Fix unaligned memory accesses on Alpha and possible other platforms.

By using a pointer to struct dos_partition, we implicitly tell the
compiler that the pointer is 4-bytes aligned, even though we know
that's not the case. The fact that we only dereference the pointer
to access a byte-wide field (field dp_ptyp) is not a guarantee that
the compiler will in fact use a byte-wide load. On some platforms
it's more efficient to use long word or quad word loads and use
bit-shifting and bit-masking to get the intended byte. On those
platforms an misaligned load will be the result.
The fix is to use byte-wide pointer arithmetic based on sizeof() and
offsetof() to avoid invalid casts which avoids that the compiler
makes invalid assumptions.

Backtrace provided by: wilko@
MFC after: 1 week

1 files changed, 4 insertions, 3 deletions

diff --git a/sys/geom/geom_gpt.c b/sys/geom/geom_gpt.c
index 8e5b583..aa074ee 100644
--- a/sys/geom/geom_gpt.c
+++ b/sys/geom/geom_gpt.c
@@ -358,8 +358,8 @@ g_gpt_ctl_remove(struct gctl_req *req, const char *flags, struct g_geom *gp,
 static int
 g_gpt_has_pmbr(struct g_consumer *cp, int *error)
 {
-	struct dos_partition *part;
 	char *buf;
+	uint8_t *typ;
 	int i, pmbr;
 	uint16_t magic;
 
@@ -373,9 +373,10 @@ g_gpt_has_pmbr(struct g_consumer *cp, int *error)
 	if (magic != DOSMAGIC)
 		goto out;
 
-	part = (struct dos_partition *)(uintptr_t)(buf + DOSPARTOFF);
 	for (i = 0; i < 4; i++) {
-		if (part[i].dp_typ != 0 && part[i].dp_typ != DOSPTYP_PMBR)
+		typ = buf + DOSPARTOFF + i * sizeof(struct dos_partition) +
+		    offsetof(struct dos_partition, dp_typ);
+		if (*typ != 0 && *typ != DOSPTYP_PMBR)
 			goto out;
 	}