Commits on Aug 28, 2024

1. sr/store: Deep copy schema_def iobufs at the interface …

   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>

   

   oleiman committed Aug 28, 2024
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