mention the data in question is junk/zero).
Just do an early bounds check and return zero if we're past the actual data limit.
v2:
* Moved check to a min expression instead
* Added lengthy comment
* Added unit test
v3:
* Fixed read_dma_bulk handling of short, unaligned read
* Added test for unaligned read
v4:
* Added another unaligned test case
(cherry picked from commit e96cc52668)
a51888694e