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ab6f762f0f
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 1b710b1b10 ("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: Lech Perczak <l.perczak@camlintechnologies.com>
Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Tested-by: Jann Horn <jannh@google.com>
Reviewed-by: Petr 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: Linus Torvalds <torvalds@linux-foundation.org>
73 lines
1.9 KiB
C
73 lines
1.9 KiB
C
/* SPDX-License-Identifier: GPL-2.0-or-later */
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/*
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* internal.h - printk internal definitions
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*/
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#include <linux/percpu.h>
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#ifdef CONFIG_PRINTK
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#define PRINTK_SAFE_CONTEXT_MASK 0x3fffffff
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#define PRINTK_NMI_DIRECT_CONTEXT_MASK 0x40000000
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#define PRINTK_NMI_CONTEXT_MASK 0x80000000
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extern raw_spinlock_t logbuf_lock;
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__printf(5, 0)
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int vprintk_store(int facility, int level,
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const char *dict, size_t dictlen,
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const char *fmt, va_list args);
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__printf(1, 0) int vprintk_default(const char *fmt, va_list args);
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__printf(1, 0) int vprintk_deferred(const char *fmt, va_list args);
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__printf(1, 0) int vprintk_func(const char *fmt, va_list args);
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void __printk_safe_enter(void);
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void __printk_safe_exit(void);
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void printk_safe_init(void);
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bool printk_percpu_data_ready(void);
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#define printk_safe_enter_irqsave(flags) \
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do { \
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local_irq_save(flags); \
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__printk_safe_enter(); \
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} while (0)
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#define printk_safe_exit_irqrestore(flags) \
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do { \
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__printk_safe_exit(); \
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local_irq_restore(flags); \
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} while (0)
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#define printk_safe_enter_irq() \
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do { \
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local_irq_disable(); \
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__printk_safe_enter(); \
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} while (0)
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#define printk_safe_exit_irq() \
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do { \
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__printk_safe_exit(); \
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local_irq_enable(); \
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} while (0)
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void defer_console_output(void);
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#else
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__printf(1, 0) int vprintk_func(const char *fmt, va_list args) { return 0; }
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/*
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* In !PRINTK builds we still export logbuf_lock spin_lock, console_sem
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* semaphore and some of console functions (console_unlock()/etc.), so
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* printk-safe must preserve the existing local IRQ guarantees.
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*/
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#define printk_safe_enter_irqsave(flags) local_irq_save(flags)
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#define printk_safe_exit_irqrestore(flags) local_irq_restore(flags)
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#define printk_safe_enter_irq() local_irq_disable()
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#define printk_safe_exit_irq() local_irq_enable()
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static inline void printk_safe_init(void) { }
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static inline bool printk_percpu_data_ready(void) { return false; }
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#endif /* CONFIG_PRINTK */
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