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If asked to decrease the size of a large (>8K) palloc chunk, AllocSetRealloc could improperly change the Valgrind state of memory beyond the new end of the chunk: it would mark data UNDEFINED as far as the old end of the chunk after having done the realloc(3) call, thus tromping on the state of memory that no longer belongs to it. One would normally expect that memory to now be marked NOACCESS, so that this mislabeling might prevent detection of later errors. If realloc() had chosen to move the chunk someplace else (unlikely, but well within its rights) we could also mismark perfectly-valid DEFINED data as UNDEFINED, causing false-positive valgrind reports later. Also, any malloc bookkeeping data placed within this area might now be wrongly marked, causing additional problems. Fix by replacing relevant uses of "oldsize" with "Min(size, oldsize)". It's sufficient to mark as far as "size" when that's smaller, because whatever remains in the new chunk size will be marked NOACCESS below, and we expect realloc() to have taken care of marking the memory beyond the new official end of the chunk. While we're here, also rename the function's "oldsize" variable to "oldchksize" to more clearly explain what it actually holds, namely the distance to the end of the chunk (that is, requested size plus trailing padding). This is more consistent with the use of "size" and "chksize" to hold the new requested size and chunk size. Add a new variable "oldsize" in the one stanza where we're actually talking about the old requested size. Oversight in commit c477f3e44. Back-patch to all supported branches, as that was, just in case anybody wants to do valgrind testing on back branches. Karina Litskevich Discussion: https://postgr.es/m/CACiT8iaAET-fmzjjZLjaJC4zwSJmrFyL7LAdHwaYyjjQOQ4hcg@mail.gmail.com
# Generating dummy probes If Postgres isn't configured with dtrace enabled, we need to generate dummy probes for the entries in probes.d, that do nothing. This is accomplished in Unix via the sed script `Gen_dummy_probes.sed`. We used to use this in MSVC builds using the perl utility `psed`, which mimicked sed. However, that utility disappeared from Windows perl distributions and so we converted the sed script to a perl script to be used in MSVC builds. We still keep the sed script as the authoritative source for generating these dummy probes because except on Windows perl is not a hard requirement when building from a tarball. So, if you need to change the way dummy probes are generated, first change the sed script, and when it's working generate the perl script. This can be accomplished by using the perl utility s2p. s2p is no longer part of the perl core, so it might not be on your system, but it is available on CPAN and also in many package systems. e.g. on Fedora it can be installed using `cpan App::s2p` or `dnf install perl-App-s2p`. The Makefile contains a recipe for regenerating Gen_dummy_probes.pl, so all you need to do is once you have s2p installed is `make Gen_dummy_probes.pl` Note that in a VPATH build this will generate the file in the vpath tree, not the source tree.