ECE 441 · Week 4

Modern Computer: Secure Infrastructure

A rebuild has an identity: the hash of its image, and one console line only that build prints.

Week 4 of 10 · Solitude

ECE 441 · Week 4 · In class

A rebuild has an identity: the hash of its image, and one console line only that build prints.

A build is reproducible when an independent rebuild from the same source gives the same bytes.
That is what lets anyone check the binary on a board against its source.
The toolchain command for a clean rebuild, and the directory it must be run in, are on the bench card the instructor posts for this week.
Copy both.

ECE 441 · Week 4 · In class

A rebuild has an identity: the hash of its image, and one console line only that build prints. (continued)

Those two are the homework.
These notes do not print a second command.

ECE 441 · Week 4 · In class

You and your partner each rebuild from a clean tree on a different bench PC and compare the two image hashes.

If they differ, find the first bytes that differ and name the cause.
Boot the board from your build, record the console line the stock image does not print, and write the commands a classmate uses to rebuild and boot it.
Reboot from the same build and confirm the same identity line.

ECE 441 · Week 4 · Lecture

Why the same bytes matter

A build is reproducible when an independent rebuild of the same source gives the same bytes.
Then anyone can check the binary on a board against its source: rebuild, hash, compare.
If the bytes differ, either the source differs or the build leaks something from the machine.
Finding and naming that leak is the work, not a failure.

ECE 441 · Week 4 · Lecture

An image's identity

sha256sum <image>
git -C <tree> rev-parse HEAD
git -C <tree> status --porcelain   # empty means clean

The hash names the bytes; the commit names the source they came from.
A tree with local edits builds a different image from the same commit.
U-Boot adds -dirty to its version when the tree has uncommitted changes.

ECE 441 · Week 4 · Lecture

What leaks into a build

Leak	How it shows	Usual fix
Build time	A date string in the image	`SOURCE_DATE_EPOCH`
Build path	Your home directory in the image	Same path, or a prefix map
Toolchain	Different code in many places	The same compiler version
Local edits	`-dirty`, or real changes	A clean tree

ECE 441 · Week 4 · Lecture

Pinning the date

# the time of the last commit, not the time of the build
export SOURCE_DATE_EPOCH=$(git -C TREE log -1 --format=%ct)
date -u -d @"$SOURCE_DATE_EPOCH"

U-Boot's build uses SOURCE_DATE_EPOCH, when set, for the date in its banner.
Both partners must set it to the same value, or the date alone makes the images differ.

ECE 441 · Week 4 · Lecture

Finding the first difference

sha256sum mine.bin theirs.bin
cmp mine.bin theirs.bin          # first differing byte
cmp -l mine.bin theirs.bin | wc -l   # how many differ
strings -t x mine.bin | grep -i '<text>'

cmp gives the offset; strings -t x shows what text sits near it.
diffoscope, if installed, explains the difference in one report.

ECE 441 · Week 4 · Lab

Lab

From a clean tree, rebuild the open bootloader on the bench PC and record the hash of the image; your partner rebuilds it independently on another bench PC.
Compare the two hashes. If they differ, find the first differing bytes and name the cause, such as a timestamp, a path, or a toolchain version. Boot the board from your build and record the console line that only that build prints. Second pass: reboot from that same build and confirm the same identity line.

ECE 441 · Week 4 · Lab

Lab (continued)

Assemble the lab hand-in: The two image hashes, identical or with the first difference and its cause, the console line that only your build prints, and the commands a classmate uses to rebuild and boot it.

ECE 441 · Week 4 · Lab

Hand in

The two image hashes, identical or with the first difference and its cause, the console line that only your build prints, and the commands a classmate uses to rebuild and boot it.

ECE 441 · Week 4 · Homework

Homework

Write the toolchain command the lecture gives for a clean rebuild, and the directory it must be run in.

ECE 441 · Week 4 · Homework

Turn in

The command and the directory.

ECE 441 · Week 4 · Read

Read

On the lab site: Module 4, Loading the bootloader into SRAM, and Module 6, Software in DRAM, describe the stages. This week grades the rebuild. The basic modules stop at Module 12.

ECE 441 · Week 4 · Next

Week 5

A method names instruments, steps, and what to record.