Install a check for mis-linking of src/port and src/common functions.

On ELF-based platforms (and maybe others?) it's possible for a shared
library, when dynamically loaded into the backend, to call the backend
versions of src/port and src/common functions rather than the frontend
versions that are actually linked into the shlib.  This is definitely
not what we want, because the frontend versions often behave slightly
differently.  Up to now it's been "slight" enough that nobody noticed;
but with the addition of SCRAM support functions in src/common, we're
observing crashes due to the difference between palloc and malloc
memory allocation rules, as reported in bug #15367 from Jeremy Evans.

The purpose of this patch is to create a direct test for this type of
mis-linking, so that we know whether any given platform requires extra
measures to prevent using the wrong functions.  If the test fails, it
will lead to connection failures in the contrib/postgres_fdw regression
test.  At the moment, *BSD platforms using ELF format are known to have
the problem and can be expected to fail; but we need to know whether
anything else does, and we need a reliable ongoing check for future
platforms.

Actually fixing the problem will be the subject of later commit(s).

Discussion: https://postgr.es/m/153626613985.23143.4743626885618266803@wrigleys.postgresql.org

src/backend/bootstrap/bootstrap.c

@@ -24,6 +24,7 @@
 #include "catalog/index.h"
 #include "catalog/pg_collation.h"
 #include "catalog/pg_type.h"
+#include "common/link-canary.h"
 #include "libpq/pqsignal.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
@@ -502,6 +503,13 @@ BootstrapModeMain(void)
 	Assert(!IsUnderPostmaster);
 	Assert(IsBootstrapProcessingMode());
 
+	/*
+	 * To ensure that src/common/link-canary.c is linked into the backend, we
+	 * must call it from somewhere.  Here is as good as anywhere.
+	 */
+	if (pg_link_canary_is_frontend())
+		elog(ERROR, "backend is incorrectly linked to frontend functions");
+
 	/*
 	 * Do backend-like initialization for bootstrap mode
 	 */

src/common/Makefile

@@ -41,7 +41,8 @@ override CPPFLAGS := -DFRONTEND $(CPPFLAGS)
 LIBS += $(PTHREAD_LIBS)
 
 OBJS_COMMON = base64.o config_info.o controldata_utils.o exec.o file_perm.o \
-	ip.o keywords.o md5.o pg_lzcompress.o pgfnames.o psprintf.o relpath.o \
+	ip.o keywords.o link-canary.o md5.o pg_lzcompress.o \
+	pgfnames.o psprintf.o relpath.o \
 	rmtree.o saslprep.o scram-common.o string.o unicode_norm.o \
 	username.o wait_error.o
 

src/common/link-canary.c

@@ -0,0 +1,36 @@
+/*-------------------------------------------------------------------------
+ * link-canary.c
+ *	  Detect whether src/common functions came from frontend or backend.
+ *
+ * Copyright (c) 2018, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/common/link-canary.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "c.h"
+
+#include "common/link-canary.h"
+
+/*
+ * This function just reports whether this file was compiled for frontend
+ * or backend environment.  We need this because in some systems, mainly
+ * ELF-based platforms, it is possible for a shlib (such as libpq) loaded
+ * into the backend to call a backend function named XYZ in preference to
+ * the shlib's own function XYZ.  That's bad if the two functions don't
+ * act identically.  This exact situation comes up for many functions in
+ * src/common and src/port, where the same function names exist in both
+ * libpq and the backend but they don't act quite identically.  To verify
+ * that appropriate measures have been taken to prevent incorrect symbol
+ * resolution, libpq should test that this function returns true.
+ */
+bool
+pg_link_canary_is_frontend(void)
+{
+#ifdef FRONTEND
+	return true;
+#else
+	return false;
+#endif
+}

src/include/common/link-canary.h

@@ -0,0 +1,17 @@
+/*-------------------------------------------------------------------------
+ *
+ * link-canary.h
+ *	  Detect whether src/common functions came from frontend or backend.
+ *
+ * Copyright (c) 2018, PostgreSQL Global Development Group
+ *
+ * src/include/common/link-canary.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef LINK_CANARY_H
+#define LINK_CANARY_H
+
+extern bool pg_link_canary_is_frontend(void);
+
+#endif							/* LINK_CANARY_H */

src/interfaces/libpq/.gitignore

@@ -25,6 +25,7 @@
 /ip.c
 /md5.c
 /base64.c
+/link-canary.c
 /scram-common.c
 /sha2.c
 /sha2_openssl.c

src/interfaces/libpq/Makefile

@@ -49,7 +49,7 @@ endif
 # src/backend/utils/mb
 OBJS += encnames.o wchar.o
 # src/common
-OBJS += base64.o ip.o md5.o scram-common.o saslprep.o unicode_norm.o
+OBJS += base64.o ip.o link-canary.o md5.o scram-common.o saslprep.o unicode_norm.o
 
 ifeq ($(with_openssl),yes)
 OBJS += fe-secure-openssl.o fe-secure-common.o sha2_openssl.o
@@ -106,7 +106,7 @@ backend_src = $(top_srcdir)/src/backend
 chklocale.c crypt.c erand48.c getaddrinfo.c getpeereid.c inet_aton.c inet_net_ntop.c noblock.c open.c system.c pgsleep.c pg_strong_random.c pgstrcasecmp.c pqsignal.c snprintf.c strerror.c strlcpy.c strnlen.c thread.c win32error.c win32setlocale.c: % : $(top_srcdir)/src/port/%
 	rm -f $@ && $(LN_S) $< .
 
-ip.c md5.c base64.c scram-common.c sha2.c sha2_openssl.c saslprep.c unicode_norm.c: % : $(top_srcdir)/src/common/%
+ip.c md5.c base64.c link-canary.c scram-common.c sha2.c sha2_openssl.c saslprep.c unicode_norm.c: % : $(top_srcdir)/src/common/%
 	rm -f $@ && $(LN_S) $< .
 
 encnames.c wchar.c: % : $(backend_src)/utils/mb/%
@@ -156,7 +156,7 @@ clean distclean: clean-lib
 	rm -f pg_config_paths.h
 # Remove files we (may have) symlinked in from src/port and other places
 	rm -f chklocale.c crypt.c erand48.c getaddrinfo.c getpeereid.c inet_aton.c inet_net_ntop.c noblock.c open.c system.c pgsleep.c pg_strong_random.c pgstrcasecmp.c pqsignal.c snprintf.c strerror.c strlcpy.c strnlen.c thread.c win32error.c win32setlocale.c
-	rm -f ip.c md5.c base64.c scram-common.c sha2.c sha2_openssl.c saslprep.c unicode_norm.c
+	rm -f ip.c md5.c base64.c link-canary.c scram-common.c sha2.c sha2_openssl.c saslprep.c unicode_norm.c
 	rm -f encnames.c wchar.c
 
 maintainer-clean: distclean maintainer-clean-lib

src/interfaces/libpq/fe-connect.c

@@ -71,6 +71,7 @@ static int ldapServiceLookup(const char *purl, PQconninfoOption *options,
 #endif
 
 #include "common/ip.h"
+#include "common/link-canary.h"
 #include "common/scram-common.h"
 #include "mb/pg_wchar.h"
 #include "port/pg_bswap.h"
@@ -1748,6 +1749,19 @@ connectDBStart(PGconn *conn)
 	if (!conn->options_valid)
 		goto connect_errReturn;
 
+	/*
+	 * Check for bad linking to backend-internal versions of src/common
+	 * functions (see comments in link-canary.c for the reason we need this).
+	 * Nobody but developers should see this message, so we don't bother
+	 * translating it.
+	 */
+	if (!pg_link_canary_is_frontend())
+	{
+		printfPQExpBuffer(&conn->errorMessage,
+						  "libpq is incorrectly linked to backend functions\n");
+		goto connect_errReturn;
+	}
+
 	/* Ensure our buffers are empty */
 	conn->inStart = conn->inCursor = conn->inEnd = 0;
 	conn->outCount = 0;

src/tools/msvc/Mkvcbuild.pm

@@ -116,7 +116,8 @@ sub mkvcbuild
 
 	our @pgcommonallfiles = qw(
 	  base64.c config_info.c controldata_utils.c exec.c file_perm.c ip.c
-	  keywords.c md5.c pg_lzcompress.c pgfnames.c psprintf.c relpath.c rmtree.c
+	  keywords.c link-canary.c md5.c
+	  pg_lzcompress.c pgfnames.c psprintf.c relpath.c rmtree.c
 	  saslprep.c scram-common.c string.c unicode_norm.c username.c
 	  wait_error.c);