The version floor I added two commits ago was one release too low.
drmtap_open_render and drmtap_convert_dmabuf are 0.4.9, but drmtap_grab_desc is
0.4.10, so a genuine 0.4.9 library passed the version gate and was then refused
by the symbol gate with a message that called it a stale or pre-release build,
which it is not. The floor is 0.4.10 now, the release where the whole split API
exists, and the test lists 0.4.9 among the rejected versions with the reason.
ExportLedger is deleted. DRM_FD_ELISION was false, so should_send_fd returned
true at its first branch and about sixty lines of eviction and epoch machinery
were unreachable, untested, in a security sensitive file. Why it was disabled
is worth keeping, so here it is: eliding the fd on an fb_id the converter has
already imported looks free, but the kernel can recycle an fb_id onto a
different buffer with identical geometry and modifier, and the exporter cannot
see the dma-buf inode that would tell the difference, so the elision can serve
a stale EGLImage. Sending it is cheap, the converter imports once per buffer and
closes the surplus fd, and libdrmtap's own cache keys on fb_id AND inode and can
only re-import when it is handed a real fd. That reasoning now lives here
instead of in dead code.
The rest:
- num_planes is clamped on the consumer before it reaches the C descriptor. The
producer normalizes it and must be root, so this is only defense in depth, but
the wire is the one place the value arrives from another process.
- warm_availability returns early on X11. Nothing there can consume a DRM
stream, and probing makes the ROOT service open DRM readers, so an X11 host
running a drm build was paying that at every startup for a path it can never
take.
- drm_cursor_id no longer clones the cursor. The cursor service polls it at
frame cadence to compare eight bytes, and a 256x256 cursor is 256 KiB.
- The premultiplied ARGB pass-through is now documented as matching the XFixes
path, since that is why it is correct rather than an oversight.
- cfg hygiene: input_service.rs uses all(target_os = "linux", feature = "drm")
like every other site, and active_uid_cached is gated with the feature too,
which also removes a dead-code warning from drm-off Linux builds.
- Nits: DrmConn is pub(crate) like its constructors, new_drm_listener is no
longer async with nothing to await, and the two anyhow! plus return Err pairs
are bail! as the codebase writes them.
- DRM_CAPTURE_SECURITY.md moves to docs/ with the other docs, and its "no
privileged child process is ever spawned" claim is corrected: an empty
helper_path is not a disable switch in the C, find_helper searches six fixed
paths and would exec one if the direct export ever failed. It is unreachable
here for two independent reasons, the root service holds CAP_SYS_ADMIN so the
direct path succeeds and the package builds no helper at all, and the paths
are root-writable only, so the accurate statement is that this package never
installs one, not that it can never happen.
- The comments that narrated the review rather than the code are rewritten to
say what the code does. One of them had also drifted: the convert context is
opened before we answer with DrmStart, not before the handshake.
Both configs build with no new warnings, 100 tests pass.
The CPU fallback moved a scanout four times: the producer packed it, the kernel
carried it, next_raw allocated and zeroed a fresh buffer to read it into, and the
consumer copied that into the slot. At 4K30 the last two are about 8 GB/s of
memory traffic that does nothing.
next_raw_into reads the body straight into a buffer the caller owns, so the
kernel copy lands where the frame is going to live, and resize costs nothing once
a buffer has seen one frame of that size. The frame buffers then circulate
instead of being freed and reallocated: whatever a new frame displaces goes back
on offer, both when the encoder consumes one and when a frame is superseded
before anyone reads it. The dma-buf path still copies once, because the convert
output is borrowed from the render context and only lives until the next convert,
but it copies into a recycled buffer and does it outside the slot lock, so a
multi-megabyte memcpy no longer holds the encoder off the slot.
Steady state is now one allocation for the whole session on both paths, and the
CPU path carries the pixels twice instead of four times.
The cursor body reads into its own buffer and is moved into the cursor cache
rather than copied; it is small and rare, so it stays out of the frame recycler.
Two tests: the raw body round trip now also covers a shorter body reusing the
buffer, so a stale tail cannot survive into it, and a new test asserts the frame
buffers circulate by allocation identity rather than by inspection. 100 tests
pass, both configs build.
M1: authorization is now done on the blocking pool. It reads the active session
uid, which on a cache miss forks loginctl, and the socket is 0666 so any local
uid can make us do it. The same call exists for _service, but this runtime is
shared by every live capture stream, so a stall here hitches frames instead of
delaying one config sync.
M2: the handshake was a loop that ignored unexpected messages, which restarted
the ten second budget on each one, so a peer sending junk just inside the timeout
held a worker thread and one of the eight connection slots for as long as it
liked, and eight of them denied DRM capture entirely. It is one receive now, and
anything that is not DrmStart closes the connection: the consumer answers the
display list with DrmStart and nothing else, so there is nothing legitimate to
skip past.
M8: dropped the extra unauthorized-connection warn. log_rejected_service_connection
inside the authorization already logs the rejection with the peer and active uid
and rate limits it to one line per five seconds, which is exactly what a
world-connectable socket needs; the second line had no throttle and handed anyone
who can connect an unbounded log write.
Both configs build, 92 tests pass.
src/ipc.rs is the file every unrelated IPC change has to be read through, and
this branch had grown it from 2227 lines to 4112. Move the DRM half out, into
the same #[path] submodule form the file already uses for ipc/auth.rs and
ipc/fs.rs, so it lands as ipc/drm.rs beside them.
What moves: the two payload structs, the producer that runs in the root
--service, and the bespoke SCM_RIGHTS framing the channel needs because
Framed/BytesCodec cannot carry ancillary data, plus their tests. What stays is
the Data variants, which belong to a shared enum and cannot live anywhere else,
and three re-exports so every existing call site keeps the path it already uses.
ipc.rs is 2285 lines now, 58 above upstream instead of 1885. The move is
content-identical: the only edits are the 39 per-item cfg attributes, redundant
now that the module is gated once at its declaration, and the test module cfg
that becomes a plain cfg(test). Checked by extracting the moved ranges from the
previous commit and comparing them line by line against the new file. Both
configs build with no new warnings and the same 92 tests pass, 14 of them the
drm ones that moved.
