Allow torch::deploy unity embed xar file of any size (#67814)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67814
There was a limitation on the xar file size we can embed into the binary previously. The payload (xar file here) is added to .data section by default using 'ld -b binary -r' command (which section the payload goes is hardcoded in ld BTW. Check code pointer [here](https://github.com/bminor/binutils-gdb/blob/binutils-2_32/bfd/binary.c#L80) ) . When we link the object file containing the payload to other parts of the executable, we will get relocation out of range error if the overall size of .test, .data, .bss etc sections exceed 2G. Some relocation entries uses 32 bit singed integer, thus the limit is 2G here.
To solve the issue and mitigate the risk, we designed a mechanism to put the payload in a customized payload section (.torch_deploy_payload.unity here). The payload section does not join the party of relocating and symbol resolution, thus in theory it can be as large as the disk space... Since we don't do relocation for the payload section, the start/end/size symbols are no longer available/valid, we have to parse the ELF file ourselves to figure out those.
The mechanism can be used to embed interprter.so as well. The interpreter.so is currently 0.5G. That will limit the other .test/.data/.bss sections of the executable to be at most 1.5G. Using this mechanim in this diff avoid the interpreter.so taking any budgets. We could also use this mechanism to ship python scripts with our binary rather than freeze them before hand. These use cases are not handled in this diff.
This diff also improves experience for those simple use cases that does not depends on extra shared libraries in the XAR file (except the shared libraries for python extensions themselves). This is mainly for fixing the stress test right now, but it also makes other simple cases easier.
ghstack-source-id: 142483327
Test Plan:
# Verify the relocation out of range issue is fixed
Add //caffe2:torch as a dependency to the macro build_unity(name="example", …) in torch/csrc/deploy/unity/TARGETS and run 'buck run mode/opt :unity_demo', it's expected to get the relocation errors like:
```
ld.lld: error:
caffe2/c10/util/intrusive_ptr.h:325:(.text._ZN11ska_ordered8detailv317sherwood_v3_tableISt4pairIN3c106IValueES4_ES4_NS3_6detail11DictKeyHashENS0_16KeyOrValueHasherIS4_S5_S7_EENS6_14DictKeyEqualToENS0_18KeyOrValueEqualityIS4_S5_SA_EESaIS5_ESaINS0_17sherwood_v3_entryIS5_EEEE15emplace_new_keyIS5_JEEES2_INSH_18templated_iteratorIS5_EEbEaPSF_OT_DpOT0_+0x4E9): relocation R_X86_64_32S out of range: 2345984168 is not in [-2147483648, 2147483647]; references c10::UndefinedTensorImpl::_singleton
>>> defined in /data/sandcastle/boxes/fbsource/fbcode/buck-out/opt/gen/caffe2/c10/c10#platform009-clang,static/libc10.a(../c10#compile-UndefinedTensorImpl.cpp.o44c44c4c,platform009-clang/core/UndefinedTensorImpl.cpp.o)
```
With the diff, the error above is resolved.
# Pass Stress Test
Also pass existing unit tests for unity.
buck test mode/opt //caffe2/torch/csrc/deploy/unity/tests:test_unity_sum -- --exact 'caffe2/torch/csrc/deploy/unity/tests:test_unity_sum - UnityTest.TestUnitySum' --run-disabled --jobs 18 --stress-runs 10 --record-results
buck test mode/opt //caffe2/torch/csrc/deploy/unity/tests:test_unity_simple_model -- --exact 'caffe2/torch/csrc/deploy/unity/tests:test_unity_simple_model - UnityTest.TestUnitySimpleModel' --run-disabled --jobs 18 --stress-runs 10 --record-results
# Verify debug sections are not messed up
Verified that debug sections are not messed up and GDB still works:
`gdb ~/fbcode/buck-out/gen/caffe2/torch/csrc/deploy/unity/unity_demo`
```
b main
run
l
c
```
Reviewed By: suo
Differential Revision: D32159644
fbshipit-source-id: a133513261b73551a71acc257f4019f7b5af34a8
