chromeos: Reverse FMAP signature constant to avoid having it in .rodata

Even though coreboot always hardcodes the FMAP offset, the same is not
possible for all other tools that manipulate ROM images. Some need to
manually find the FMAP by searching for it's magic number (ASCII
"__FMAP__"). If we do something like 'memcmp(fmap_buffer, "__FMAP__",
...) in coreboot code, it has the unfortunate side effect that the
compiler will output that very same magic number as a constant in the
.rodata section to compare against. Other tools may mistake this for the
"real" FMAP location and get confused.

This patch reverses the constant defined in coreboot and changes the
only use of it correspondingly. It is not impossible but extremely
unlikely (at the current state of the art) that any compiler would be
clever enough to understand this pattern and optimize it back to a
straight memcmp() (GCC 4.9 definitely doesn't), so it should solve the
problem at least for another few years/decades.

BRANCH=veyron
BUG=chromium:447051
TEST=Made sure the new binaries actually contain "__PAMF__" in their
.rodata. Booted Pinky. Independently corrupted both the first and the
last byte of the FMAP signature with a hex editor and confirmed that
signature check fails in both cases.

Change-Id: I314b5e7e4d78352f409e73a3ed0e71d1b56fe774
Signed-off-by: Stefan Reinauer <reinauer@chromium.org>
Original-Commit-Id: 1359d2d4502eb34a043dffab35cf4a5b033ed65a
Original-Change-Id: I725652ef2a77f7f99884b46498428c3d68cd0945
Original-Signed-off-by: Julius Werner <jwerner@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/240723
Original-Reviewed-by: Daisuke Nojiri <dnojiri@chromium.org>
Original-Reviewed-by: David Hendricks <dhendrix@chromium.org>
Reviewed-on: http://review.coreboot.org/9562
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

2 files changed, 10 insertions, 5 deletions

diff --git a/src/vendorcode/google/chromeos/fmap.c b/src/vendorcode/google/chromeos/fmap.c
index 50b0248..21d439e 100644
--- a/src/vendorcode/google/chromeos/fmap.c
+++ b/src/vendorcode/google/chromeos/fmap.c
@@ -26,10 +26,15 @@
 
 static int is_fmap_signature_valid(const struct fmap *fmap)
 {
-	if (memcmp(fmap, FMAP_SIGNATURE, sizeof(FMAP_SIGNATURE) - 1)) {
-		printk(BIOS_ERR, "No FMAP found at %p.\n", fmap);
-		return 1;
-	}
+	const char reversed_sig[] = FMAP_REVERSED_SIGNATURE;
+	const char *p2 = reversed_sig + sizeof(FMAP_REVERSED_SIGNATURE) - 2;
+	const char *p1 = (char *)fmap;
+
+	while (p2 >= reversed_sig)
+		if (*p1++ != *p2--) {
+			printk(BIOS_ERR, "No FMAP found at %p.\n", fmap);
+			return 1;
+		}
 
 	printk(BIOS_DEBUG, "FMAP: Found \"%s\" version %d.%d at %p.\n",
 	       fmap->name, fmap->ver_major, fmap->ver_minor, fmap);
diff --git a/src/vendorcode/google/chromeos/fmap.h b/src/vendorcode/google/chromeos/fmap.h
index a3d2abd..05d3fb6 100644
--- a/src/vendorcode/google/chromeos/fmap.h
+++ b/src/vendorcode/google/chromeos/fmap.h
@@ -38,7 +38,7 @@
 
 #include <stdint.h>
 
-#define FMAP_SIGNATURE		"__FMAP__"
+#define FMAP_REVERSED_SIGNATURE	"__PAMF__" /* avoid magic number in .rodata */
 #define FMAP_VER_MAJOR		1	/* this header's FMAP minor version */
 #define FMAP_VER_MINOR		1	/* this header's FMAP minor version */
 #define FMAP_STRLEN		32	/* maximum length for strings, */