// SPDX-License-Identifier: GPL-2.0-or-later
/*
* printk_safe.c - Safe printk for printk-deadlock-prone contexts
*/
#include <linux/preempt.h>
#include <linux/spinlock.h>
#include <linux/debug_locks.h>
#include <linux/smp.h>
#include <linux/cpumask.h>
#include <linux/irq_work.h>
#include <linux/printk.h>
#include "internal.h"
/*
* printk() could not take logbuf_lock in NMI context. Instead,
* it uses an alternative implementation that temporary stores
* the strings into a per-CPU buffer. The content of the buffer
* is later flushed into the main ring buffer via IRQ work.
*
* The alternative implementation is chosen transparently
* by examinig current printk() context mask stored in @printk_context
* per-CPU variable.
*
* The implementation allows to flush the strings also from another CPU.
* There are situations when we want to make sure that all buffers
* were handled or when IRQs are blocked.
*/
static int printk_safe_irq_ready __read_mostly;
#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) - \
sizeof(atomic_t) - \
sizeof(atomic_t) - \
sizeof(struct irq_work))
struct printk_safe_seq_buf {
atomic_t len; /* length of written data */
atomic_t message_lost;
struct irq_work work; /* IRQ work that flushes the buffer */
unsigned char buffer[SAFE_LOG_BUF_LEN];
};
static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
static DEFINE_PER_CPU(int, printk_context);
#ifdef CONFIG_PRINTK_NMI
static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
#endif
/* Get flushed in a more safe context. */
static void queue_flush_work(struct printk_safe_seq_buf *s)
{
if (printk_safe_irq_ready)
irq_work_queue(&s->work);
}
/*
* Add a message to per-CPU context-dependent buffer. NMI and printk-safe
* have dedicated buffers, because otherwise printk-safe preempted by
* NMI-printk would have overwritten the NMI messages.
*
* The messages are flushed from irq work (or from panic()), possibly,
* from other CPU, concurrently with printk_safe_log_store(). Should this
* happen, printk_safe_log_store() will notice the buffer->len mismatch
* and repeat the write.
*/
static __printf(2, 0) int printk_safe_log_store(struct printk_safe_seq_buf *s,
const char *fmt, va_list args)
{
int add;
size_t len;
va_list ap;
again:
len = atomic_read(&s->len);
/* The trailing '\0' is not counted into len. */
if (len >= sizeof(s->buffer) - 1) {
atomic_inc(&s->message_lost);
queue_flush_work(s);
return 0;
}
/*
* Make sure that all old data have been read before the buffer
* was reset. This is not needed when we just append data.
*/
if (!len)
smp_rmb();
va_copy(ap, args);
add = vscnprintf(s->buffer + len, sizeof(s->buffer) - len, fmt, ap);
va_end(ap);
if (!add)
return 0;
/*
* Do it once again if the buffer has been flushed in the meantime.
* Note that atomic_cmpxchg() is an implicit memory barrier that
* makes sure that the data were written before updating s->len.
*/
if (atomic_cmpxchg(&s->len, len, len + add) != len)
goto again;
queue_flush_work(s);
return add;
}
static inline void printk_safe_flush_line(const char *text, int len)
{
/*
* Avoid any console drivers calls from here, because we may be
* in NMI or printk_safe context (when in panic). The messages
* must go only into the ring buffer at this stage. Consoles will
* get explicitly called later when a crashdump is not generated.
*/
printk_deferred("%.*s", len, text);
}
/* printk part of the temporary buffer line by line */
static int printk_safe_flush_buffer(const char *start, size_t len)
{
const char *c, *end;
bool header;
c = start;
end = start + len;
header = true;
/* Print line by line. */
while (c < end) {
if (*c == '\n') {
printk_safe_flush_line(start, c - start + 1);
start = ++c;
header = true;
continue;
}
/* Handle continuous lines or missing new line. */
if ((c + 1 < end) && printk_get_level(c)) {
if (header) {
c = printk_skip_level(c);
continue;
}
printk_safe_flush_line(start, c - start);
start = c++;
header = true;
continue;
}
header = false;
c++;
}
/* Check if there was a partial line. Ignore pure header. */
if (start < end && !header) {
static const char newline[] = KERN_CONT "\n";
printk_safe_flush_line(start, end - start);
printk_safe_flush_line(newline, strlen(newline));
}
return len;
}
static void report_message_lost(struct printk_safe_seq_buf *s)
{
int lost = atomic_xchg(&s->message_lost, 0);
if (lost)
printk_deferred("Lost %d message(s)!\n", lost);
}
/*
* Flush data from the associated per-CPU buffer. The function
* can be called either via IRQ work or independently.
*/
static void __printk_safe_flush(struct irq_work *work)
{
static raw_spinlock_t read_lock =
__RAW_SPIN_LOCK_INITIALIZER(read_lock);
struct printk_safe_seq_buf *s =
container_of(work, struct printk_safe_seq_buf, work);
unsigned long flags;
size_t len;
int i;
/*
* The lock has two functions. First, one reader has to flush all
* available message to make the lockless synchronization with
* writers easier. Second, we do not want to mix messages from
* different CPUs. This is especially important when printing
* a backtrace.
*/
raw_spin_lock_irqsave(&read_lock, flags);
i = 0;
more:
len = atomic_read(&s->len);
/*
* This is just a paranoid check that nobody has manipulated
* the buffer an unexpected way. If we printed something then
* @len must only increase. Also it should never overflow the
* buffer size.
*/
if ((i && i >= len) || len > sizeof(s->buffer)) {
const char *msg = "printk_safe_flush: internal error\n";
printk_safe_flush_line(msg, strlen(msg));
len = 0;
}
if (!len)
goto out; /* Someone else has already flushed the buffer. */
/* Make sure that data has been written up to the @len */
smp_rmb();
i += printk_safe_flush_buffer(s->buffer + i, len - i);
/*
* Check that nothing has got added in the meantime and truncate
* the buffer. Note that atomic_cmpxchg() is an implicit memory
* barrier that makes sure that the data were copied before
* updating s->len.
*/
if (atomic_cmpxchg(&s->len, len, 0) != len)
goto more;
out:
report_message_lost(s);
raw_spin_unlock_irqrestore(&read_lock, flags);
}
/**
* printk_safe_flush - flush all per-cpu nmi buffers.
*
* The buffers are flushed automatically via IRQ work. This function
* is useful only when someone wants to be sure that all buffers have
* been flushed at some point.
*/
void printk_safe_flush(void)
{
int cpu;
for_each_possible_cpu(cpu) {
#ifdef CONFIG_PRINTK_NMI
__printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
#endif
__printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
}
}
/**
* printk_safe_flush_on_panic - flush all per-cpu nmi buffers when the system
* goes down.
*
* Similar to printk_safe_flush() but it can be called even in NMI context when
* the system goes down. It does the best effort to get NMI messages into
* the main ring buffer.
*
* Note that it could try harder when there is only one CPU online.
*/
void printk_safe_flush_on_panic(void)
{
/*
* Make sure that we could access the main ring buffer.
* Do not risk a double release when more CPUs are up.
*/
if (raw_spin_is_locked(&logbuf_lock)) {
if (num_online_cpus() > 1)
return;
debug_locks_off();
raw_spin_lock_init(&logbuf_lock);
}
printk_safe_flush();
}
#ifdef CONFIG_PRINTK_NMI
/*
* Safe printk() for NMI context. It uses a per-CPU buffer to
* store the message. NMIs are not nested, so there is always only
* one writer running. But the buffer might get flushed from another
* CPU, so we need to be careful.
*/
static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
{
struct printk_safe_seq_buf *s = this_cpu_ptr(&nmi_print_seq);
return printk_safe_log_store(s, fmt, args);
}
void notrace printk_nmi_enter(void)
{
this_cpu_or(printk_context, PRINTK_NMI_CONTEXT_MASK);
}
void notrace printk_nmi_exit(void)
{
this_cpu_and(printk_context, ~PRINTK_NMI_CONTEXT_MASK);
}
/*
* Marks a code that might produce many messages in NMI context
* and the risk of losing them is more critical than eventual
* reordering.
*
* It has effect only when called in NMI context. Then printk()
* will try to store the messages into the main logbuf directly
* and use the per-CPU buffers only as a fallback when the lock
* is not available.
*/
void printk_nmi_direct_enter(void)
{
if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
this_cpu_or(printk_context, PRINTK_NMI_DIRECT_CONTEXT_MASK);
}
void printk_nmi_direct_exit(void)
{
this_cpu_and(printk_context, ~PRINTK_NMI_DIRECT_CONTEXT_MASK);
}
#else
static __printf(1, 0) int vprintk_nmi(const char *fmt, va_list args)
{
return 0;
}
#endif /* CONFIG_PRINTK_NMI */
/*
* Lock-less printk(), to avoid deadlocks should the printk() recurse
* into itself. It uses a per-CPU buffer to store the message, just like
* NMI.
*/
static __printf(1, 0) int vprintk_safe(const char *fmt, va_list args)
{
struct printk_safe_seq_buf *s = this_cpu_ptr(&safe_print_seq);
return printk_safe_log_store(s, fmt, args);
}
/* Can be preempted by NMI. */
void __printk_safe_enter(void)
{
this_cpu_inc(printk_context);
}
/* Can be preempted by NMI. */
void __printk_safe_exit(void)
{
this_cpu_dec(printk_context);
}
__printf(1, 0) int vprintk_func(const char *fmt, va_list args)
{
/*
* Try to use the main logbuf even in NMI. But avoid calling console
* drivers that might have their own locks.
*/
if ((this_cpu_read(printk_context) & PRINTK_NMI_DIRECT_CONTEXT_MASK) &&
raw_spin_trylock(&logbuf_lock)) {
int len;
len = vprintk_store(0, LOGLEVEL_DEFAULT, NULL, 0, fmt, args);
raw_spin_unlock(&logbuf_lock);
defer_console_output();
return len;
}
/* Use extra buffer in NMI when logbuf_lock is taken or in safe mode. */
if (this_cpu_read(printk_context) & PRINTK_NMI_CONTEXT_MASK)
return vprintk_nmi(fmt, args);
/* Use extra buffer to prevent a recursion deadlock in safe mode. */
if (this_cpu_read(printk_context) & PRINTK_SAFE_CONTEXT_MASK)
return vprintk_safe(fmt, args);
/* No obstacles. */
return vprintk_default(fmt, args);
}
void __init printk_safe_init(void)
{
int cpu;
for_each_possible_cpu(cpu) {
struct printk_safe_seq_buf *s;
s = &per_cpu(safe_print_seq, cpu);
init_irq_work(&s->work, __printk_safe_flush);
#ifdef CONFIG_PRINTK_NMI
s = &per_cpu(nmi_print_seq, cpu);
init_irq_work(&s->work, __printk_safe_flush);
#endif
}
/*
* In the highly unlikely event that a NMI were to trigger at
* this moment. Make sure IRQ work is set up before this
* variable is set.
*/
barrier();
printk_safe_irq_ready = 1;
/* Flush pending messages that did not have scheduled IRQ works. */
printk_safe_flush();
}
