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, 26 insertions, 0 deletions

diff --git a/src/include/c.h b/src/include/c.h
index 8633657aeb..81d8e642da 100644
--- a/src/include/c.h
+++ b/src/include/c.h
@@ -949,6 +949,32 @@ typedef NameData *Name;
  * ----------------------------------------------------------------
  */
 
+/*
+ * Use this, not "char buf[BLCKSZ]", to declare a field or local variable
+ * holding a page buffer, if that page might be accessed as a page and not
+ * just a string of bytes.  Otherwise the variable might be under-aligned,
+ * causing problems on alignment-picky hardware.  (In some places, we use
+ * this to declare buffers even though we only pass them to read() and
+ * write(), because copying to/from aligned buffers is usually faster than
+ * using unaligned buffers.)  We include both "double" and "int64" in the
+ * union to ensure that the compiler knows the value must be MAXALIGN'ed
+ * (cf. configure's computation of MAXIMUM_ALIGNOF).
+ */
+typedef union PGAlignedBlock
+{
+	char		data[BLCKSZ];
+	double		force_align_d;
+	int64		force_align_i64;
+} PGAlignedBlock;
+
+/* Same, but for an XLOG_BLCKSZ-sized buffer */
+typedef union PGAlignedXLogBlock
+{
+	char		data[XLOG_BLCKSZ];
+	double		force_align_d;
+	int64		force_align_i64;
+} PGAlignedXLogBlock;
+
 /* msb for char */
 #define HIGHBIT					(0x80)
 #define IS_HIGHBIT_SET(ch)		((unsigned char)(ch) & HIGHBIT)