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As reported in erlang-bugs, the following sequence of events crashes the VM:
1. Module M1 is loaded and in native mode.
2. Module M2 is not loaded, in emulated mode, and has an on_load function.
3. M1 calls some function in M2. This works.
4. M1 again calls some function in M2. This segfaults.
The reason for the crash is that when the beam loader fixes up export
entries after a successful on_load function call, it erroneously clears
the ->code[3] field in that module's export entries. This is redundant
(no code in beam relies on ->code[3] being NULL), inconsistent with
modules without on_load functions (there ->code[3] remains a valid beam
instruction after the module is loaded), and breaks native code which needs
the old ->address value in an export entry to remain valid after a module
load step (before the load ->address points to ->code[3], after the load
->address points to the real code but uses of the old ->address value
remain so ->code[3] must remain valid).
Thus the fix for the crash is to simply not clear ->code[3].
This patch fixes R14A and should also fix R13B04.
(There does exist a performance bug in this area, but it is unrelated
to the on_load feature so will be fixed separately.)
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Merging the three off-heap lists (binaries, funs and externals) into
one list. This reduces memory consumption by two words (pointers) per
ETS object.
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For cleanliness, use BeamInstr instead of the UWord
data type to any machine-sized words that are used
for BEAM instructions. Only use UWord for untyped
words in general.
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Store Erlang terms in 32-bit entities on the heap, expanding the
pointers to 64-bit when needed. This works because all terms are stored
on addresses in the 32-bit address range (the 32 most significant bits
of pointers to term data are always 0).
Introduce a new datatype called UWord (along with its companion SWord),
which is an integer having the exact same size as the machine word
(a void *), but might be larger than Eterm/Uint.
Store code as machine words, as the instructions are pointers to
executable code which might reside outside the 32-bit address range.
Continuation pointers are stored on the 32-bit stack and hence must
point to addresses in the low range, which means that loaded beam code
much be placed in the low 32-bit address range (but, as said earlier,
the instructions themselves are full words).
No Erlang term data can be stored on C stacks (enforced by an
earlier commit).
This version gives a prompt, but test cases still fail (and dump core).
The loader (and emulator loop) has instruction packing disabled.
The main issues has been in rewriting loader and actual virtual
machine. Subsystems (like distribution) does not work yet.
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