1 // SPDX-License-Identifier: GPL-2.0-only
2 // Copyright (C) 2022 Linutronix GmbH, John Ogness
3 // Copyright (C) 2022 Intel, Thomas Gleixner
4
5 #include <linux/kernel.h>
6 #include <linux/console.h>
7 #include <linux/delay.h>
8 #include <linux/slab.h>
9 #include "internal.h"
10 /*
11 * Printk console printing implementation for consoles which does not depend
12 * on the legacy style console_lock mechanism.
13 *
14 * The state of the console is maintained in the "nbcon_state" atomic
15 * variable.
16 *
17 * The console is locked when:
18 *
19 * - The 'prio' field contains the priority of the context that owns the
20 * console. Only higher priority contexts are allowed to take over the
21 * lock. A value of 0 (NBCON_PRIO_NONE) means the console is not locked.
22 *
23 * - The 'cpu' field denotes on which CPU the console is locked. It is used
24 * to prevent busy waiting on the same CPU. Also it informs the lock owner
25 * that it has lost the lock in a more complex scenario when the lock was
26 * taken over by a higher priority context, released, and taken on another
27 * CPU with the same priority as the interrupted owner.
28 *
29 * The acquire mechanism uses a few more fields:
30 *
31 * - The 'req_prio' field is used by the handover approach to make the
32 * current owner aware that there is a context with a higher priority
33 * waiting for the friendly handover.
34 *
35 * - The 'unsafe' field allows to take over the console in a safe way in the
36 * middle of emitting a message. The field is set only when accessing some
37 * shared resources or when the console device is manipulated. It can be
38 * cleared, for example, after emitting one character when the console
39 * device is in a consistent state.
40 *
41 * - The 'unsafe_takeover' field is set when a hostile takeover took the
42 * console in an unsafe state. The console will stay in the unsafe state
43 * until re-initialized.
44 *
45 * The acquire mechanism uses three approaches:
46 *
47 * 1) Direct acquire when the console is not owned or is owned by a lower
48 * priority context and is in a safe state.
49 *
50 * 2) Friendly handover mechanism uses a request/grant handshake. It is used
51 * when the current owner has lower priority and the console is in an
52 * unsafe state.
53 *
54 * The requesting context:
55 *
56 * a) Sets its priority into the 'req_prio' field.
57 *
58 * b) Waits (with a timeout) for the owning context to unlock the
59 * console.
60 *
61 * c) Takes the lock and clears the 'req_prio' field.
62 *
63 * The owning context:
64 *
65 * a) Observes the 'req_prio' field set on exit from the unsafe
66 * console state.
67 *
68 * b) Gives up console ownership by clearing the 'prio' field.
69 *
70 * 3) Unsafe hostile takeover allows to take over the lock even when the
71 * console is an unsafe state. It is used only in panic() by the final
72 * attempt to flush consoles in a try and hope mode.
73 *
74 * Note that separate record buffers are used in panic(). As a result,
75 * the messages can be read and formatted without any risk even after
76 * using the hostile takeover in unsafe state.
77 *
78 * The release function simply clears the 'prio' field.
79 *
80 * All operations on @console::nbcon_state are atomic cmpxchg based to
81 * handle concurrency.
82 *
83 * The acquire/release functions implement only minimal policies:
84 *
85 * - Preference for higher priority contexts.
86 * - Protection of the panic CPU.
87 *
88 * All other policy decisions must be made at the call sites:
89 *
90 * - What is marked as an unsafe section.
91 * - Whether to spin-wait if there is already an owner and the console is
92 * in an unsafe state.
93 * - Whether to attempt an unsafe hostile takeover.
94 *
95 * The design allows to implement the well known:
96 *
97 * acquire()
98 * output_one_printk_record()
99 * release()
100 *
101 * The output of one printk record might be interrupted with a higher priority
102 * context. The new owner is supposed to reprint the entire interrupted record
103 * from scratch.
104 */
105
106 /**
107 * nbcon_state_set - Helper function to set the console state
108 * @con: Console to update
109 * @new: The new state to write
110 *
111 * Only to be used when the console is not yet or no longer visible in the
112 * system. Otherwise use nbcon_state_try_cmpxchg().
113 */
114 static inline void nbcon_state_set(struct console *con, struct nbcon_state *new)
115 {
116 atomic_set(&ACCESS_PRIVATE(con, nbcon_state), new->atom);
117 }
118
119 /**
120 * nbcon_state_read - Helper function to read the console state
121 * @con: Console to read
122 * @state: The state to store the result
123 */
124 static inline void nbcon_state_read(struct console *con, struct nbcon_state *state)
125 {
126 state->atom = atomic_read(&ACCESS_PRIVATE(con, nbcon_state));
127 }
128
129 /**
130 * nbcon_state_try_cmpxchg() - Helper function for atomic_try_cmpxchg() on console state
131 * @con: Console to update
132 * @cur: Old/expected state
133 * @new: New state
134 *
135 * Return: True on success. False on fail and @cur is updated.
136 */
137 static inline bool nbcon_state_try_cmpxchg(struct console *con, struct nbcon_state *cur,
138 struct nbcon_state *new)
139 {
140 return atomic_try_cmpxchg(&ACCESS_PRIVATE(con, nbcon_state), &cur->atom, new->atom);
141 }
142
143 /**
144 * nbcon_seq_read - Read the current console sequence
145 * @con: Console to read the sequence of
146 *
147 * Return: Sequence number of the next record to print on @con.
148 */
149 u64 nbcon_seq_read(struct console *con)
150 {
151 unsigned long nbcon_seq = atomic_long_read(&ACCESS_PRIVATE(con, nbcon_seq));
152
153 return __ulseq_to_u64seq(prb, nbcon_seq);
154 }
155
156 /**
157 * nbcon_seq_force - Force console sequence to a specific value
158 * @con: Console to work on
159 * @seq: Sequence number value to set
160 *
161 * Only to be used during init (before registration) or in extreme situations
162 * (such as panic with CONSOLE_REPLAY_ALL).
163 */
164 void nbcon_seq_force(struct console *con, u64 seq)
165 {
166 /*
167 * If the specified record no longer exists, the oldest available record
168 * is chosen. This is especially important on 32bit systems because only
169 * the lower 32 bits of the sequence number are stored. The upper 32 bits
170 * are derived from the sequence numbers available in the ringbuffer.
171 */
172 u64 valid_seq = max_t(u64, seq, prb_first_valid_seq(prb));
173
174 atomic_long_set(&ACCESS_PRIVATE(con, nbcon_seq), __u64seq_to_ulseq(valid_seq));
175
176 /* Clear con->seq since nbcon consoles use con->nbcon_seq instead. */
177 con->seq = 0;
178 }
179
180 /**
181 * nbcon_seq_try_update - Try to update the console sequence number
182 * @ctxt: Pointer to an acquire context that contains
183 * all information about the acquire mode
184 * @new_seq: The new sequence number to set
185 *
186 * @ctxt->seq is updated to the new value of @con::nbcon_seq (expanded to
187 * the 64bit value). This could be a different value than @new_seq if
188 * nbcon_seq_force() was used or the current context no longer owns the
189 * console. In the later case, it will stop printing anyway.
190 */
191 static void nbcon_seq_try_update(struct nbcon_context *ctxt, u64 new_seq)
192 {
193 unsigned long nbcon_seq = __u64seq_to_ulseq(ctxt->seq);
194 struct console *con = ctxt->console;
195
196 if (atomic_long_try_cmpxchg(&ACCESS_PRIVATE(con, nbcon_seq), &nbcon_seq,
197 __u64seq_to_ulseq(new_seq))) {
198 ctxt->seq = new_seq;
199 } else {
200 ctxt->seq = nbcon_seq_read(con);
201 }
202 }
203
204 /**
205 * nbcon_context_try_acquire_direct - Try to acquire directly
206 * @ctxt: The context of the caller
207 * @cur: The current console state
208 *
209 * Acquire the console when it is released. Also acquire the console when
210 * the current owner has a lower priority and the console is in a safe state.
211 *
212 * Return: 0 on success. Otherwise, an error code on failure. Also @cur
213 * is updated to the latest state when failed to modify it.
214 *
215 * Errors:
216 *
217 * -EPERM: A panic is in progress and this is not the panic CPU.
218 * Or the current owner or waiter has the same or higher
219 * priority. No acquire method can be successful in
220 * this case.
221 *
222 * -EBUSY: The current owner has a lower priority but the console
223 * in an unsafe state. The caller should try using
224 * the handover acquire method.
225 */
226 static int nbcon_context_try_acquire_direct(struct nbcon_context *ctxt,
227 struct nbcon_state *cur)
228 {
229 unsigned int cpu = smp_processor_id();
230 struct console *con = ctxt->console;
231 struct nbcon_state new;
232
233 do {
234 if (other_cpu_in_panic())
235 return -EPERM;
236
237 if (ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio)
238 return -EPERM;
239
240 if (cur->unsafe)
241 return -EBUSY;
242
243 /*
244 * The console should never be safe for a direct acquire
245 * if an unsafe hostile takeover has ever happened.
246 */
247 WARN_ON_ONCE(cur->unsafe_takeover);
248
249 new.atom = cur->atom;
250 new.prio = ctxt->prio;
251 new.req_prio = NBCON_PRIO_NONE;
252 new.unsafe = cur->unsafe_takeover;
253 new.cpu = cpu;
254
255 } while (!nbcon_state_try_cmpxchg(con, cur, &new));
256
257 return 0;
258 }
259
260 static bool nbcon_waiter_matches(struct nbcon_state *cur, int expected_prio)
261 {
262 /*
263 * The request context is well defined by the @req_prio because:
264 *
265 * - Only a context with a higher priority can take over the request.
266 * - There are only three priorities.
267 * - Only one CPU is allowed to request PANIC priority.
268 * - Lower priorities are ignored during panic() until reboot.
269 *
270 * As a result, the following scenario is *not* possible:
271 *
272 * 1. Another context with a higher priority directly takes ownership.
273 * 2. The higher priority context releases the ownership.
274 * 3. A lower priority context takes the ownership.
275 * 4. Another context with the same priority as this context
276 * creates a request and starts waiting.
277 */
278
279 return (cur->req_prio == expected_prio);
280 }
281
282 /**
283 * nbcon_context_try_acquire_requested - Try to acquire after having
284 * requested a handover
285 * @ctxt: The context of the caller
286 * @cur: The current console state
287 *
288 * This is a helper function for nbcon_context_try_acquire_handover().
289 * It is called when the console is in an unsafe state. The current
290 * owner will release the console on exit from the unsafe region.
291 *
292 * Return: 0 on success and @cur is updated to the new console state.
293 * Otherwise an error code on failure.
294 *
295 * Errors:
296 *
297 * -EPERM: A panic is in progress and this is not the panic CPU
298 * or this context is no longer the waiter.
299 *
300 * -EBUSY: The console is still locked. The caller should
301 * continue waiting.
302 *
303 * Note: The caller must still remove the request when an error has occurred
304 * except when this context is no longer the waiter.
305 */
306 static int nbcon_context_try_acquire_requested(struct nbcon_context *ctxt,
307 struct nbcon_state *cur)
308 {
309 unsigned int cpu = smp_processor_id();
310 struct console *con = ctxt->console;
311 struct nbcon_state new;
312
313 /* Note that the caller must still remove the request! */
314 if (other_cpu_in_panic())
315 return -EPERM;
316
317 /*
318 * Note that the waiter will also change if there was an unsafe
319 * hostile takeover.
320 */
321 if (!nbcon_waiter_matches(cur, ctxt->prio))
322 return -EPERM;
323
324 /* If still locked, caller should continue waiting. */
325 if (cur->prio != NBCON_PRIO_NONE)
326 return -EBUSY;
327
328 /*
329 * The previous owner should have never released ownership
330 * in an unsafe region.
331 */
332 WARN_ON_ONCE(cur->unsafe);
333
334 new.atom = cur->atom;
335 new.prio = ctxt->prio;
336 new.req_prio = NBCON_PRIO_NONE;
337 new.unsafe = cur->unsafe_takeover;
338 new.cpu = cpu;
339
340 if (!nbcon_state_try_cmpxchg(con, cur, &new)) {
341 /*
342 * The acquire could fail only when it has been taken
343 * over by a higher priority context.
344 */
345 WARN_ON_ONCE(nbcon_waiter_matches(cur, ctxt->prio));
346 return -EPERM;
347 }
348
349 /* Handover success. This context now owns the console. */
350 return 0;
351 }
352
353 /**
354 * nbcon_context_try_acquire_handover - Try to acquire via handover
355 * @ctxt: The context of the caller
356 * @cur: The current console state
357 *
358 * The function must be called only when the context has higher priority
359 * than the current owner and the console is in an unsafe state.
360 * It is the case when nbcon_context_try_acquire_direct() returns -EBUSY.
361 *
362 * The function sets "req_prio" field to make the current owner aware of
363 * the request. Then it waits until the current owner releases the console,
364 * or an even higher context takes over the request, or timeout expires.
365 *
366 * The current owner checks the "req_prio" field on exit from the unsafe
367 * region and releases the console. It does not touch the "req_prio" field
368 * so that the console stays reserved for the waiter.
369 *
370 * Return: 0 on success. Otherwise, an error code on failure. Also @cur
371 * is updated to the latest state when failed to modify it.
372 *
373 * Errors:
374 *
375 * -EPERM: A panic is in progress and this is not the panic CPU.
376 * Or a higher priority context has taken over the
377 * console or the handover request.
378 *
379 * -EBUSY: The current owner is on the same CPU so that the hand
380 * shake could not work. Or the current owner is not
381 * willing to wait (zero timeout). Or the console does
382 * not enter the safe state before timeout passed. The
383 * caller might still use the unsafe hostile takeover
384 * when allowed.
385 *
386 * -EAGAIN: @cur has changed when creating the handover request.
387 * The caller should retry with direct acquire.
388 */
389 static int nbcon_context_try_acquire_handover(struct nbcon_context *ctxt,
390 struct nbcon_state *cur)
391 {
392 unsigned int cpu = smp_processor_id();
393 struct console *con = ctxt->console;
394 struct nbcon_state new;
395 int timeout;
396 int request_err = -EBUSY;
397
398 /*
399 * Check that the handover is called when the direct acquire failed
400 * with -EBUSY.
401 */
402 WARN_ON_ONCE(ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio);
403 WARN_ON_ONCE(!cur->unsafe);
404
405 /* Handover is not possible on the same CPU. */
406 if (cur->cpu == cpu)
407 return -EBUSY;
408
409 /*
410 * Console stays unsafe after an unsafe takeover until re-initialized.
411 * Waiting is not going to help in this case.
412 */
413 if (cur->unsafe_takeover)
414 return -EBUSY;
415
416 /* Is the caller willing to wait? */
417 if (ctxt->spinwait_max_us == 0)
418 return -EBUSY;
419
420 /*
421 * Setup a request for the handover. The caller should try to acquire
422 * the console directly when the current state has been modified.
423 */
424 new.atom = cur->atom;
425 new.req_prio = ctxt->prio;
426 if (!nbcon_state_try_cmpxchg(con, cur, &new))
427 return -EAGAIN;
428
429 cur->atom = new.atom;
430
431 /* Wait until there is no owner and then acquire the console. */
432 for (timeout = ctxt->spinwait_max_us; timeout >= 0; timeout--) {
433 /* On successful acquire, this request is cleared. */
434 request_err = nbcon_context_try_acquire_requested(ctxt, cur);
435 if (!request_err)
436 return 0;
437
438 /*
439 * If the acquire should be aborted, it must be ensured
440 * that the request is removed before returning to caller.
441 */
442 if (request_err == -EPERM)
443 break;
444
445 udelay(1);
446
447 /* Re-read the state because some time has passed. */
448 nbcon_state_read(con, cur);
449 }
450
451 /* Timed out or aborted. Carefully remove handover request. */
452 do {
453 /*
454 * No need to remove request if there is a new waiter. This
455 * can only happen if a higher priority context has taken over
456 * the console or the handover request.
457 */
458 if (!nbcon_waiter_matches(cur, ctxt->prio))
459 return -EPERM;
460
461 /* Unset request for handover. */
462 new.atom = cur->atom;
463 new.req_prio = NBCON_PRIO_NONE;
464 if (nbcon_state_try_cmpxchg(con, cur, &new)) {
465 /*
466 * Request successfully unset. Report failure of
467 * acquiring via handover.
468 */
469 cur->atom = new.atom;
470 return request_err;
471 }
472
473 /*
474 * Unable to remove request. Try to acquire in case
475 * the owner has released the lock.
476 */
477 } while (nbcon_context_try_acquire_requested(ctxt, cur));
478
479 /* Lucky timing. The acquire succeeded while removing the request. */
480 return 0;
481 }
482
483 /**
484 * nbcon_context_try_acquire_hostile - Acquire via unsafe hostile takeover
485 * @ctxt: The context of the caller
486 * @cur: The current console state
487 *
488 * Acquire the console even in the unsafe state.
489 *
490 * It can be permitted by setting the 'allow_unsafe_takeover' field only
491 * by the final attempt to flush messages in panic().
492 *
493 * Return: 0 on success. -EPERM when not allowed by the context.
494 */
495 static int nbcon_context_try_acquire_hostile(struct nbcon_context *ctxt,
496 struct nbcon_state *cur)
497 {
498 unsigned int cpu = smp_processor_id();
499 struct console *con = ctxt->console;
500 struct nbcon_state new;
501
502 if (!ctxt->allow_unsafe_takeover)
503 return -EPERM;
504
505 /* Ensure caller is allowed to perform unsafe hostile takeovers. */
506 if (WARN_ON_ONCE(ctxt->prio != NBCON_PRIO_PANIC))
507 return -EPERM;
508
509 /*
510 * Check that try_acquire_direct() and try_acquire_handover() returned
511 * -EBUSY in the right situation.
512 */
513 WARN_ON_ONCE(ctxt->prio <= cur->prio || ctxt->prio <= cur->req_prio);
514 WARN_ON_ONCE(cur->unsafe != true);
515
516 do {
517 new.atom = cur->atom;
518 new.cpu = cpu;
519 new.prio = ctxt->prio;
520 new.unsafe |= cur->unsafe_takeover;
521 new.unsafe_takeover |= cur->unsafe;
522
523 } while (!nbcon_state_try_cmpxchg(con, cur, &new));
524
525 return 0;
526 }
527
528 static struct printk_buffers panic_nbcon_pbufs;
529
530 /**
531 * nbcon_context_try_acquire - Try to acquire nbcon console
532 * @ctxt: The context of the caller
533 *
534 * Return: True if the console was acquired. False otherwise.
535 *
536 * If the caller allowed an unsafe hostile takeover, on success the
537 * caller should check the current console state to see if it is
538 * in an unsafe state. Otherwise, on success the caller may assume
539 * the console is not in an unsafe state.
540 */
541 __maybe_unused
542 static bool nbcon_context_try_acquire(struct nbcon_context *ctxt)
543 {
544 unsigned int cpu = smp_processor_id();
545 struct console *con = ctxt->console;
546 struct nbcon_state cur;
547 int err;
548
549 nbcon_state_read(con, &cur);
550 try_again:
551 err = nbcon_context_try_acquire_direct(ctxt, &cur);
552 if (err != -EBUSY)
553 goto out;
554
555 err = nbcon_context_try_acquire_handover(ctxt, &cur);
556 if (err == -EAGAIN)
557 goto try_again;
558 if (err != -EBUSY)
559 goto out;
560
561 err = nbcon_context_try_acquire_hostile(ctxt, &cur);
562 out:
563 if (err)
564 return false;
565
566 /* Acquire succeeded. */
567
568 /* Assign the appropriate buffer for this context. */
569 if (atomic_read(&panic_cpu) == cpu)
570 ctxt->pbufs = &panic_nbcon_pbufs;
571 else
572 ctxt->pbufs = con->pbufs;
573
574 /* Set the record sequence for this context to print. */
575 ctxt->seq = nbcon_seq_read(ctxt->console);
576
577 return true;
578 }
579
580 static bool nbcon_owner_matches(struct nbcon_state *cur, int expected_cpu,
581 int expected_prio)
582 {
583 /*
584 * Since consoles can only be acquired by higher priorities,
585 * owning contexts are uniquely identified by @prio. However,
586 * since contexts can unexpectedly lose ownership, it is
587 * possible that later another owner appears with the same
588 * priority. For this reason @cpu is also needed.
589 */
590
591 if (cur->prio != expected_prio)
592 return false;
593
594 if (cur->cpu != expected_cpu)
595 return false;
596
597 return true;
598 }
599
600 /**
601 * nbcon_context_release - Release the console
602 * @ctxt: The nbcon context from nbcon_context_try_acquire()
603 */
604 static void nbcon_context_release(struct nbcon_context *ctxt)
605 {
606 unsigned int cpu = smp_processor_id();
607 struct console *con = ctxt->console;
608 struct nbcon_state cur;
609 struct nbcon_state new;
610
611 nbcon_state_read(con, &cur);
612
613 do {
614 if (!nbcon_owner_matches(&cur, cpu, ctxt->prio))
615 break;
616
617 new.atom = cur.atom;
618 new.prio = NBCON_PRIO_NONE;
619
620 /*
621 * If @unsafe_takeover is set, it is kept set so that
622 * the state remains permanently unsafe.
623 */
624 new.unsafe |= cur.unsafe_takeover;
625
626 } while (!nbcon_state_try_cmpxchg(con, &cur, &new));
627
628 ctxt->pbufs = NULL;
629 }
630
631 /**
632 * nbcon_context_can_proceed - Check whether ownership can proceed
633 * @ctxt: The nbcon context from nbcon_context_try_acquire()
634 * @cur: The current console state
635 *
636 * Return: True if this context still owns the console. False if
637 * ownership was handed over or taken.
638 *
639 * Must be invoked when entering the unsafe state to make sure that it still
640 * owns the lock. Also must be invoked when exiting the unsafe context
641 * to eventually free the lock for a higher priority context which asked
642 * for the friendly handover.
643 *
644 * It can be called inside an unsafe section when the console is just
645 * temporary in safe state instead of exiting and entering the unsafe
646 * state.
647 *
648 * Also it can be called in the safe context before doing an expensive
649 * safe operation. It does not make sense to do the operation when
650 * a higher priority context took the lock.
651 *
652 * When this function returns false then the calling context no longer owns
653 * the console and is no longer allowed to go forward. In this case it must
654 * back out immediately and carefully. The buffer content is also no longer
655 * trusted since it no longer belongs to the calling context.
656 */
657 static bool nbcon_context_can_proceed(struct nbcon_context *ctxt, struct nbcon_state *cur)
658 {
659 unsigned int cpu = smp_processor_id();
660
661 /* Make sure this context still owns the console. */
662 if (!nbcon_owner_matches(cur, cpu, ctxt->prio))
663 return false;
664
665 /* The console owner can proceed if there is no waiter. */
666 if (cur->req_prio == NBCON_PRIO_NONE)
667 return true;
668
669 /*
670 * A console owner within an unsafe region is always allowed to
671 * proceed, even if there are waiters. It can perform a handover
672 * when exiting the unsafe region. Otherwise the waiter will
673 * need to perform an unsafe hostile takeover.
674 */
675 if (cur->unsafe)
676 return true;
677
678 /* Waiters always have higher priorities than owners. */
679 WARN_ON_ONCE(cur->req_prio <= cur->prio);
680
681 /*
682 * Having a safe point for take over and eventually a few
683 * duplicated characters or a full line is way better than a
684 * hostile takeover. Post processing can take care of the garbage.
685 * Release and hand over.
686 */
687 nbcon_context_release(ctxt);
688
689 /*
690 * It is not clear whether the waiter really took over ownership. The
691 * outermost callsite must make the final decision whether console
692 * ownership is needed for it to proceed. If yes, it must reacquire
693 * ownership (possibly hostile) before carefully proceeding.
694 *
695 * The calling context no longer owns the console so go back all the
696 * way instead of trying to implement reacquire heuristics in tons of
697 * places.
698 */
699 return false;
700 }
701
702 /**
703 * nbcon_can_proceed - Check whether ownership can proceed
704 * @wctxt: The write context that was handed to the write function
705 *
706 * Return: True if this context still owns the console. False if
707 * ownership was handed over or taken.
708 *
709 * It is used in nbcon_enter_unsafe() to make sure that it still owns the
710 * lock. Also it is used in nbcon_exit_unsafe() to eventually free the lock
711 * for a higher priority context which asked for the friendly handover.
712 *
713 * It can be called inside an unsafe section when the console is just
714 * temporary in safe state instead of exiting and entering the unsafe state.
715 *
716 * Also it can be called in the safe context before doing an expensive safe
717 * operation. It does not make sense to do the operation when a higher
718 * priority context took the lock.
719 *
720 * When this function returns false then the calling context no longer owns
721 * the console and is no longer allowed to go forward. In this case it must
722 * back out immediately and carefully. The buffer content is also no longer
723 * trusted since it no longer belongs to the calling context.
724 */
725 bool nbcon_can_proceed(struct nbcon_write_context *wctxt)
726 {
727 struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
728 struct console *con = ctxt->console;
729 struct nbcon_state cur;
730
731 nbcon_state_read(con, &cur);
732
733 return nbcon_context_can_proceed(ctxt, &cur);
734 }
735 EXPORT_SYMBOL_GPL(nbcon_can_proceed);
736
737 #define nbcon_context_enter_unsafe(c) __nbcon_context_update_unsafe(c, true)
738 #define nbcon_context_exit_unsafe(c) __nbcon_context_update_unsafe(c, false)
739
740 /**
741 * __nbcon_context_update_unsafe - Update the unsafe bit in @con->nbcon_state
742 * @ctxt: The nbcon context from nbcon_context_try_acquire()
743 * @unsafe: The new value for the unsafe bit
744 *
745 * Return: True if the unsafe state was updated and this context still
746 * owns the console. Otherwise false if ownership was handed
747 * over or taken.
748 *
749 * This function allows console owners to modify the unsafe status of the
750 * console.
751 *
752 * When this function returns false then the calling context no longer owns
753 * the console and is no longer allowed to go forward. In this case it must
754 * back out immediately and carefully. The buffer content is also no longer
755 * trusted since it no longer belongs to the calling context.
756 *
757 * Internal helper to avoid duplicated code.
758 */
759 static bool __nbcon_context_update_unsafe(struct nbcon_context *ctxt, bool unsafe)
760 {
761 struct console *con = ctxt->console;
762 struct nbcon_state cur;
763 struct nbcon_state new;
764
765 nbcon_state_read(con, &cur);
766
767 do {
768 /*
769 * The unsafe bit must not be cleared if an
770 * unsafe hostile takeover has occurred.
771 */
772 if (!unsafe && cur.unsafe_takeover)
773 goto out;
774
775 if (!nbcon_context_can_proceed(ctxt, &cur))
776 return false;
777
778 new.atom = cur.atom;
779 new.unsafe = unsafe;
780 } while (!nbcon_state_try_cmpxchg(con, &cur, &new));
781
782 cur.atom = new.atom;
783 out:
784 return nbcon_context_can_proceed(ctxt, &cur);
785 }
786
787 /**
788 * nbcon_enter_unsafe - Enter an unsafe region in the driver
789 * @wctxt: The write context that was handed to the write function
790 *
791 * Return: True if this context still owns the console. False if
792 * ownership was handed over or taken.
793 *
794 * When this function returns false then the calling context no longer owns
795 * the console and is no longer allowed to go forward. In this case it must
796 * back out immediately and carefully. The buffer content is also no longer
797 * trusted since it no longer belongs to the calling context.
798 */
799 bool nbcon_enter_unsafe(struct nbcon_write_context *wctxt)
800 {
801 struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
802
803 return nbcon_context_enter_unsafe(ctxt);
804 }
805 EXPORT_SYMBOL_GPL(nbcon_enter_unsafe);
806
807 /**
808 * nbcon_exit_unsafe - Exit an unsafe region in the driver
809 * @wctxt: The write context that was handed to the write function
810 *
811 * Return: True if this context still owns the console. False if
812 * ownership was handed over or taken.
813 *
814 * When this function returns false then the calling context no longer owns
815 * the console and is no longer allowed to go forward. In this case it must
816 * back out immediately and carefully. The buffer content is also no longer
817 * trusted since it no longer belongs to the calling context.
818 */
819 bool nbcon_exit_unsafe(struct nbcon_write_context *wctxt)
820 {
821 struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
822
823 return nbcon_context_exit_unsafe(ctxt);
824 }
825 EXPORT_SYMBOL_GPL(nbcon_exit_unsafe);
826
827 /**
828 * nbcon_emit_next_record - Emit a record in the acquired context
829 * @wctxt: The write context that will be handed to the write function
830 *
831 * Return: True if this context still owns the console. False if
832 * ownership was handed over or taken.
833 *
834 * When this function returns false then the calling context no longer owns
835 * the console and is no longer allowed to go forward. In this case it must
836 * back out immediately and carefully. The buffer content is also no longer
837 * trusted since it no longer belongs to the calling context. If the caller
838 * wants to do more it must reacquire the console first.
839 *
840 * When true is returned, @wctxt->ctxt.backlog indicates whether there are
841 * still records pending in the ringbuffer,
842 */
843 __maybe_unused
844 static bool nbcon_emit_next_record(struct nbcon_write_context *wctxt)
845 {
846 struct nbcon_context *ctxt = &ACCESS_PRIVATE(wctxt, ctxt);
847 struct console *con = ctxt->console;
848 bool is_extended = console_srcu_read_flags(con) & CON_EXTENDED;
849 struct printk_message pmsg = {
850 .pbufs = ctxt->pbufs,
851 };
852 unsigned long con_dropped;
853 struct nbcon_state cur;
854 unsigned long dropped;
855 bool done;
856
857 /*
858 * The printk buffers are filled within an unsafe section. This
859 * prevents NBCON_PRIO_NORMAL and NBCON_PRIO_EMERGENCY from
860 * clobbering each other.
861 */
862
863 if (!nbcon_context_enter_unsafe(ctxt))
864 return false;
865
866 ctxt->backlog = printk_get_next_message(&pmsg, ctxt->seq, is_extended, true);
867 if (!ctxt->backlog)
868 return nbcon_context_exit_unsafe(ctxt);
869
870 /*
871 * @con->dropped is not protected in case of an unsafe hostile
872 * takeover. In that situation the update can be racy so
873 * annotate it accordingly.
874 */
875 con_dropped = data_race(READ_ONCE(con->dropped));
876
877 dropped = con_dropped + pmsg.dropped;
878 if (dropped && !is_extended)
879 console_prepend_dropped(&pmsg, dropped);
880
881 if (!nbcon_context_exit_unsafe(ctxt))
882 return false;
883
884 /* For skipped records just update seq/dropped in @con. */
885 if (pmsg.outbuf_len == 0)
886 goto update_con;
887
888 /* Initialize the write context for driver callbacks. */
889 wctxt->outbuf = &pmsg.pbufs->outbuf[0];
890 wctxt->len = pmsg.outbuf_len;
891 nbcon_state_read(con, &cur);
892 wctxt->unsafe_takeover = cur.unsafe_takeover;
893
894 if (con->write_atomic) {
895 done = con->write_atomic(con, wctxt);
896 } else {
897 nbcon_context_release(ctxt);
898 WARN_ON_ONCE(1);
899 done = false;
900 }
901
902 /* If not done, the emit was aborted. */
903 if (!done)
904 return false;
905
906 /*
907 * Since any dropped message was successfully output, reset the
908 * dropped count for the console.
909 */
910 dropped = 0;
911 update_con:
912 /*
913 * The dropped count and the sequence number are updated within an
914 * unsafe section. This limits update races to the panic context and
915 * allows the panic context to win.
916 */
917
918 if (!nbcon_context_enter_unsafe(ctxt))
919 return false;
920
921 if (dropped != con_dropped) {
922 /* Counterpart to the READ_ONCE() above. */
923 WRITE_ONCE(con->dropped, dropped);
924 }
925
926 nbcon_seq_try_update(ctxt, pmsg.seq + 1);
927
928 return nbcon_context_exit_unsafe(ctxt);
929 }
930
931 /**
932 * nbcon_alloc - Allocate buffers needed by the nbcon console
933 * @con: Console to allocate buffers for
934 *
935 * Return: True on success. False otherwise and the console cannot
936 * be used.
937 *
938 * This is not part of nbcon_init() because buffer allocation must
939 * be performed earlier in the console registration process.
940 */
941 bool nbcon_alloc(struct console *con)
942 {
943 if (con->flags & CON_BOOT) {
944 /*
945 * Boot console printing is synchronized with legacy console
946 * printing, so boot consoles can share the same global printk
947 * buffers.
948 */
949 con->pbufs = &printk_shared_pbufs;
950 } else {
951 con->pbufs = kmalloc(sizeof(*con->pbufs), GFP_KERNEL);
952 if (!con->pbufs) {
953 con_printk(KERN_ERR, con, "failed to allocate printing buffer\n");
954 return false;
955 }
956 }
957
958 return true;
959 }
960
961 /**
962 * nbcon_init - Initialize the nbcon console specific data
963 * @con: Console to initialize
964 *
965 * nbcon_alloc() *must* be called and succeed before this function
966 * is called.
967 *
968 * This function expects that the legacy @con->seq has been set.
969 */
970 void nbcon_init(struct console *con)
971 {
972 struct nbcon_state state = { };
973
974 /* nbcon_alloc() must have been called and successful! */
975 BUG_ON(!con->pbufs);
976
977 nbcon_seq_force(con, con->seq);
978 nbcon_state_set(con, &state);
979 }
980
981 /**
982 * nbcon_free - Free and cleanup the nbcon console specific data
983 * @con: Console to free/cleanup nbcon data
984 */
985 void nbcon_free(struct console *con)
986 {
987 struct nbcon_state state = { };
988
989 nbcon_state_set(con, &state);
990
991 /* Boot consoles share global printk buffers. */
992 if (!(con->flags & CON_BOOT))
993 kfree(con->pbufs);
994
995 con->pbufs = NULL;
996 }
997
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