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linux-brain/include/linux/kdb.h
Petr Mladek 34aaff40b4 kdb: call vkdb_printf() from vprintk_default() only when wanted 
kdb_trap_printk allows to pass normal printk() messages to kdb via
vkdb_printk().  For example, it is used to get backtrace using the
classic show_stack(), see kdb_show_stack().

vkdb_printf() tries to avoid a potential infinite loop by disabling the
trap.  But this approach is racy, for example:

CPU1					CPU2

vkdb_printf()
  // assume that kdb_trap_printk == 0
  saved_trap_printk = kdb_trap_printk;
  kdb_trap_printk = 0;

					kdb_show_stack()
					  kdb_trap_printk++;

Problem1: Now, a nested printk() on CPU0 calls vkdb_printf()
	  even when it should have been disabled. It will not
	  cause a deadlock but...

   // using the outdated saved value: 0
   kdb_trap_printk = saved_trap_printk;

					  kdb_trap_printk--;

Problem2: Now, kdb_trap_printk == -1 and will stay like this.
   It means that all messages will get passed to kdb from
   now on.

This patch removes the racy saved_trap_printk handling.  Instead, the
recursion is prevented by a check for the locked CPU.

The solution is still kind of racy.  A non-related printk(), from
another process, might get trapped by vkdb_printf().  And the wanted
printk() might not get trapped because kdb_printf_cpu is assigned.  But
this problem existed even with the original code.

A proper solution would be to get_cpu() before setting kdb_trap_printk
and trap messages only from this CPU.  I am not sure if it is worth the
effort, though.

In fact, the race is very theoretical.  When kdb is running any of the
commands that use kdb_trap_printk there is a single active CPU and the
other CPUs should be in a holding pen inside kgdb_cpu_enter().

The only time this is violated is when there is a timeout waiting for
the other CPUs to report to the holding pen.

Finally, note that the situation is a bit schizophrenic.  vkdb_printf()
explicitly allows recursion but only from KDB code that calls
kdb_printf() directly.  On the other hand, the generic printk()
recursion is not allowed because it might cause an infinite loop.  This
is why we could not hide the decision inside vkdb_printf() easily.

Link: http://lkml.kernel.org/r/1480412276-16690-4-git-send-email-pmladek@suse.com
Signed-off-by: Petr Mladek <pmladek@suse.com>
Cc: Daniel Thompson <daniel.thompson@linaro.org>
Cc: Jason Wessel <jason.wessel@windriver.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2016-12-14 16:04:08 -08:00

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#ifndef _KDB_H
#define _KDB_H
/*
* Kernel Debugger Architecture Independent Global Headers
*
* This file is subject to the terms and conditions of the GNU General Public
* License. See the file "COPYING" in the main directory of this archive
* for more details.
*
* Copyright (c) 2000-2007 Silicon Graphics, Inc. All Rights Reserved.
* Copyright (C) 2000 Stephane Eranian <eranian@hpl.hp.com>
* Copyright (C) 2009 Jason Wessel <jason.wessel@windriver.com>
*/
/* Shifted versions of the command enable bits are be used if the command
* has no arguments (see kdb_check_flags). This allows commands, such as
* go, to have different permissions depending upon whether it is called
* with an argument.
*/
#define KDB_ENABLE_NO_ARGS_SHIFT 10
typedef enum {
KDB_ENABLE_ALL = (1 << 0), /* Enable everything */
KDB_ENABLE_MEM_READ = (1 << 1),
KDB_ENABLE_MEM_WRITE = (1 << 2),
KDB_ENABLE_REG_READ = (1 << 3),
KDB_ENABLE_REG_WRITE = (1 << 4),
KDB_ENABLE_INSPECT = (1 << 5),
KDB_ENABLE_FLOW_CTRL = (1 << 6),
KDB_ENABLE_SIGNAL = (1 << 7),
KDB_ENABLE_REBOOT = (1 << 8),
/* User exposed values stop here, all remaining flags are
* exclusively used to describe a commands behaviour.
*/
KDB_ENABLE_ALWAYS_SAFE = (1 << 9),
KDB_ENABLE_MASK = (1 << KDB_ENABLE_NO_ARGS_SHIFT) - 1,
KDB_ENABLE_ALL_NO_ARGS = KDB_ENABLE_ALL << KDB_ENABLE_NO_ARGS_SHIFT,
KDB_ENABLE_MEM_READ_NO_ARGS = KDB_ENABLE_MEM_READ
<< KDB_ENABLE_NO_ARGS_SHIFT,
KDB_ENABLE_MEM_WRITE_NO_ARGS = KDB_ENABLE_MEM_WRITE
<< KDB_ENABLE_NO_ARGS_SHIFT,
KDB_ENABLE_REG_READ_NO_ARGS = KDB_ENABLE_REG_READ
<< KDB_ENABLE_NO_ARGS_SHIFT,
KDB_ENABLE_REG_WRITE_NO_ARGS = KDB_ENABLE_REG_WRITE
<< KDB_ENABLE_NO_ARGS_SHIFT,
KDB_ENABLE_INSPECT_NO_ARGS = KDB_ENABLE_INSPECT
<< KDB_ENABLE_NO_ARGS_SHIFT,
KDB_ENABLE_FLOW_CTRL_NO_ARGS = KDB_ENABLE_FLOW_CTRL
<< KDB_ENABLE_NO_ARGS_SHIFT,
KDB_ENABLE_SIGNAL_NO_ARGS = KDB_ENABLE_SIGNAL
<< KDB_ENABLE_NO_ARGS_SHIFT,
KDB_ENABLE_REBOOT_NO_ARGS = KDB_ENABLE_REBOOT
<< KDB_ENABLE_NO_ARGS_SHIFT,
KDB_ENABLE_ALWAYS_SAFE_NO_ARGS = KDB_ENABLE_ALWAYS_SAFE
<< KDB_ENABLE_NO_ARGS_SHIFT,
KDB_ENABLE_MASK_NO_ARGS = KDB_ENABLE_MASK << KDB_ENABLE_NO_ARGS_SHIFT,
KDB_REPEAT_NO_ARGS = 0x40000000, /* Repeat the command w/o arguments */
KDB_REPEAT_WITH_ARGS = 0x80000000, /* Repeat the command with args */
} kdb_cmdflags_t;
typedef int (*kdb_func_t)(int, const char **);
#ifdef CONFIG_KGDB_KDB
#include <linux/init.h>
#include <linux/sched.h>
#include <linux/atomic.h>
#define KDB_POLL_FUNC_MAX 5
extern int kdb_poll_idx;
/*
* kdb_initial_cpu is initialized to -1, and is set to the cpu
* number whenever the kernel debugger is entered.
*/
extern int kdb_initial_cpu;
/* Types and messages used for dynamically added kdb shell commands */
#define KDB_MAXARGS 16 /* Maximum number of arguments to a function */
/* KDB return codes from a command or internal kdb function */
#define KDB_NOTFOUND (-1)
#define KDB_ARGCOUNT (-2)
#define KDB_BADWIDTH (-3)
#define KDB_BADRADIX (-4)
#define KDB_NOTENV (-5)
#define KDB_NOENVVALUE (-6)
#define KDB_NOTIMP (-7)
#define KDB_ENVFULL (-8)
#define KDB_ENVBUFFULL (-9)
#define KDB_TOOMANYBPT (-10)
#define KDB_TOOMANYDBREGS (-11)
#define KDB_DUPBPT (-12)
#define KDB_BPTNOTFOUND (-13)
#define KDB_BADMODE (-14)
#define KDB_BADINT (-15)
#define KDB_INVADDRFMT (-16)
#define KDB_BADREG (-17)
#define KDB_BADCPUNUM (-18)
#define KDB_BADLENGTH (-19)
#define KDB_NOBP (-20)
#define KDB_BADADDR (-21)
#define KDB_NOPERM (-22)
/*
* kdb_diemsg
*
* Contains a pointer to the last string supplied to the
* kernel 'die' panic function.
*/
extern const char *kdb_diemsg;
#define KDB_FLAG_EARLYKDB (1 << 0) /* set from boot parameter kdb=early */
#define KDB_FLAG_CATASTROPHIC (1 << 1) /* A catastrophic event has occurred */
#define KDB_FLAG_CMD_INTERRUPT (1 << 2) /* Previous command was interrupted */
#define KDB_FLAG_NOIPI (1 << 3) /* Do not send IPIs */
#define KDB_FLAG_NO_CONSOLE (1 << 5) /* No console is available,
* kdb is disabled */
#define KDB_FLAG_NO_VT_CONSOLE (1 << 6) /* No VT console is available, do
* not use keyboard */
#define KDB_FLAG_NO_I8042 (1 << 7) /* No i8042 chip is available, do
* not use keyboard */
extern int kdb_flags; /* Global flags, see kdb_state for per cpu state */
extern void kdb_save_flags(void);
extern void kdb_restore_flags(void);
#define KDB_FLAG(flag) (kdb_flags & KDB_FLAG_##flag)
#define KDB_FLAG_SET(flag) ((void)(kdb_flags |= KDB_FLAG_##flag))
#define KDB_FLAG_CLEAR(flag) ((void)(kdb_flags &= ~KDB_FLAG_##flag))
/*
* External entry point for the kernel debugger. The pt_regs
* at the time of entry are supplied along with the reason for
* entry to the kernel debugger.
*/
typedef enum {
KDB_REASON_ENTER = 1, /* KDB_ENTER() trap/fault - regs valid */
KDB_REASON_ENTER_SLAVE, /* KDB_ENTER_SLAVE() trap/fault - regs valid */
KDB_REASON_BREAK, /* Breakpoint inst. - regs valid */
KDB_REASON_DEBUG, /* Debug Fault - regs valid */
KDB_REASON_OOPS, /* Kernel Oops - regs valid */
KDB_REASON_SWITCH, /* CPU switch - regs valid*/
KDB_REASON_KEYBOARD, /* Keyboard entry - regs valid */
KDB_REASON_NMI, /* Non-maskable interrupt; regs valid */
KDB_REASON_RECURSE, /* Recursive entry to kdb;
* regs probably valid */
KDB_REASON_SSTEP, /* Single Step trap. - regs valid */
KDB_REASON_SYSTEM_NMI, /* In NMI due to SYSTEM cmd; regs valid */
} kdb_reason_t;
enum kdb_msgsrc {
KDB_MSGSRC_INTERNAL, /* direct call to kdb_printf() */
KDB_MSGSRC_PRINTK, /* trapped from printk() */
};
extern int kdb_trap_printk;
extern int kdb_printf_cpu;
extern __printf(2, 0) int vkdb_printf(enum kdb_msgsrc src, const char *fmt,
va_list args);
extern __printf(1, 2) int kdb_printf(const char *, ...);
typedef __printf(1, 2) int (*kdb_printf_t)(const char *, ...);
extern void kdb_init(int level);
/* Access to kdb specific polling devices */
typedef int (*get_char_func)(void);
extern get_char_func kdb_poll_funcs[];
extern int kdb_get_kbd_char(void);
static inline
int kdb_process_cpu(const struct task_struct *p)
{
unsigned int cpu = task_cpu(p);
if (cpu > num_possible_cpus())
cpu = 0;
return cpu;
}
/* kdb access to register set for stack dumping */
extern struct pt_regs *kdb_current_regs;
#ifdef CONFIG_KALLSYMS
extern const char *kdb_walk_kallsyms(loff_t *pos);
#else /* ! CONFIG_KALLSYMS */
static inline const char *kdb_walk_kallsyms(loff_t *pos)
{
return NULL;
}
#endif /* ! CONFIG_KALLSYMS */
/* Dynamic kdb shell command registration */
extern int kdb_register(char *, kdb_func_t, char *, char *, short);
extern int kdb_register_flags(char *, kdb_func_t, char *, char *,
short, kdb_cmdflags_t);
extern int kdb_unregister(char *);
#else /* ! CONFIG_KGDB_KDB */
static inline __printf(1, 2) int kdb_printf(const char *fmt, ...) { return 0; }
static inline void kdb_init(int level) {}
static inline int kdb_register(char *cmd, kdb_func_t func, char *usage,
char *help, short minlen) { return 0; }
static inline int kdb_register_flags(char *cmd, kdb_func_t func, char *usage,
char *help, short minlen,
kdb_cmdflags_t flags) { return 0; }
static inline int kdb_unregister(char *cmd) { return 0; }
#endif /* CONFIG_KGDB_KDB */
enum {
KDB_NOT_INITIALIZED,
KDB_INIT_EARLY,
KDB_INIT_FULL,
};
extern int kdbgetintenv(const char *, int *);
extern int kdb_set(int, const char **);
#endif /* !_KDB_H */
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