[v24.2.x] [CORE-7061] sr/store: Deep copy schema_def iobufs at the interface #23136
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sharded_store's main purpose is to supply a consistent interface to an underlying (sharded) in-memory store. So to process a given request, it must first work out the home shard for the desired schema ID or subject, dispatching requests to that shard. Recently, schema registry was refactored to store schema definitions as iobufs (rather than contiguous strings) to avoid large allocations. To avoid semantically unnecessary _copies_, get_schema_definition was implemented to return a iobuf share across a shard boundary. This is an unsafe operation, as the reference counting and deletion machinery of the underlying temporary buffers is totally unsynchronized. This is UB in principle (data race) and leads to memory access faults (UAF, etc.) in practice. So this commit changes 'store::get_schema_definition' to return a copy of the stored schema definition (iobuf), eliminating any cross-shard sharing of the underlying temp buffers. This approach is rather blunt and carries some cost at deallocation time (seastar allocator must work out the appropriate site for memory reclamation for each component temp buffer of a given schema def), but it should be safe since the component temp buffers are never live on multiple shards simultaneously. Note that this path could be optimized somewhat by returning a shared iobuf managed by foreign_ptr and carrying out the necessary copy (a read-only operation) on the requesting shard. This commit also includes a "unit test" that is more of a memory access canary, of sorts. Reverting the change in 'store::gets_schema_definition' will cause the test to fail reliably, producing memory faults in line with the reasoning above. With the change in place, this test reliably runs to completion without issue. Signed-off-by: Oren Leiman <oren.leiman@redpanda.com> (cherry picked from commit 420bd28)
michael-redpanda
approved these changes
Aug 29, 2024
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Backport of PR #23114