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c6e0fe1f2 recently changed the way we store headers for allocated chunks of memory. Prior to that commit, we stored a pointer to the owning MemoryContext directly prior to the pointer to the allocated memory. That's no longer true and c6e0fe1f2 neglected to update MemoryContextContains() so that it correctly obtains the owning context with the new method. A side effect of this change and c6e0fe1f2, in general, is that it's even less safe than it was previously to pass MemoryContextContains() an arbitrary pointer which was not allocated by one of our MemoryContexts. Previously some comments in MemoryContextContains() seemed to indicate that the worst that could happen by passing an arbitrary pointer would be a false positive return value. It seems to me that this was a rather wishful outlook as we subsequently proceeded to subtract sizeof(void *) from the given pointer and then dereferenced that memory. So it seems quite likely that we could have segfaulted instead of returning a false positive. However, it's not impossible that the memory sizeof(void *) bytes before the pointer could have been owned by the process, but it's far less likely to work now as obtaining a pointer to the owning MemoryContext is less direct than before c6e0fe1f2 and will access memory that's possibly much further away to obtain the owning MemoryContext. Because of this, I took the liberty of updating the comment to warn against any future usages of the function and checked the existing core usages to ensure that we only ever pass in a pointer to memory allocated by a MemoryContext. Extension authors updating their code for PG16 who are using MemoryContextContains should check to ensure that only NULL pointers and pointers to chunks allocated with a MemoryContext will ever be passed to MemoryContextContains. Reported-by: Andres Freund Discussion: https://postgr.es/m/20220905230949.kb3x2fkpfwtngz43@awork3.anarazel.de
PostgreSQL Database Management System ===================================== This directory contains the source code distribution of the PostgreSQL database management system. PostgreSQL is an advanced object-relational database management system that supports an extended subset of the SQL standard, including transactions, foreign keys, subqueries, triggers, user-defined types and functions. This distribution also contains C language bindings. PostgreSQL has many language interfaces, many of which are listed here: https://www.postgresql.org/download/ See the file INSTALL for instructions on how to build and install PostgreSQL. That file also lists supported operating systems and hardware platforms and contains information regarding any other software packages that are required to build or run the PostgreSQL system. Copyright and license information can be found in the file COPYRIGHT. A comprehensive documentation set is included in this distribution; it can be read as described in the installation instructions. The latest version of this software may be obtained at https://www.postgresql.org/download/. For more information look at our web site located at https://www.postgresql.org/.