Commit ab6f762f authored by Sergey Senozhatsky's avatar Sergey Senozhatsky Committed by Linus Torvalds
Browse files

printk: queue wake_up_klogd irq_work only if per-CPU areas are ready 

printk_deferred(), similarly to printk_safe/printk_nmi, does not
immediately attempt to print a new message on the consoles, avoiding
calls into non-reentrant kernel paths, e.g. scheduler or timekeeping,
which potentially can deadlock the system.

Those printk() flavors, instead, rely on per-CPU flush irq_work to print
messages from safer contexts.  For same reasons (recursive scheduler or
timekeeping calls) printk() uses per-CPU irq_work in order to wake up
user space syslog/kmsg readers.

However, only printk_safe/printk_nmi do make sure that per-CPU areas
have been initialised and that it's safe to modify per-CPU irq_work.
This means that, for instance, should printk_deferred() be invoked "too
early", that is before per-CPU areas are initialised, printk_deferred()
will perform illegal per-CPU access.

Lech Perczak [0] reports that after commit 1b710b1b ("char/random:
silence a lockdep splat with printk()") user-space syslog/kmsg readers
are not able to read new kernel messages.

The reason is printk_deferred() being called too early (as was pointed
out by Petr and John).

Fix printk_deferred() and do not queue per-CPU irq_work before per-CPU
areas are initialized.

Link: https://lore.kernel.org/lkml/aa0732c6-5c4e-8a8b-a1c1-75ebe3dca05b@camlintechnologies.com/


Reported-by: default avatarLech Perczak <l.perczak@camlintechnologies.com>
Signed-off-by: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
Tested-by: default avatarJann Horn <jannh@google.com>
Reviewed-by: default avatarPetr Mladek <pmladek@suse.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: John Ogness <john.ogness@linutronix.de>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 87ad46e6

include/linux/printk.h

+0 −5
Original line number Original line Diff line number Diff line
@@ -202,7 +202,6 @@ __printf(1, 2) void dump_stack_set_arch_desc(const char *fmt, ...); 
void dump_stack_print_info(const char *log_lvl);

void dump_stack_print_info(const char *log_lvl);

void show_regs_print_info(const char *log_lvl);

void show_regs_print_info(const char *log_lvl);

extern asmlinkage void dump_stack(void) __cold;

extern asmlinkage void dump_stack(void) __cold;

extern void printk_safe_init(void);

extern void printk_safe_flush(void);

extern void printk_safe_flush(void);

extern void printk_safe_flush_on_panic(void);

extern void printk_safe_flush_on_panic(void);

#else

#else
@@ -269,10 +268,6 @@ static inline void dump_stack(void) 
{

{

}

}





static inline void printk_safe_init(void)

{

}



static inline void printk_safe_flush(void)

static inline void printk_safe_flush(void)

{

{

}

}

init/main.c

+0 −1
Original line number Original line Diff line number Diff line
@@ -913,7 +913,6 @@ asmlinkage __visible void __init start_kernel(void) 
	boot_init_stack_canary();

	boot_init_stack_canary();





	time_init();

	time_init();

	printk_safe_init();

	perf_event_init();

	perf_event_init();

	profile_init();

	profile_init();

	call_function_init();

	call_function_init();

kernel/printk/internal.h

+5 −0
Original line number Original line Diff line number Diff line
@@ -23,6 +23,9 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args); 
void __printk_safe_enter(void);

void __printk_safe_enter(void);

void __printk_safe_exit(void);

void __printk_safe_exit(void);





void printk_safe_init(void);

bool printk_percpu_data_ready(void);



#define printk_safe_enter_irqsave(flags)	\

#define printk_safe_enter_irqsave(flags)	\

	do {					\

	do {					\

		local_irq_save(flags);		\

		local_irq_save(flags);		\
@@ -64,4 +67,6 @@ __printf(1, 0) int vprintk_func(const char *fmt, va_list args) { return 0; } 
#define printk_safe_enter_irq() local_irq_disable()

#define printk_safe_enter_irq() local_irq_disable()

#define printk_safe_exit_irq() local_irq_enable()

#define printk_safe_exit_irq() local_irq_enable()





static inline void printk_safe_init(void) { }

static inline bool printk_percpu_data_ready(void) { return false; }

#endif /* CONFIG_PRINTK */
 
#endif /* CONFIG_PRINTK */

kernel/printk/printk.c

+34 −0
Original line number Original line Diff line number Diff line
@@ -460,6 +460,18 @@ static char __log_buf[__LOG_BUF_LEN] __aligned(LOG_ALIGN); 
static char *log_buf = __log_buf;

static char *log_buf = __log_buf;

static u32 log_buf_len = __LOG_BUF_LEN;

static u32 log_buf_len = __LOG_BUF_LEN;





/*

 * We cannot access per-CPU data (e.g. per-CPU flush irq_work) before

 * per_cpu_areas are initialised. This variable is set to true when

 * it's safe to access per-CPU data.

 */

static bool __printk_percpu_data_ready __read_mostly;



bool printk_percpu_data_ready(void)

{

	return __printk_percpu_data_ready;

}



/* Return log buffer address */

/* Return log buffer address */

char *log_buf_addr_get(void)

char *log_buf_addr_get(void)

{

{
@@ -1146,12 +1158,28 @@ static void __init log_buf_add_cpu(void) 
static inline void log_buf_add_cpu(void) {}

static inline void log_buf_add_cpu(void) {}

#endif /* CONFIG_SMP */

#endif /* CONFIG_SMP */





static void __init set_percpu_data_ready(void)

{

	printk_safe_init();

	/* Make sure we set this flag only after printk_safe() init is done */

	barrier();

	__printk_percpu_data_ready = true;

}



void __init setup_log_buf(int early)

void __init setup_log_buf(int early)

{

{

	unsigned long flags;

	unsigned long flags;

	char *new_log_buf;

	char *new_log_buf;

	unsigned int free;

	unsigned int free;





	/*

	 * Some archs call setup_log_buf() multiple times - first is very

	 * early, e.g. from setup_arch(), and second - when percpu_areas

	 * are initialised.

	 */

	if (!early)

		set_percpu_data_ready();



	if (log_buf != __log_buf)

	if (log_buf != __log_buf)

		return;

		return;
@@ -2975,6 +3003,9 @@ static DEFINE_PER_CPU(struct irq_work, wake_up_klogd_work) = { 




void wake_up_klogd(void)

void wake_up_klogd(void)

{

{

	if (!printk_percpu_data_ready())

		return;



	preempt_disable();

	preempt_disable();

	if (waitqueue_active(&log_wait)) {

	if (waitqueue_active(&log_wait)) {

		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);

		this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
@@ -2985,6 +3016,9 @@ void wake_up_klogd(void) 




void defer_console_output(void)

void defer_console_output(void)

{

{

	if (!printk_percpu_data_ready())

		return;



	preempt_disable();

	preempt_disable();

	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);

	__this_cpu_or(printk_pending, PRINTK_PENDING_OUTPUT);

	irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));

	irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));

kernel/printk/printk_safe.c

+1 −10
Original line number Original line Diff line number Diff line
@@ -27,7 +27,6 @@ 
 * There are situations when we want to make sure that all buffers

 * There are situations when we want to make sure that all buffers

 * were handled or when IRQs are blocked.

 * 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) -	\

#define SAFE_LOG_BUF_LEN ((1 << CONFIG_PRINTK_SAFE_LOG_BUF_SHIFT) -	\

				sizeof(atomic_t) -			\

				sizeof(atomic_t) -			\
@@ -51,7 +50,7 @@ static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq); 
/* Get flushed in a more safe context. */

/* Get flushed in a more safe context. */

static void queue_flush_work(struct printk_safe_seq_buf *s)

static void queue_flush_work(struct printk_safe_seq_buf *s)

{

{

	if (printk_safe_irq_ready)

	if (printk_percpu_data_ready())

		irq_work_queue(&s->work);

		irq_work_queue(&s->work);

}

}
@@ -402,14 +401,6 @@ void __init printk_safe_init(void) 
#endif

#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. */

	/* Flush pending messages that did not have scheduled IRQ works. */

	printk_safe_flush();

	printk_safe_flush();

}
 
}

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