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From ad...@apache.org
Subject [1/4] incubator-mynewt-site git commit: Updated bootloader doc
Date Wed, 14 Dec 2016 01:17:53 GMT
Repository: incubator-mynewt-site
Updated Branches:
  refs/heads/asf-site f78bd4195 -> 6e429bad9


http://git-wip-us.apache.org/repos/asf/incubator-mynewt-site/blob/6e429bad/latest/os/modules/bootloader/bootloader/index.html
----------------------------------------------------------------------
diff --git a/latest/os/modules/bootloader/bootloader/index.html b/latest/os/modules/bootloader/bootloader/index.html
index c8d2b3f..4ebc284 100644
--- a/latest/os/modules/bootloader/bootloader/index.html
+++ b/latest/os/modules/bootloader/bootloader/index.html
@@ -527,21 +527,26 @@
                         </div>
                         
                             <h1 id="bootloader">Bootloader</h1>
-<p>The "bootloader" is the code that loads the Mynewt OS image into memory and conducts
some checks before allowing the OS to be run. The bootloader in the Apache Mynewt project
verifies the cryptographic signature of the firmware image before running it. It maintains
a detailed status log for each stage of the boot process. For verification of the authenticity
of the OS image, it:</p>
+<p>The "bootloader" is the code that loads the Mynewt OS image into memory and conducts
some checks before allowing the OS to be run. It manages images for the embedded system and
upgrades of those images using protocols over various interfaces (e.g. serial, BLE etc.).
Typically, systems with bootloaders have at least two program images coexisting on the same
microcontroller, and hence must include branch code that performs a check to see if an attempt
to update software is already underway and manage the progress of the process.</p>
+<p>The bootloader in the Apache Mynewt project verifies the cryptographic signature
of the firmware image before running it. It maintains a detailed status log for each stage
of the boot process. For verification of the authenticity of the OS image, it:</p>
 <ul>
 <li>Calculates hash of the image.</li>
 <li>Uses public key to uncover hash value from included signature. </li>
 <li>Compares the calculated and uncovered hashes for a match.</li>
 </ul>
 <p>The "secure bootloader" should be placed in protected memory on a given microcontroller.</p>
-<p>The Mynewt code is structured so that the generic bootutil library performs most
of the functions of a boot loader. The final step of actually jumping to the main image is
kept out of the bootutil library.  This last step should instead be implemented in an
+<p>The Mynewt bootloader comprises two packages:</p>
+<ul>
+<li>The bootutil library (boot/bootutil)</li>
+<li>The boot application (apps/boot)</li>
+</ul>
+<p>The Mynewt code is thus structured so that the generic bootutil library performs
most of the functions of a boot loader. The final step of actually jumping to the main image
is kept out of the bootutil library.  This last step should instead be implemented in an
 architecture-specific project.  Boot loader functionality is separated in this
 manner for the following two reasons:</p>
 <ol>
 <li>By keeping architecture-dependent code separate, the bootutil library can be
    reused among several boot loaders.</li>
-<li>By excluding the last boot step from the library, the rest of the code can
-   be tested in a sim environment.</li>
+<li>By excluding the last boot step from the library, the bootloader can be unit tested
since a library can be unit tested but an applicant can't.</li>
 </ol>
 <h3 id="limitations">Limitations</h3>
 <p>The boot loader currently only supports images with the following
@@ -568,7 +573,15 @@ characteristics:</p>
     <span style="color: #A90D91">uint16_t</span> <span style="color: #000000">ih_tlv_size</span>;
<span style="color: #177500">/* Trailing TLVs */</span>
     <span style="color: #A90D91">uint8_t</span>  <span style="color: #000000">ih_key_id</span>;
     <span style="color: #A90D91">uint8_t</span>  <span style="color: #000000">_pad1</span>;
-    <span style="color: #A90D91">uint16_t</span> <span style="color: #000000">ih_hdr_size</span>;
+    <span style="color: #A90D91">uint16_t</span> <span style="color: #000000">ih_hdr_s&lt;</span>
<span style="color: #000000">bok@bok</span>.<span style="color: #000000">net</span>
+<span style="color: #1C01CE">35</span><span style="color: #000000">d33</span>
+<span style="color: #000000">&lt;</span> <span style="color: #000000">ericmanganaro@gmail</span>.<span
style="color: #000000">com</span>
+<span style="color: #1C01CE">42</span><span style="color: #000000">d39</span>
+<span style="color: #000000">&lt;</span> <span style="color: #000000">tam@proxy</span>.<span
style="color: #000000">co</span>
+<span style="color: #1C01CE">55</span><span style="color: #000000">d51</span>
+<span style="color: #000000">&lt;</span> <span style="color: #000000">nathan@natb1</span>.<span
style="color: #000000">com</span>
+<span style="color: #1C01CE">110</span><span style="color: #000000">d105</span>
+<span style="color: #000000">&lt;</span> <span style="color: #000000">rvs@apache</span>.<span
style="color: #000000">orgize</span>;
     <span style="color: #A90D91">uint16_t</span> <span style="color: #000000">_pad2</span>;
     <span style="color: #A90D91">uint32_t</span> <span style="color: #000000">ih_img_size</span>;
<span style="color: #177500">/* Does not include header. */</span>
     <span style="color: #A90D91">uint32_t</span> <span style="color: #000000">ih_flags</span>;
@@ -591,8 +604,16 @@ case of changes to the format of the image header.</p>
 </pre></div>
 
 
-<p>Security data gets added as a footer at the end of the image.</p>
-<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%"><span
style="color: #177500">/*</span>
+<p>Optional type-length-value records (TLVs) containing image metadata are placed
+after the end of the image. For example, security data gets added as a footer at the end
of the image.</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%"><span
style="color: #177500">/** Image trailer TLV format. All fields in little endian. */</span>
+<span style="color: #A90D91">struct</span> <span style="color: #000000">image_tlv</span>
{
+    <span style="color: #A90D91">uint8_t</span>  <span style="color: #000000">it_type</span>;
  <span style="color: #177500">/* IMAGE_TLV_[...]. */</span>
+    <span style="color: #A90D91">uint8_t</span>  <span style="color: #000000">_pad</span>;
+    <span style="color: #A90D91">uint16_t</span> <span style="color: #000000">it_len</span>
    <span style="color: #177500">/* Data length (not including TLV header). */</span>
+};
+
+<span style="color: #177500">/*</span>
 <span style="color: #177500"> * Image trailer TLV types.</span>
 <span style="color: #177500"> */</span>
 <span style="color: #633820">#define IMAGE_TLV_SHA256            1   </span><span
style="color: #177500">/* SHA256 of image hdr and body */</span><span style="color:
#633820"></span>
@@ -601,13 +622,23 @@ case of changes to the format of the image header.</p>
 </pre></div>
 
 
-<h3 id="flash-areas">Flash Areas</h3>
-<p>Bootutil uses the same concept of "flash areas" as the nffs file system.
-Briefly, an area is a region of disk with the following properties:
-1. An area can be fully erased without affecting any other areas.
-2. A write to one area does not restrict writes to other areas.</p>
-<p>The areas used for image data must not be used for anything else.  In
-particular, these areas must be kept separate from the nffs file system.</p>
+<h3 id="flash-map">Flash Map</h3>
+<p>A Mynewt device's flash is partitioned according to its <em>flash map</em>.
 At a high
+level, the flash map maps numeric IDs to <em>flash areas</em>.  A flash area
is a
+region of disk with the following properties:</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%">1.
An area can be fully erased without affecting any other areas.
+2. A write to one area does not restrict writes to other areas.
+</pre></div>
+
+
+<p>The boot loader uses the following flash areas:</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%"><span
style="color: #633820">#define FLASH_AREA_BOOTLOADER                    0</span>
+<span style="color: #633820">#define FLASH_AREA_IMAGE_0                       1</span>
+<span style="color: #633820">#define FLASH_AREA_IMAGE_1                       2</span>
+<span style="color: #633820">#define FLASH_AREA_IMAGE_SCRATCH                 3</span>
+</pre></div>
+
+
 <h3 id="image-slots">Image Slots</h3>
 <p>A portion of the flash memory is partitioned into two image slots: a primary
 slot and a secondary slot.  The boot loader will only run an image from the
@@ -616,65 +647,398 @@ location in flash.  If the boot loader needs to run the image resident
in the
 secondary slot, it must swap the two images in flash prior to booting.</p>
 <p>In addition to the two image slots, the boot loader requires a scratch area to
 allow for reliable image swapping.</p>
-<p>All areas used by image data (including the scratch area) must be the same
-size.</p>
+<h3 id="boot-states">Boot States</h3>
+<p>Logically, you can think of a pair of flags associated with each image slot:
+pending and confirmed.  On startup, the boot loader determines the state of the
+device by inspecting each pair of flags.  These flags have the following
+meanings:</p>
+<ul>
+<li>pending: image gets tested on next reboot; absent subsequent confirm command,
+           revert to original image on second reboot.</li>
+<li>confirmed: always use image unless excluded by a test image.</li>
+</ul>
+<p>In English, when the user wants to run the secondary image, they set the
+pending flag for the second slot and reboot the device.  On startup, the boot
+loader will swap the two images in flash, clear the secondary slot's pending
+flag, and run the newly-copied image in slot 0.  This is a temporary state; if
+the device reboots again, the boot loader swaps the images back to their
+original slots and boots into the original image.  If the user doesn't want to
+revert to the original state, they can make the current state permanent by
+setting the confirmed flag in slot 0.</p>
+<p>Switching to an alternate image is a two-step process (set + confirm) to
+prevent a device from becoming "bricked" by bad firmware.  If the device
+crashes immediately upon booting the second image, the boot loader reverts to
+the working image, rather than repeatedly rebooting into the bad image.</p>
+<p>The following set of tables illustrate the three possible states that the
+device can be in:</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%">
              | slot-0 | slot-1 |
+---------------+--------+--------|
+       pending |        |        |
+     confirmed |   X    |        |
+---------------+--------+--------&#39;
+Image 0 confirmed;               |
+No change on reboot              |
+---------------------------------&#39;
+
+               | slot-0 | slot-1 |
+---------------+--------+--------|
+       pending |        |   X    |
+     confirmed |   X    |        |
+---------------+--------+--------&#39;
+Image 0 confirmed;               |
+Test image 1 on next reboot      |
+---------------------------------&#39;
+
+               | slot-0 | slot-1 |
+---------------+--------+--------|
+       pending |        |        |
+     confirmed |        |   X    |
+---------------+--------+--------&#39;
+Testing image 0;                 |
+Revert to image 1 on next reboot |
+---------------------------------&#39;
+</pre></div>
+
+
 <h3 id="boot-vector">Boot Vector</h3>
-<p>If a vector file contains a version which doesn't correspond to an image
-actually present in flash, the boot loader deletes the file and procedes as
-though the file was not present.</p>
-<h3 id="boot-status">Boot Status</h3>
-<p>The boot status record allows the boot loader to recover in case it was reset
-while in the middle of an image swap operation.  Image swapping is discussed
-later in this document. </p>
-<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%"><span
style="color: #A90D91">struct</span> <span style="color: #000000">boot_status_table</span>
{
-    <span style="color: #177500">/**</span>
-<span style="color: #177500">     * For each field, a value of 0 means &quot;any&quot;.</span>
-<span style="color: #177500">     */</span>
-    <span style="color: #A90D91">uint8_t</span> <span style="color: #000000">bst_magic_slot0</span>;
-    <span style="color: #A90D91">uint8_t</span> <span style="color: #000000">bst_magic_scratch</span>;
-    <span style="color: #A90D91">uint8_t</span> <span style="color: #000000">bst_copy_done_slot0</span>;
-    <span style="color: #A90D91">uint8_t</span> <span style="color: #000000">bst_status_source</span>;
-};
+<p>At startup, the boot loader determines which of the above three boot states a device
is in by inspecting the boot vector.  The boot vector consists of two
+records (called "image trailers"), one written at the end of each image slot.
+An image trailer has the following structure:</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%">
0                   1                   2                   3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+~                       MAGIC (16 octets)                       ~
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+~                                                               ~
+~             Swap status (128 * min-write-size * 3)            ~
+~                                                               ~
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|   Copy done   |     0xff padding (up to min-write-sz - 1)     ~
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|   Image OK    |     0xff padding (up to min-write-sz - 1)     ~
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 </pre></div>
 
 
-<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%"><span
style="color: #A90D91">struct</span> <span style="color: #000000">boot_status</span>
{
-    <span style="color: #A90D91">uint32_t</span> <span style="color: #000000">idx</span>;
      <span style="color: #177500">/* Which area we&#39;re operating on */</span>
-    <span style="color: #A90D91">uint8_t</span> <span style="color: #000000">state</span>;
     <span style="color: #177500">/* Which part of the swapping process are we at */</span>
-};
+<p>These records are at the end of each image slot.  The offset immediately
+following such a record represents the start of the next flash area.</p>
+<p>Note: "min-write-size" is a property of the flash hardware.  If the hardware
+allows individual bytes to be written at arbitrary addresses, then
+min-write-size is 1.  If the hardware only allows writes at even addresses,
+then min-write-size is 2, and so on.</p>
+<p>The fields are defined as follows:</p>
+<ol>
+<li>
+<p>MAGIC: The following 16 bytes, written in host-byte-order:</p>
+<p>const uint32_t boot_img_magic[4] = {
+    0xf395c277,
+    0x7fefd260,
+    0x0f505235,
+    0x8079b62c,
+};</p>
+</li>
+<li>
+<p>Swap status: A series of single-byte records.  Each record corresponds to a
+flash sector in an image slot.  A swap status byte indicate the location of
+the corresponding sector data.  During an image swap, image data is moved one
+sector at a time.  The swap status is necessary for resuming a swap operation
+if the device rebooted before a swap operation completed.</p>
+</li>
+<li>
+<p>Copy done: A single byte indicating whether the image in this slot is
+complete (0x01=d&lt; bok@bok.net
+35d33
+&lt; ericmanganaro@gmail.com
+42d39
+&lt; tam@proxy.co
+55d51
+&lt; nathan@natb1.com
+110d105
+&lt; rvs@apache.orgone; 0xff=not done).</p>
+</li>
+<li>
+<p>Image OK: A single byte indicating whether the image in this slot has been
+confirmed as good by the user (0x01=confirmed; 0xff=not confirmed).</p>
+</li>
+</ol>
+<p>The boot vector records are structured around the limitations imposed by flash
+hardware.  As a consequence, they do not have a very intuitive design, and it
+is difficult to get a sense of the state of the device just by looking at the
+boot vector.  It is better to map all the possible vector states to the swap types (None,
Test, Revert)
+via a set of tables.  These tables are reproduced below.
+In these tables, the "pending" and "confirmed" flags are shown for illustrative
+purposes; they are not actually present in the boot vector.</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%">State
I
+                 | slot-0 | slot-1 |
+-----------------+--------+--------|
+           magic | Unset  | Unset  |
+        image-ok | Any    | N/A    |
+-----------------+--------+--------&#39;
+         pending |        |        |
+      confirmed  |   X    |        |
+-----------------+--------+--------&#39;
+ swap: none                        |
+-----------------------------------&#39;
+
+
+State II
+                 | slot-0 | slot-1 |
+-----------------+--------+--------|
+           magic | Any    | Good   |
+        image-ok | Any    | N/A    |
+-----------------+--------+--------&#39;
+         pending |        |   X    |
+      confirmed  |   X    |        |
+-----------------+--------+--------&#39;
+ swap: test                        |
+-----------------------------------&#39;
+
+
+State III
+                 | slot-0 | slot-1 |
+-----------------+--------+--------|
+           magic | Good   | Unset  |
+        image-ok | 0xff   | N/A    |
+-----------------+--------+--------&#39;
+         pending |        |        |
+      confirmed  |        |   X    |
+-----------------+--------+--------&#39;
+ swap: revert (test image running) |
+-----------------------------------&#39;
+
+
+State IV
+                 | slot-0 | slot-1 |
+-----------------+--------+--------|
+           magic | Good   | Unset  |
+        image-ok | 0x01   | N/A    |
+-----------------+--------+--------&#39;
+         pending |        |        |
+      confirmed  |   X    |        |
+-----------------+--------+--------&#39;
+ swap: none (confirmed test image) |
+-----------------------------------&#39;
 </pre></div>
 
 
+<h3 id="high-level-operation">High-level Operation</h3>
+<p>With the terms defined, we can now explore the boot loader's operation.  First,
+a high-level overview of the boot process is presented.  Then, the following
+sections describe each step of the process in more detail.</p>
+<p>Procedure:</p>
+<p>A. Inspect swap status region; is an interrupted swap is being resumed?
+    Yes: Complete the partial swap operation; skip to step C.
+    No: Proceed to step B.</p>
+<p>B. Insect boot vector; is a swap requested?
+    Yes.
+        1. Is the requested image valid (integrity and security check)?
+            Yes.
+                a. Perform swap operation.
+                b. Persist completion of swap procedure to boot vector.
+                c. Proceed to step C.
+            No.
+                a. Erase invalid image.
+                b. Persist failure of swap procedure to boot vector.
+                c. Proceed to step C.
+    No: Proceed to step C.</p>
+<p>C. Boot into image in slot 0.</p>
 <h3 id="image-swapping">Image Swapping</h3>
+<p>The boot loader swaps the contents of the two image slots for two reasons:
+    * User has issued an "image test" operation; the image in slot-1 should be
+      run once (state II).
+    * Test image rebooted without being confirmed; the boot loader should
+      revert to the original image currently in slot-1 (state III).</p>
 <p>If the boot vector indicates that the image in the secondary slot should be
 run, the boot loader needs to copy it to the primary slot.  The image currently
 in the primary slot also needs to be retained in flash so that it can be used
 later.  Furthermore, both images need to be recoverable if the boot loader
-resets in the middle of the process.  The two images are swapped according to
-the following procedure:</p>
-<h3 id="verifying-integrity-of-image">Verifying integrity of image</h3>
+resets in the middle of the swap operation.  The two images are swapped
+according to the following procedure:</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%">1.
Determine how many flash sectors each image slot consists of.  This
+   number must be the same for both slots.
+2. Iterate the list of sector indices in descending order (i.e., starting
+   with the greatest index); current element = &quot;index&quot;.
+    b. Erase scratch area.
+    c. Copy slot0[index] to scratch area.
+    d. Write updated swap status (i).
+
+    e. Erase slot1[index]
+    f. Copy slot0[index] to slot1[index]
+        - If these are the last sectors (i.e., first swap being perfomed),
+          copy the full sector *except* the image trailer.
+        - Else, copy entire sector contents.
+    g. Write updated swap status (ii).
+
+    h. Erase slot0[index].
+    i. Copy scratch area slot0[index].
+    j. Write updated swap status (iii).
+
+3. Persist completion of swap procedure to slot 0 image trailer.
+</pre></div>
+
+
+<p>The additional caveats in step 2f are necessary so that the slot 1 image trailer
+can be written by the user at a later time.  With the image trailer unwritten,
+the user can test the image in slot 1 (i.e., transition to state II).</p>
+<p>The particulars of step 3 vary depending on whether an image is being tested or
+reverted:
+    * test:
+        o Write slot0.copy_done = 1
+        (should now be in state III)</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%">*
revert:
+    o Write slot0.magic = BOOT_MAGIC
+    o Write slot0.copy_done = 1
+    o Write slot0.image_ok = 1
+    (should now be in state IV)
+</pre></div>
+
+
+<h3 id="swap-status">Swap Status</h3>
+<p>The swap status region allows the boot loader to recover in case it restarts in
+the middle of an image swap operation.  The swap status region consists of a
+series of single-byte records.  These records are written independently, and
+therefore must be padded according to the minimum write size imposed by the
+flash hardware.  In the below figure, a min-write-size of 1 is assumed for
+simplicity.  The structure of the swap status region is illustrated below.  In
+this figure, a min-write-size of 1 is assumed for simplicity.</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%">
0                   1                   2                   3
+ 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|sec127,state 0 |sec127,state 1 |sec127,state 2 |sec126,state 0 |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|sec126,state 1 |sec126,state 2 |sec125,state 0 |sec125,state 1 |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+|sec125,state 2 |                                               |
++-+-+-+-+-+-+-+-+                                               +
+~                                                               ~
+~               [Records for indices 124 through 1              ~
+~                                                               ~
+~               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+~               |sec000,state 0 |sec000,state 1 |sec000,state 2 |
++-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
+</pre></div>
+
+
+<p>And now, in English...</p>
+<p>Each image slot is partitioned into a sequence of flash sectors.  If we were to
+enumerate the sectors in a single slot, starting at 0, we would have a list of
+sector indices.  Since there are two image slots, each sector index would
+correspond to a pair of sectors.  For example, sector index 0 corresponds to
+the first sector in slot 0 and the first sector in slot 1.  Furthermore, we
+impose a limit of 128 indices.  If an image slot consists of more than 128
+sectors, the flash layout is not compatible with this boot loader.  Finally,
+reverse the list of indices such that the list starts with index 127 and ends
+with 0.  The swap status region is a representation of this reversed list.</p>
+<p>During a swap operation, each sector index transitions through four separate
+states:
+    0. slot 0: image 0,   slot 1: image 1,   scratch: N/A
+    1. slot 0: image 0,   slot 1: N/A,       scratch: image 1 (1-&gt;s, erase 1)
+    2. slot 0: N/A,       slot 1: image 0,   scratch: image 1 (0-&gt;1, erase 0)
+    3. slot 0: image 1,   slot 1: image 0,   scratch: N/A     (s-&gt;0)</p>
+<p>Each time a sector index transitions to a new state, the boot loader writes a
+record to the swap status region.  Logically, the boot loader only needs one
+record per sector index to keep track of the current swap state.  However, due
+to limitations imposed by flash hardware, a record cannot be overwritten when
+an index's state changes.  To solve this problem, the boot loader uses three
+records per sector index rather than just one.</p>
+<p>Each sector-state pair is represented as a set of three records.  The record
+values map to the above four states as follows</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%">
       | rec0 | rec1 | rec2
+--------+------+------+------
+state 0 | 0xff | 0xff | 0xff
+state 1 | 0x01 | 0xff | 0xff
+state 2 | 0x01 | 0x02 | 0xff
+state 3 | 0x01 | 0x02 | 0x03
+</pre></div>
+
+
+<p>The swap status region can accommodate 128 sector indices.  Hence, the size of
+the region, in bytes, is 128 * min-write-size * 3.  The number 128 is chosen
+somewhat arbitrarily and will likely be made configurable.  The only
+requirement for the index count is that is is great enough to account for a
+maximum-sized image (i.e., at least as great as the total sector count in an
+image slot).  If a device's image slots use less than 128 sectors, the first
+record that gets written will be somewhere in the middle of the region.  For
+example, if a slot uses 64 sectors, the first sector index that gets swapped is
+63, which corresponds to the exact halfway point within the region.</p>
 <h3 id="reset-recovery">Reset Recovery</h3>
 <p>If the boot loader resets in the middle of a swap operation, the two images may
 be discontiguous in flash.  Bootutil recovers from this condition by using the
-boot status file to determine how the image parts are placed in flash.</p>
-<p>If the boot status file indicates that the images are not contiguous, bootutil
-completes the swap operation that was in progress when the system was reset.
-In other words, it applies the procedure defined in the previous section,
-moving image 1 into slot 0 and image 0 into slot 1.  If the boot status file
-indicates that an image part is present in the scratch area, this part is
-copied into the correct location by starting at step e or step h in the
+boot vector to determine how the image parts are distributed in flash.</p>
+<p>The first step is determine where the relevant swap status region is located.
+Because this region is embedded within the image slots, its location in flash
+changes during a swap operation.  The below set of tables map boot vector
+contents to swap status location.  In these tables, the "source" field
+indicates where the swap status region is located.</p>
+<div class="codehilite" style="background: #ffffff"><pre style="line-height: 125%">
         | slot-0     | scratch    |
+----------+------------+------------|
+    magic | Good       | Any        |
+copy-done | 0x01       | N/A        |
+----------+------------+------------&#39;
+source: none                        |
+------------------------------------&#39;
+
+          | slot-0     | scratch    |
+----------+------------+------------|
+    magic | Good       | Any        |
+copy-done | 0xff       | N/A        |
+----------+------------+------------&#39;
+source: slot 0                      |
+------------------------------------&#39;
+
+          | slot-0     | scratch    |
+----------+------------+------------|
+    magic | Any        | Good       |
+copy-done | Any        | N/A        |
+----------+------------+------------&#39;
+source: scratch                     |
+------------------------------------&#39;
+
+          | slot-0     | scratch    |
+----------+------------+------------|
+    magic | Unset      | Any        |
+copy-done | 0xff       | N/A        |
+----------+------------+------------|
+source: varies                      |
+------------------------------------+------------------------------+
+This represents one of two cases:                                  |
+o No swaps ever (no status to read, so no harm in checking).       |
+o Mid-revert; status in slot 0.                                    |
+-------------------------------------------------------------------&#39;
+</pre></div>
+
+
+<p>If the swap status region indicates that the images are not contiguous,
+bootutil completes the swap operation that was in progress when the system was
+reset.  In other words, it applies the procedure defined in the previous
+section, moving image 1 into slot 0 and image 0 into slot 1.  If the boot
+status file indicates that an image part is present in the scratch area, this
+part is copied into the correct location by starting at step e or step h in the
 area-swap procedure, depending on whether the part belongs to image 0 or image
 1.</p>
 <p>After the swap operation has been completed, the boot loader proceeds as though
 it had just been started.</p>
-<h3 id="api">API</h3>
-<h3 id="example">Example</h3>
-<h3 id="dependencies">Dependencies</h3>
-<p>The bootloader depends on the following OS kernel functions:</p>
-<p>The bootloader does not depend on any flash file system.</p>
-<h3 id="list-of-functions">List of Functions</h3>
-<p><Comments such as these instructions are placed within angle brackets. List all
the functions here. Note how the anchors work. You put the text you want to show up as a link
within [] and the relevant #heading within (). Note that the header has to have at least 2
words for the anchor to work - that's how it is. "no-highlight" disables syntax highlighting.
You can enable it for a particular language by specifying what the language is instead of
"no-highlight". Be warned that this highlighting or no-highlighting specification may not
show up nicely on Mou.></p>
-<p>The functions available in bootloader are:</p>
+<h3 id="integrity-check">Integrity Check</h3>
+<p>An image is checked for integrity immediately before it gets copied into the
+primary slot.  If the boot loader doesn't perform an image swap, then it
+doesn't perform an integrity check.</p>
+<p>During the integrity check, the boot loader verifies the following aspects of
+an image:
+    * 32-bit magic number must be correct (0x96f3b83c).
+    * Image must contain a SHA256 TLV.
+    * Calculated SHA256 must matche SHA256 TLV contents.
+    * Image <em>may</em> contain a signature TLV.  If it does, its contents must
be
+      verifiable using a key embedded in the boot loader.</p>
+<h3 id="image-signing-and-verification">Image Signing and Verification</h3>
+<p>As indicated above, the final step of the integrity check is signature
+verification.  The boot loader can have one or more public keys embedded in it
+at build time.  During signature verification, the boot loader verifies that an
+image was signed with a private key that corresponds to one of its public keys.
+The image signature TLV indicates the index of the key that is has been signed
+with.  The boot loader uses this index to identify the corresponding public
+key.</p>
+<p>For information on embedding public keys in the boot loader, as well as
+producing signed images, see: boot/bootutil/signed_images.md </p>
                         
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