From 3d70e706407072f7d19dfdba5edd02f1446888d3 Mon Sep 17 00:00:00 2001
From: Michael Davidsaver <mdavidsaver@gmail.com>
Date: Fri, 14 Jun 2024 14:49:30 -0700
Subject: [PATCH] Add dbPvt2Rec() cast

Reverse of dbRec2Pvt()

pacify -D_FORTIFY_SOURCE=3 and __builtin_object_size() as
"&precord->common" does not know than common as actually
the prefix of a variable sized struct.
---
 modules/database/src/ioc/db/dbCommonPvt.h       | 12 ++++++++++--
 modules/database/src/ioc/dbStatic/dbStaticRun.c | 16 +++++++++++++---
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/modules/database/src/ioc/db/dbCommonPvt.h b/modules/database/src/ioc/db/dbCommonPvt.h
index eda8b18a7..4d41902a2 100644
--- a/modules/database/src/ioc/db/dbCommonPvt.h
+++ b/modules/database/src/ioc/db/dbCommonPvt.h
@@ -15,13 +15,21 @@ typedef struct dbCommonPvt {
     /* Thread which is currently processing this record */
     struct epicsThreadOSD* procThread;
 
-    struct dbCommon common;
+    /* actually followed by:
+     * struct dbCommon common;
+     */
 } dbCommonPvt;
 
 static EPICS_ALWAYS_INLINE
 dbCommonPvt* dbRec2Pvt(struct dbCommon *prec)
 {
-    return CONTAINER(prec, dbCommonPvt, common);
+    return (dbCommonPvt*)((char*)prec - sizeof(dbCommonPvt));
+}
+
+static EPICS_ALWAYS_INLINE
+dbCommon* dbPvt2Rec(struct dbCommonPvt *pvt)
+{
+    return (dbCommon*)&pvt[1];
 }
 
 #endif // DBCOMMONPVT_H
diff --git a/modules/database/src/ioc/dbStatic/dbStaticRun.c b/modules/database/src/ioc/dbStatic/dbStaticRun.c
index ee9480c54..a4e93d5ca 100644
--- a/modules/database/src/ioc/dbStatic/dbStaticRun.c
+++ b/modules/database/src/ioc/dbStatic/dbStaticRun.c
@@ -66,7 +66,17 @@ void devExtend(dsxt *pdsxt)
         pthisDevSup->pdsxt = pdsxt;
     }
 }
-
+
+/* Ensure that: sizeof(dbCommonPvt) + pdbRecordType->rec_size
+ * accounts for necessary alignment of record type struct
+ */
+typedef struct {
+    dbCommonPvt prefix;
+    /* ensure no padding needed... */
+    dbCommon suffix;
+} dbCommonPvtAlignmentTest;
+STATIC_ASSERT(offsetof(dbCommonPvtAlignmentTest, suffix)==sizeof(dbCommonPvt));
+
 long dbAllocRecord(DBENTRY *pdbentry,const char *precordName)
 {
     dbRecordType    *pdbRecordType = pdbentry->precordType;
@@ -90,8 +100,8 @@ long dbAllocRecord(DBENTRY *pdbentry,const char *precordName)
                     precordName, pdbRecordType->name, pdbRecordType->rec_size);
         return(S_dbLib_noRecSup);
     }
-    ppvt = dbCalloc(1, offsetof(dbCommonPvt, common) + pdbRecordType->rec_size);
-    precord = &ppvt->common;
+    ppvt = dbCalloc(1, sizeof(dbCommonPvt) + pdbRecordType->rec_size);
+    precord = dbPvt2Rec(ppvt);
     ppvt->recnode = precnode;
     precord->rdes = pdbRecordType;
     precnode->precord = precord;