From ba04cedf1d262f3d7dfe6950b13a7ed1d80a9502 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Fri, 18 Dec 2020 15:46:44 -0500 Subject: [PATCH] Avoid memcpy() with same source and destination during relmapper init. A narrow reading of the C standard says that memcpy(x,x,n) is undefined, although it's hard to envision an implementation that would really misbehave. However, analysis tools such as valgrind might whine about this; accordingly, let's band-aid relmapper.c to not do it. See also 5b630501e, d3f4e8a8a, ad7b48ea0, and other similar fixes. Apparently, none of those folk tried valgrinding initdb? This has been like this for long enough that I'm surprised it hasn't been reported before. Back-patch, just in case anybody wants to use a back branch on a platform that complains about this; we back-patched those earlier fixes too. Discussion: https://postgr.es/m/161790.1608310142@sss.pgh.pa.us --- src/backend/utils/cache/relmapper.c | 11 +++++++++-- 1 file changed, 9 insertions(+), 2 deletions(-) diff --git a/src/backend/utils/cache/relmapper.c b/src/backend/utils/cache/relmapper.c index 3b4f21bc54..897a4deb14 100644 --- a/src/backend/utils/cache/relmapper.c +++ b/src/backend/utils/cache/relmapper.c @@ -928,8 +928,15 @@ write_relmap_file(bool shared, RelMapFile *newmap, } } - /* Success, update permanent copy */ - memcpy(realmap, newmap, sizeof(RelMapFile)); + /* + * Success, update permanent copy. During bootstrap, we might be working + * on the permanent copy itself, in which case skip the memcpy() to avoid + * invoking nominally-undefined behavior. + */ + if (realmap != newmap) + memcpy(realmap, newmap, sizeof(RelMapFile)); + else + Assert(!send_sinval); /* must be bootstrapping */ /* Critical section done */ if (write_wal)