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Before commit c6e0fe1f2, functions such as AllocSetFree could pretty safely presume that they were given a valid chunk pointer for their own type of context, because the indirect call through a memory context object and method struct would be very unlikely to work otherwise. But now, if pfree() is mistakenly invoked on a pointer to garbage, we have three chances in eight of ending up at one of these functions. That means we need to take extra measures to verify that we are looking at what we're supposed to be looking at, especially in debug builds. Hence, add code to verify that the chunk's back-link to a block header leads to a memory context object that satisfies the right sort of IsA() check. This is still a bit weaker than what we did before, but for the moment assume that an IsA() check is sufficient. As a compromise between speed and safety, implement these checks as Asserts when dealing with small chunks but plain test-and-elogs when dealing with large (external) chunks. The latter case should not be too performance-critical, but the former case probably is. In slab.c, all chunks are small; but nonetheless use a plain test in SlabRealloc, because that is certainly not performance-critical, indeed we should be suspicious that it's being called in error. In aset.c, additionally add some assertions that the "value" field of the chunk header is within the small range allowed for freelist indexes. Without that, we might find ourselves trying to wipe most of memory when CLOBBER_FREED_MEMORY is enabled, or scribbling on a "freelist header" that's far away from the context object. Eventually, field experience might show us that it's smarter for these tests to be active always, but for now we'll try to get away with just having them as assertions. While at it, also be more uniform about asserting that context objects passed as parameters are of the type we expect. Some places missed that altogether, and slab.c was for no very good reason doing it differently from the other allocators. Discussion: https://postgr.es/m/3578387.1665244345@sss.pgh.pa.us
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/.