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authorJulius Werner <jwerner@chromium.org>2015-01-14 13:12:10 -0800
committerPatrick Georgi <pgeorgi@google.com>2015-04-13 12:22:55 +0200
commita7902edf56244d92e7529afc9391e8d49913914f (patch)
treeea6bf2e562a35bf2d022032fd87d9b170298a4ea /src/vendorcode/google/chromeos
parent9e381a140f206d7bdec3a71fafafabcde4182b5d (diff)
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>
Diffstat (limited to 'src/vendorcode/google/chromeos')
-rw-r--r--src/vendorcode/google/chromeos/fmap.c13
-rw-r--r--src/vendorcode/google/chromeos/fmap.h2
2 files changed, 10 insertions, 5 deletions
diff --git a/src/vendorcode/google/chromeos/fmap.c b/src/vendorcode/google/chromeos/fmap.c
index 50b0248783..21d439e07d 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 a3d2abd582..05d3fb6e1d 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, */