postgres.git / summary / log / commit / refs

commit    b70cab8a2c835f37cabcbe85ad480b7ce3889ee1
Author:   Tom Lane <tgl@sss.pgh.pa.us>
Date:     Fri Dec 18 20:46:44 2020 +0000
First released in: 14.0, 9.5.25, 9.6.21, 10.16, 11.11, 12.6, 13.2

    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 8d4215cb981..e4d59774850 100644
--- a/src/backend/utils/cache/relmapper.c
+++ b/src/backend/utils/cache/relmapper.c
@@ -846,8 +846,15 @@ 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)


[parent: 9595b06ef736]