Jorijn van der Graaf
93692c9505
packaging/make-sysroot.sh populates a minimal Alpine aarch64 rootfs with apk-tools-static, so a cross build needs no root, no qemu and no phone. Adapted from imsd's, plus glib-dev for the bus layer that is coming. Building against a real Alpine sysroot means the binary links dynamically against the phone's own musl and libc++ -- libc++, libc++abi, libunwind and libgcc_s are all already on pmOS. The harness this daemon replaces had to be built -static, but only because it was built with the host's glibc toolchain; that constraint was never about the target. The pipeline is proven end to end rather than assumed: the cross-built binary runs on the phone, and all five core suites pass there too -- on the real hardware, not just under qemu-aarch64. That matters for modules that are almost entirely little-endian field packing and offset arithmetic.
107 lines
4.9 KiB
Markdown
107 lines
4.9 KiB
Markdown
# fingerprintd
|
|
|
|
Fingerprint daemon for the Fairphone 6 (`milos`, SM7635) on mainline Linux.
|
|
|
|
## Why a daemon
|
|
|
|
The sensor is a FocalTech FT9391 on a TrustZone-owned SPI bus. `spi@a88000` is
|
|
`disabled` in both the mainline and the stock Android device tree, and the pads
|
|
are XPU-protected — touching them from the normal world is an instant SError
|
|
reboot. Every pixel the sensor produces stays inside the TEE: capture,
|
|
preprocessing, the classifier, enrolment and matching all run in the `focal64`
|
|
trustlet, which reports a matched finger id and nothing else. A libfprint-style
|
|
driver cannot exist on this device.
|
|
|
|
So the normal world's job is narrower than usual, and none of it is per-request
|
|
work:
|
|
|
|
* **Power the sensor.** Rail on gpio29, reset on gpio74, interrupt on gpio75 —
|
|
the same division of labour the downstream driver uses. One sensor reset buys
|
|
exactly one trustlet init, so whatever powers the sensor must also hold the
|
|
session open.
|
|
* **Be QTEE's filesystem.** QTEE cannot reach storage. When the trustlet saves
|
|
or loads a template it calls back into the normal world through the `gpfile`
|
|
(0x7000) and RPMB (0x2000) listeners, and expects them served. QTEE does the
|
|
crypto and the anti-rollback; this side moves opaque bytes and performs the
|
|
authenticated RPMB transactions against the UFS device.
|
|
* **Speak a biometrics API.** The daemon owns `net.reactivated.Fprint`, so
|
|
`pam_fprintd`, the Plasma fingerprint KCM and `fprintd-enroll(1)` work
|
|
against it unmodified.
|
|
|
|
A listener registration is held for as long as the process lives and QTEE's
|
|
listener table is global to the boot, so this has to be one long-lived process
|
|
rather than a tool spawned per request.
|
|
|
|
## Layout
|
|
|
|
```
|
|
interfaces/ Fingerprintd{,-Sfs}.cppm the core: pure C++ modules
|
|
implementations/main.cpp the daemon shell
|
|
tests/ one suite per core module
|
|
```
|
|
|
|
`fingerprintd-core` is a static library with no GLib, no libqcomtee and no
|
|
system headers. Everything in it is a wire format or a state machine that was
|
|
recovered by reverse-engineering, so all of it is pinned by tests that run on a
|
|
dev box with no phone, no TEE and no sensor. The daemon shell holds everything
|
|
that touches hardware.
|
|
|
|
## Build
|
|
|
|
```sh
|
|
crafter-build # bin/fingerprintd-<target>-<march>/fingerprintd
|
|
crafter-build test # the unit suites
|
|
```
|
|
|
|
Cross-compiling for the phone:
|
|
|
|
```sh
|
|
packaging/make-sysroot.sh # once; no root, no qemu, no device
|
|
crafter-build -- --target=aarch64-alpine-linux-musl \
|
|
--sysroot=~/.cache/fingerprintd/sysroot-aarch64-alpine \
|
|
--march=armv8-a --mtune=generic
|
|
crafter-build test --target=aarch64-alpine-linux-musl --sysroot=... \
|
|
--march=armv8-a --mtune=generic # runs the suites under qemu-aarch64
|
|
```
|
|
|
|
The result links dynamically against the phone's own musl and libc++
|
|
(`libc++`, `libc++abi`, `libunwind`, `libgcc_s`, all already present on pmOS).
|
|
The research harness this replaces had to be built `-static`, but only because
|
|
it was built with the host's glibc toolchain — that constraint does not apply
|
|
to a real Alpine sysroot.
|
|
|
|
Verified on the device: all five suites pass cross-built and run **on the phone
|
|
itself**, not only under emulation.
|
|
|
|
## Status
|
|
|
|
**The core is complete; the daemon does not run yet.** Everything was ported
|
|
out of the research harness that first made the sensor work (`utilities/fpta.c`
|
|
in the fp6 repo), one module at a time, each landing with its tests before the
|
|
next started.
|
|
|
|
| module | what it holds |
|
|
|---|---|
|
|
| `:Sfs` | the gpfile frame — the read/write offset split, the `O_TRUNC` guard, root mapping, path-traversal rejection |
|
|
| `:Rpmb` | request/reply framing, the bytes-transferred out-parameter, JEDEC result codes, chunking, the one-time-programmable key guard |
|
|
| `:Ta` | command surface, the 740-byte event context, capture flags, SAVE_DATA masks, enrol/auth payloads, the error table, the verdict rule |
|
|
| `:Engine` | baseline calibration, touch edges, enrolment progress, and the accounting |
|
|
| `:Store` | the finger name map |
|
|
|
|
Every constant that was recovered by reverse-engineering carries where it came
|
|
from, and the tests are written to fail if it is undone rather than to restate
|
|
it. Several replay real captures: two SFS containers off the phone, and three
|
|
recorded authentication runs.
|
|
|
|
Next is the I/O shell — the TEE session, the sensor rail, the RPMB device and
|
|
the bus — which is the first part that cannot be validated without hardware.
|
|
|
|
The working reference enrols a finger, keeps it across a reboot, and matches it
|
|
with zero false accepts; the port exists to turn that into a service rather
|
|
than to rediscover it.
|
|
|
|
## Runtime dependencies, not carried here
|
|
|
|
The `focal64` trustlet is proprietary and is **not** in this repo. It is
|
|
extracted from the device's own stock Android partition on first boot by the
|
|
`fp6-vendor-blobs` mechanism, the same way the audio firmware is.
|