Avoid using potentially-under-aligned page buffers.

There's a project policy against using plain "char buf[BLCKSZ]" local
or static variables as page buffers; preferred style is to palloc or
malloc each buffer to ensure it is MAXALIGN'd.  However, that policy's
been ignored in an increasing number of places.  We've apparently got
away with it so far, probably because (a) relatively few people use
platforms on which misalignment causes core dumps and/or (b) the
variables chance to be sufficiently aligned anyway.  But this is not
something to rely on.  Moreover, even if we don't get a core dump,
we might be paying a lot of cycles for misaligned accesses.

To fix, invent new union types PGAlignedBlock and PGAlignedXLogBlock
that the compiler must allocate with sufficient alignment, and use
those in place of plain char arrays.

I used these types even for variables where there's no risk of a
misaligned access, since ensuring proper alignment should make
kernel data transfers faster.  I also changed some places where
we had been palloc'ing short-lived buffers, for coding style
uniformity and to save palloc/pfree overhead.

Since this seems to be a live portability hazard (despite the lack
of field reports), back-patch to all supported versions.

Patch by me; thanks to Michael Paquier for review.

Discussion: https://postgr.es/m/1535618100.1286.3.camel@credativ.de

1 files changed, 7 insertions, 7 deletions

diff --git a/src/backend/access/transam/xloginsert.c b/src/backend/access/transam/xloginsert.c
index 5bea073a2b7..34d4db42977 100644
--- a/src/backend/access/transam/xloginsert.c
+++ b/src/backend/access/transam/xloginsert.c
@@ -809,12 +809,12 @@ XLogCompressBackupBlock(char *page, uint16 hole_offset, uint16 hole_length,
 	int32		len;
 	int32		extra_bytes = 0;
 	char	   *source;
-	char		tmp[BLCKSZ];
+	PGAlignedBlock tmp;
 
 	if (hole_length != 0)
 	{
 		/* must skip the hole */
-		source = tmp;
+		source = tmp.data;
 		memcpy(source, page, hole_offset);
 		memcpy(source + hole_offset,
 			   page + (hole_offset + hole_length),
@@ -917,7 +917,7 @@ XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
 	if (lsn <= RedoRecPtr)
 	{
 		int			flags;
-		char		copied_buffer[BLCKSZ];
+		PGAlignedBlock copied_buffer;
 		char	   *origdata = (char *) BufferGetBlock(buffer);
 		RelFileNode rnode;
 		ForkNumber	forkno;
@@ -935,11 +935,11 @@ XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
 			uint16		lower = ((PageHeader) page)->pd_lower;
 			uint16		upper = ((PageHeader) page)->pd_upper;
 
-			memcpy(copied_buffer, origdata, lower);
-			memcpy(copied_buffer + upper, origdata + upper, BLCKSZ - upper);
+			memcpy(copied_buffer.data, origdata, lower);
+			memcpy(copied_buffer.data + upper, origdata + upper, BLCKSZ - upper);
 		}
 		else
-			memcpy(copied_buffer, origdata, BLCKSZ);
+			memcpy(copied_buffer.data, origdata, BLCKSZ);
 
 		XLogBeginInsert();
 
@@ -948,7 +948,7 @@ XLogSaveBufferForHint(Buffer buffer, bool buffer_std)
 			flags |= REGBUF_STANDARD;
 
 		BufferGetTag(buffer, &rnode, &forkno, &blkno);
-		XLogRegisterBlock(0, &rnode, forkno, blkno, copied_buffer, flags);
+		XLogRegisterBlock(0, &rnode, forkno, blkno, copied_buffer.data, flags);
 
 		recptr = XLogInsert(RM_XLOG_ID, XLOG_FPI_FOR_HINT);
 	}