YJIT: Fix potential infinite loop when OOM #13186
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rianmcguire
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Apr 27, 2025
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Thank you so much for the investigation and the thorough fix! 🙏🏼 |
XrXr
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Avoid generating an infinite loop in the case where: 1. Block `first` is adjacent to block `second`, and the branch from `first` to `second` is a fallthrough, and 2. Block `second` immediately exits to the interpreter, and 3. Block `second` is invalidated and YJIT is OOM While pondering how to fix this, I think I've stumbled on another related edge case: 1. Block `incoming_one` and `incoming_two` both branch to block `second`. Block `incoming_one` has a fallthrough 2. Block `second` immediately exits to the interpreter (so it starts with its exit) 3. When Block `second` is invalidated, the incoming fallthrough branch from `incoming_one` might be rewritten first, which overwrites the start of block `second` with a jump to a new branch stub. 4. YJIT runs of out memory 5. The incoming branch from `incoming_two` is then rewritten, but because we're OOM we can't generate a new stub, so we use `second`'s exit as the branch target. However `second`'s exit was already overwritten with a jump to the branch stub for `incoming_one`, so `incoming_two` will end up jumping to `incoming_one`'s branch stub. Backport [Bug #21257]
k0kubun
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Apr 28, 2025
Avoid generating an infinite loop in the case where: 1. Block `first` is adjacent to block `second`, and the branch from `first` to `second` is a fallthrough, and 2. Block `second` immediately exits to the interpreter, and 3. Block `second` is invalidated and YJIT is OOM While pondering how to fix this, I think I've stumbled on another related edge case: 1. Block `incoming_one` and `incoming_two` both branch to block `second`. Block `incoming_one` has a fallthrough 2. Block `second` immediately exits to the interpreter (so it starts with its exit) 3. When Block `second` is invalidated, the incoming fallthrough branch from `incoming_one` might be rewritten first, which overwrites the start of block `second` with a jump to a new branch stub. 4. YJIT runs of out memory 5. The incoming branch from `incoming_two` is then rewritten, but because we're OOM we can't generate a new stub, so we use `second`'s exit as the branch target. However `second`'s exit was already overwritten with a jump to the branch stub for `incoming_one`, so `incoming_two` will end up jumping to `incoming_one`'s branch stub. Backport [Bug #21257]
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Fixes https://bugs.ruby-lang.org/issues/21257 [Bug #21257]
Avoid generating an infinite loop in the case where:
firstis adjacent to blocksecond, and the branch fromfirsttosecondis a fallthrough, andsecondimmediately exits to the interpreter, andsecondis invalidated and YJIT is OOMWhile pondering how to fix this, I think I've stumbled on another related edge case:
incoming_oneandincoming_twoboth branch to blocksecond. Blockincoming_onehas a fallthroughsecondimmediately exits to the interpreter (so it starts with its exit)secondis invalidated, the incoming fallthrough branch fromincoming_onemight be rewritten first, which overwrites the start of blocksecondwith a jump to a new branch stub.incoming_twois then rewritten, but because we're OOM we can't generate a new stub, so we usesecond's exit as the branch target. Howeversecond's exit was already overwritten with a jump to the branch stub forincoming_one, soincoming_twowill end up jumping toincoming_one's branch stub.I'm not sure what the consequences of calling the wrong branch stub are, but the comments in invalidate_block_version suggest the stubs are supposed to be unique.
I've attempted to address both of these cases with this change, but I can split it up if that would be preferable.