index-pack, unpack-objects: use get_be32() for reading pack header

Both of these commands read the incoming pack into a static unsigned
char buffer in BSS, and then parse it by casting the start of the buffer
to a struct pack_header. This can result in SIGBUS on some platforms if
the compiler doesn't place the buffer in a position that is properly
aligned for 4-byte integers.

This reportedly happens with unpack-objects (but not index-pack) on
sparc64 when compiled with clang (but not gcc). But we are definitely in
the wrong in both spots; since the buffer's type is unsigned char, we
can't depend on larger alignment. When it works it is only because we
are lucky.

We'll fix this by switching to get_be32() to read the headers (just like
the last few commits similarly switched us to put_be32() for writing
into the same buffer).

It would be nice to factor this out into a common helper function, but
the interface ends up quite awkward. Either the caller needs to hardcode
how many bytes we'll need, or it needs to pass us its fill()/use()
functions as pointers. So I've just fixed both spots in the same way;
this is not code that is likely to be repeated a third time (most of the
pack reading code uses an mmap'd buffer, which should be properly
aligned).

I did make one tweak to the shared code: our pack_version_ok() macro
expects us to pass the big-endian value we'd get by casting. We can
introduce a "native" variant which uses the host integer ordering.

Reported-by: Koakuma <[email protected]>
Signed-off-by: Jeff King <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

builtin/index-pack.c

@@ -363,16 +363,18 @@ static const char *open_pack_file(const char *pack_name)
 
 static void parse_pack_header(void)
 {
-	struct pack_header *hdr = fill(sizeof(struct pack_header));
+	unsigned char *hdr = fill(sizeof(struct pack_header));
 
 	/* Header consistency check */
-	if (hdr->hdr_signature != htonl(PACK_SIGNATURE))
+	if (get_be32(hdr) != PACK_SIGNATURE)
 		die(_("pack signature mismatch"));
-	if (!pack_version_ok(hdr->hdr_version))
+	hdr += 4;
+	if (!pack_version_ok_native(get_be32(hdr)))
 		die(_("pack version %"PRIu32" unsupported"),
-			ntohl(hdr->hdr_version));
+		    get_be32(hdr));
+	hdr += 4;
 
-	nr_objects = ntohl(hdr->hdr_entries);
+	nr_objects = get_be32(hdr);
 	use(sizeof(struct pack_header));
 }
 

builtin/unpack-objects.c

@@ -576,15 +576,16 @@ static void unpack_one(unsigned nr)
 static void unpack_all(void)
 {
 	int i;
-	struct pack_header *hdr = fill(sizeof(struct pack_header));
+	unsigned char *hdr = fill(sizeof(struct pack_header));
 
-	nr_objects = ntohl(hdr->hdr_entries);
-
-	if (ntohl(hdr->hdr_signature) != PACK_SIGNATURE)
+	if (get_be32(hdr) != PACK_SIGNATURE)
 		die("bad pack file");
-	if (!pack_version_ok(hdr->hdr_version))
+	hdr += 4;
+	if (!pack_version_ok_native(get_be32(hdr)))
 		die("unknown pack file version %"PRIu32,
-			ntohl(hdr->hdr_version));
+		    get_be32(hdr));
+	hdr += 4;
+	nr_objects = get_be32(hdr);
 	use(sizeof(struct pack_header));
 
 	if (!quiet)

pack.h

@@ -13,7 +13,8 @@ struct repository;
  */
 #define PACK_SIGNATURE 0x5041434b	/* "PACK" */
 #define PACK_VERSION 2
-#define pack_version_ok(v) ((v) == htonl(2) || (v) == htonl(3))
+#define pack_version_ok(v) pack_version_ok_native(ntohl(v))
+#define pack_version_ok_native(v) ((v) == 2 || (v) == 3)
 struct pack_header {
 	uint32_t hdr_signature;
 	uint32_t hdr_version;