cbfstool: Make the add action choose an aligned entries capacity

This fixes an inconsistency between `cbfstool create` and `cbfstool add` that
was resulting in confusing claims about the amount of free space at the end of a
CBFS. Calls to `cbfstool add` check whether a file fits under a given empty file
entry by testing whether it would collide with the beginning of the *subsequent*
file header; thus, if a file's end is unaligned, its reported size will not
match the actual available capacity. Although deleted entries always end on an
alignment boundary because `cbfstool remove` expands them to fill the available
space, `cbfstool create` doesn't necessarily size a new entries region to result
in an empty entry with an aligned end.
This problem never resulted in clobbering important data because cbfstool would
blindly reserve 64B (or the selected alignment) of free space immediately after
the all-inclusive empty file entry. This change alters the way this reservation
is reported: only the overhang past the alignment is used as hidden padding, and
the empty entry's capacity is always reported such that it ends at an aligned
address.
Much of the time that went into this patch was spent building trust in the
trickery cbfstool employs to avoid explicitly tracking the image's total
capacity for entries, so below are two proofs of correctness to save others time
and discourage inadvertent breakage:

OBSERVATION (A): A check in cbfs_image_create() guarantees that an aligned CBFS
empty file header is small enough that it won't cross another aligned address.

OBSERVATION (B): In cbfs_image_create(), the initial empty entry is sized such
that its contents end on an aligned address.

THM. 1: Placing a new file within an empty entry located below an existing file
entry will never leave an aligned flash address containing neither the beginning
of a file header nor part of a file.
 We can prove this by contradiction: assume a newly-added file neither fills to
 the end of the preexisting empty entry nor leaves room for another aligned
 empty header after it. Then the first aligned address after the end of the
 newly-inserted file...
  - CASE 1: ...already contains a preexisting file entry header.
    + Then that address contains a file header.
  - CASE 2: ...does not already house a file entry header.
    + Then because CBFS content doesn't fall outside headers, the area between
      there and the *next* aligned address after that is unused.
    + By (A), we can fit a file header without clobbering anything.
    + Then that address now contains a file header.

THM. 2: Placing a new file in an empty entry at the very end of the image such
that it fits, but leaves no room for a final header, is guaranteed not to change
the total amount of space for entries, even if that new file is later removed
from the CBFS.
 Again, we use contradiction: assume that creating such a file causes a
 permanent...
  - CASE 1: ...increase in the amount of available space.
    + Then the combination of the inserted file, its header, and any padding
      must have exceeded the empty entry in size enough for it to cross at
      least one additional aligned address, since aligned addresses are how
      the limit on an entry's capacity is determined.
    + But adding the file couldn't have caused us to write past any further
      aligned addresses because they are the boundary's used when verifying
      that sufficient capacity exists; furthermore, by (B), no entry can ever
      terminate beyond where the initial empty entry did when the CBFS was
      first created.
    + Then the creation of the file did not result in a space increase.
  - CASE 2: ...decrease in the amount of available space.
    + Then the end of the new file entry crosses at least one fewer aligned
      address than did the empty file entry.
    + Then by (A), there is room to place a new file entry that describes the
      remaining available space at the first available aligned address.
    + Then there is now a new record showing the same amount of available space.
    + Then the creation of the file did not result in a space decrease.

BUG=chromium:473726
TEST=Had the following conversation with cbfstool:
$ ./cbfstool test.image create -s 0x100000 -m arm
Created CBFS image (capacity = 1048408 bytes)
$ ./cbfstool test.image print
test.image: 1024 kB, bootblocksize 0, romsize 1048576, offset 0x40
alignment: 64 bytes, architecture: arm

Name                           Offset     Type         Size
(empty)                        0x40       null         1048408
$ dd if=/dev/zero of=toobigmed.bin bs=1048409 count=1
1+0 records in
1+0 records out
1048409 bytes (1.0 MB) copied, 0.0057865 s, 181 MB/s
$ ./cbfstool test.image add -t 0x50 -f toobigmed.bin -n toobig
E: Could not add [toobigmed.bin, 1048409 bytes (1023 KB)@0x0]; too big?
E: Failed to add 'toobigmed.bin' into ROM image.
$ truncate -s -1 toobigmed.bin
$ ./cbfstool test.image add -t 0x50 -f toobigmed.bin -n toobig
$ ./cbfstool test.image print
test.image: 1024 kB, bootblocksize 0, romsize 1048576, offset 0x40
alignment: 64 bytes, architecture: arm

Name                           Offset     Type         Size
toobig                         0x40       raw          1048408
$ ./cbfstool test.image remove
-n toobig
$ ./cbfstool test.image print
test.image: 1024 kB, bootblocksize 0, romsize 1048576, offset 0x40
alignment: 64 bytes, architecture: arm

Name                           Offset     Type         Size
(empty)                        0x40       deleted      1048408
$ ./cbfstool test.image print
test.image: 1024 kB, bootblocksize 0, romsize 1048576, offset 0x40
alignment: 64 bytes, architecture: arm

Name                           Offset     Type         Size
(empty)                        0x40       deleted      1048408
BRANCH=None

Change-Id: I118743e37469ef0226970decc900db5d9b92c5df
Signed-off-by: Sol Boucher <solb@chromium.org>
Original-Commit-Id: e317ddca14bc36bc36e6406b758378c88e9ae04e
Original-Change-Id: I294ee489b4918646c359b06aa1581918f2d8badc
Original-Signed-off-by: Sol Boucher <solb@chromium.org>
Original-Reviewed-on: https://chromium-review.googlesource.com/263962
Original-Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Original-Reviewed-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/9939
Tested-by: build bot (Jenkins)

util/cbfstool/cbfs_image.c

@@ -28,6 +28,24 @@
 #include "common.h"
 #include "cbfs_image.h"
 
+/* Even though the file-adding functions---cbfs_add_entry() and
+ * cbfs_add_entry_at()---perform their sizing checks against the beginning of
+ * the subsequent section rather than a stable recorded value such as an empty
+ * file header's len field, it's possible to prove two interesting properties
+ * about their behavior:
+ *  - Placing a new file within an empty entry located below an existing file
+ *    entry will never leave an aligned flash address containing neither the
+ *    beginning of a file header nor part of a file.
+ *  - Placing a new file in an empty entry at the very end of the image such
+ *    that it fits, but leaves no room for a final header, is guaranteed not to
+ *    change the total amount of space for entries, even if that new file is
+ *    later removed from the CBFS.
+ * These properties are somewhat nonobvious from the implementation, so the
+ * reader is encouraged to blame this comment and examine the full proofs
+ * in the commit message before making significant changes that would risk
+ * removing said guarantees.
+ */
+
 /* The file name align is not defined in CBFS spec -- only a preference by
  * (old) cbfstool. */
 #define CBFS_FILENAME_ALIGN	(16)
@@ -185,6 +203,16 @@ int cbfs_image_create(struct cbfs_image *image,
 	      bootblock_offset, bootblock->size,
 	      header_offset, sizeof(header), entries_offset);
 
+	// This attribute must be given in order to prove that this module
+	// correctly preserves certain CBFS properties. See the block comment
+	// near the top of this file (and the associated commit message).
+	size_t empty_header_len = cbfs_calculate_file_header_size("");
+	if (align < empty_header_len) {
+		ERROR("CBFS must be aligned to at least %zu bytes\n",
+							empty_header_len);
+		return -1;
+	}
+
 	if (buffer_create(&image->buffer, size, "(new)") != 0)
 		return -1;
 	if ((image->header = malloc(sizeof(*image->header))) == NULL)
@@ -263,7 +291,11 @@ int cbfs_image_create(struct cbfs_image *image,
 		cbfs_len = bootblock_offset;
 	if (header_offset > entries_offset && header_offset < cbfs_len)
 		cbfs_len = header_offset;
-	cbfs_len -= entries_offset + align + entry_header_len;
+	// This alignment is necessary in order to prove that this module
+	// correctly preserves certain CBFS properties. See the block comment
+	// near the top of this file (and the associated commit message).
+	cbfs_len -= cbfs_len % align;
+	cbfs_len -= entries_offset + entry_header_len;
 	cbfs_create_empty_entry(entry, cbfs_len, "");
 	LOG("Created CBFS image (capacity = %d bytes)\n", cbfs_len);
 	return 0;
@@ -462,8 +494,12 @@ static int cbfs_add_entry_at(struct cbfs_image *image,
 
 	assert(addr < addr_next);
 	if (addr_next - addr < min_entry_size) {
-		DEBUG("No space after content to keep CBFS structure.\n");
-		return -1;
+		DEBUG("No need for new \"empty\" entry\n");
+		/* No need to increase the size of the just
+		 * stored file to extend to next file. Alignment
+		 * of next file takes care of this.
+		 */
+		return 0;
 	}
 
 	len = addr_next - addr - min_entry_size;