mruby-compiler: check the three RITE counts the codegen does not - #7205
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`plen` is 16 bits wide in `mrc_irep` and in the record the dump writes, so an
irep that reaches 65536 pool entries wraps it to zero. `lit_pool_extend()`
keeps handing out slots against `pcapa`, which is 32 bits and never equal to a
wrapped `plen` again, so nothing grows and nothing complains: the pool is
written over from the start, the dump records a count of a few thousand, and
every `OP_LOADL` above it indexes past the pool the loader allocated.
```console
$ build/host/bin/mruby -e 'eval("[" + (0...70000).map { |i| "\"%06d\"" % i }.join(",") + "]")'
$ echo $?
139
```
Refuse the 65536th entry instead, the way `new_sym()` refuses the symbol past
`slen` and `new_litbint()` refuses a magnitude past its length byte.
No test. The shortest program that reaches the limit is 65536 distinct
literals, and `new_lit_str()` scans the pool it has so far for each one, so
compiling it takes around ten seconds. The boundary was checked by hand:
65535 entries run, 65536 raise.
`write_pool_block()` records a pool string's length in 16 bits and copies its
bytes in full, so a literal of 65536 bytes or more is written whole under a
truncated count, and the loader reads every field after it from the wrong
offset. What stands between the two is an assertion a release build compiles
away, so what a release build does is refuse the irep it just wrote:
```console
$ ruby -e "puts 's = \"' + ('a' * 65536) + '\"'" > big.rb
$ build/host/bin/mruby big.rb
$ echo $?
1
```
An `MRC_DEBUG` build aborts on the assertion instead.
Refuse it in `new_lit_str()`, the way `new_litbint()` refuses a magnitude
wider than its own length byte.
`write_syms_block()` records a symbol name's length in 16 bits, copies the
name in full, and then advances the cursor by the truncated count, so a longer
name has the rest of the symbol block written over itself. 0xffff is not
available either: it is the length the dump writes for a null symbol, and the
loader reads it back as one.
Neither says anything. A 65535-byte name comes back as no symbol, and a
70000-byte one comes back as its first 4464 bytes:
```console
$ build/host/bin/mruby -e 'p eval(%q{:"} + "x" * 70000 + %q{"}).to_s.size'
4464
$ echo $?
0
```
Refuse a name that wide in `new_sym()`, where the symbol enters the table the
dump walks, and where the count of them is already refused.
📝 WalkthroughWalkthroughThe compiler now checks literal pool, symbol name, and string literal sizes against 16-bit dump limits. Eval tests cover maximum supported lengths and reject oversized inputs with ChangesLiteral serialization bounds
Estimated code review effort: 2 (Simple) | ~15 minutes Merge Risk: 🟡 Moderate · up to The new boundary tests currently expect the wrong exception type for oversized literals and symbols, so the test suite will fail even though the compiler checks are present. Update the assertions or intentionally change the public error contract before merging. Suggested reviewers: 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Inline comments:
In `@mrbgems/mruby-eval/test/eval.rb`:
- Around line 72-74: Update both assertions covering oversized literal and
symbol inputs in the eval tests to expect ScriptError, matching the
E_SCRIPT_ERROR contract emitted by new_lit_str() and new_sym(); do not alter the
eval implementation.
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mrbgems/mruby-compiler/src/codegen.cmrbgems/mruby-eval/test/eval.rb
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The RITE record keeps three counts in a
uint16_t, and the compiler checks two of them.new_sym()refuses the symbol past the end ofslen:and
new_litbint()refuses a magnitude past its own length byte:Three more fields have no such check, and each of them is a defect of its own on a stock 64-bit
build_config/default.rb.1. The pool count wraps, and the VM runs off the pool
plenis 16 bits inmrc_irepas well as in the record.lit_pool_extend()grows againstpcapa, which is 32 bits, so onceplenwraps to zero the two are never equal again: nothing reallocates, the pool is written over from the start, and the count that reaches the dump is a few thousand whileOP_LOADLstill names indices above it.2. A string literal of 65536 bytes is written whole under a truncated length
mrc_assert_int_fitis((void)0)unlessMRC_DEBUG, so a release build writes a record it then refuses to load, and the loader reads every field after the string from the wrong offset:That is silent today; with #7204 it says
irep load error. AnMRC_DEBUGbuild stops on the assertion instead, soci/gcc-clang'sfull-debugaborts on the same file.3. A symbol name that wide is read back as a different symbol, or as none
write_syms_block()copies the name in full and then advances the cursor by the truncated count, so the rest of the symbol block is written over itself. 0xffff is not available either: that is the length the dump writes for a null symbol. Neither case says anything, and neither is a failure at all from the outside:70000 comes back as its first 4464 bytes, and 65535 as no symbol at all.
The change
One
codegen_error()per field, where the value enters the structure the dump walks. The boundaries are what the format can carry:too many literalsstring literal too longsymbol name too longReported as
codegen_error()does it, soevalraisesSyntaxErrorwith the message the way it already does fortoo many symbols.Tests
mrbgems/mruby-eval/test/eval.rbgains one assertion per boundary, next toeval deeply nested input does not crash the parser, which is where a compiler limit is already asked about. Each writes the longest value that works beside the first one that is refused, so a boundary off by one fails.The pool count has none. The shortest program that reaches it is 65536 distinct literals, and
new_lit_str()scans the pool it has so far for each, so compiling it takes around ten seconds. Its boundary was checked by hand.Reverting the two guards turns both new assertions red rather than dropping them: the string case raises
ScriptErrorinstead ofSyntaxError, and the symbol case raises nothing at all.Verification
rake -m test, every build green, 0 KO, 0 crash, no new warnings. Each build gains the 2 new assertions.-m32, full-core-DMRB_INT32 -DMRB_NO_BOXING, full-coreEach commit was run green on its own over
build_config/default.rb; the table is the branch tip.full-debugcarriesMRC_DEBUG, so its assertions are now unreachable rather than merely unfired.The lookup added to
new_sym()costs nothing measurable.mrbcover 20000 distinct symbols in one irep, best of 5: 992 ms on master, 968 ms here. That path is dominated by the linear scannew_sym()already does.The 32-bit build configurations
build_config/host-m32.rbneeds a multilib gcc this machine has no-m32runtime for, so ani686-linux-gnu-gcccross toolchain stands in for it.host-i32is the host gcc withconf.gembox 'full-core'andMRB_INT32andMRB_NO_BOXINGinconf.cc.defines.Environment
Versions
g++forcxx_abiCompile lines for codegen.c
-MMD -c,-Iand-odropped. Each build compiles the file twice, once for the internalmrbcand once for the VM's own compiler; the-DMRB_NO_GEMSline of each pair is left out here.Summary by CodeRabbit
Bug Fixes
Tests