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ruby: Fix object cache lookups on 32-bit platforms (#13494) (#13580)
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#13204 refactored the Ruby object cache to use a key of `LL2NUM(key_val)` instead of `LL2NUM(key_val >> 2)`. On 32-bit systems, `LL2NUM(key_val)` returns inconsistent results because a large value has to be stored as a Bignum on the heap. This causes cache lookups to fail.

This commit restores the previous behavior of using `ObjectCache_GetKey`, which discards the lower 2 bits, which are zero. This enables a key to be stored as a Fixnum on both 32 and 64-bit platforms.

As https://patshaughnessy.net/2014/1/9/how-big-is-a-bignum describes, a Fixnum uses:

* 1 bit for the `FIXNUM_FLAG`.
* 1 bit for the sign flag.

Therefore the largest possible Fixnum value on a 64-bit value is 4611686018427387903 (2^62 - 1). On a 32-bit system, the largest value  is 1073741823 (2^30 - 1).

For example, a possible VALUE pointer address on a 32-bit system:

0xff5b4af8 => 4284173048

Dropping the lower 2 bits makes up for the loss of range to these flags. In the example above, we see that shifting by 2 turns the value into a 30-bit number, which can be represented as a Fixnum:

(0xff5b4af8 >> 2) => 1071043262

This bug can also manifest on a 64-bit system if the upper bits are 0xff.

Closes #13481

Closes #13494

COPYBARA_INTEGRATE_REVIEW=#13494 from stanhu:sh-fix-ruby-protobuf-32bit d63122a
PiperOrigin-RevId: 557211768

Co-authored-by: Stan Hu <stanhu@gmail.com>
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esrauchg and stanhu authored Aug 17, 2023
1 parent 7520eff commit 3094b49
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33 changes: 33 additions & 0 deletions .github/workflows/test_ruby.yml
Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,39 @@ jobs:
bazel-cache: ruby_linux/${{ matrix.ruby }}_${{ matrix.bazel }}
bazel: test //ruby/... //ruby/tests:ruby_version --test_env=KOKORO_RUBY_VERSION

linux-32bit:
name: Linux 32-bit
runs-on: ubuntu-latest
steps:
- name: Checkout pending changes
uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # v3.3.0
with:
ref: ${{ inputs.safe-checkout }}
submodules: recursive

- name: Cross compile protoc for i386
id: cross-compile
uses: protocolbuffers/protobuf-ci/cross-compile-protoc@v2
with:
image: us-docker.pkg.dev/protobuf-build/containers/common/linux/bazel:6.3.0-91a0ac83e968068672bc6001a4d474cfd9a50f1d
credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
architecture: linux-i386

- name: Run tests
uses: protocolbuffers/protobuf-ci/docker@v2
with:
image: i386/ruby:2.7.3-buster
skip-staleness-check: true
credentials: ${{ secrets.GAR_SERVICE_ACCOUNT }}
command: >-
/bin/bash -cex '
gem install bundler;
cd /workspace/ruby;
bundle;
PROTOC=/workspace/${{ steps.cross-compile.outputs.protoc }} rake;
rake clobber_package gem;
PROTOC=/workspace/${{ steps.cross-compile.outputs.protoc }} rake test'
linux-aarch64:
name: Linux aarch64
runs-on: ubuntu-latest
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18 changes: 13 additions & 5 deletions ruby/ext/google/protobuf_c/protobuf.c
Original file line number Diff line number Diff line change
Expand Up @@ -276,17 +276,25 @@ static void ObjectCache_Init(VALUE protobuf) {
rb_const_set(protobuf, rb_intern("SIZEOF_VALUE"), INT2NUM(SIZEOF_VALUE));
}

VALUE ObjectCache_TryAdd(const void *key, VALUE val) {
static VALUE ObjectCache_GetKey(const void *key) {
VALUE key_val = (VALUE)key;
PBRUBY_ASSERT((key_val & 3) == 0);
return rb_funcall(weak_obj_cache, item_try_add, 2, LL2NUM(key_val), val);
// Ensure the key can be stored as a Fixnum since 1 bit is needed for
// FIXNUM_FLAG and 1 bit is needed for the sign bit.
VALUE new_key = LL2NUM(key_val >> 2);
PBRUBY_ASSERT(FIXNUM_P(new_key));
return new_key;
}

VALUE ObjectCache_TryAdd(const void *key, VALUE val) {
VALUE key_val = ObjectCache_GetKey(key);
return rb_funcall(weak_obj_cache, item_try_add, 2, key_val, val);
}

// Returns the cached object for this key, if any. Otherwise returns Qnil.
VALUE ObjectCache_Get(const void *key) {
VALUE key_val = (VALUE)key;
PBRUBY_ASSERT((key_val & 3) == 0);
return rb_funcall(weak_obj_cache, item_get, 1, LL2NUM(key_val));
VALUE key_val = ObjectCache_GetKey(key);
return rb_funcall(weak_obj_cache, item_get, 1, key_val);
}

/*
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