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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>
This commit is contained in:
Julius Werner 2015-01-14 13:12:10 -08:00 committed by Patrick Georgi
parent 9e381a140f
commit a7902edf56
2 changed files with 10 additions and 5 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
src/vendorcode/google/chromeos/fmap.c
View File

@ -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);

2
src/vendorcode/google/chromeos/fmap.h
View File

@ -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, */