[PATCH v2 3/3] kdb: Call vkdb_printf() from vprintk_default() only when wanted

From: Petr Mladek
Date: Tue Nov 29 2016 - 04:39:06 EST

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:


// assume that kdb_trap_printk == 0
saved_trap_printk = kdb_trap_printk;
kdb_trap_printk = 0;


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// using the outdated saved value: 0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;


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()

Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
include/linux/kdb.h | 1 +
kernel/debug/kdb/kdb_io.c | 9 ++-------
kernel/printk/printk.c | 3 ++-
3 files changed, 5 insertions(+), 8 deletions(-)

diff --git a/include/linux/kdb.h b/include/linux/kdb.h
index eb706188dc23..68bd88223417 100644
--- a/include/linux/kdb.h
+++ b/include/linux/kdb.h
@@ -161,6 +161,7 @@ enum kdb_msgsrc {

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 *, ...);
diff --git a/kernel/debug/kdb/kdb_io.c b/kernel/debug/kdb/kdb_io.c
index cf495c7a8519..351b4f785270 100644
--- a/kernel/debug/kdb/kdb_io.c
+++ b/kernel/debug/kdb/kdb_io.c
@@ -30,6 +30,7 @@
char kdb_prompt_str[CMD_BUFLEN];

int kdb_trap_printk;
+int kdb_printf_cpu = -1;

static int kgdb_transition_check(char *buffer)
@@ -554,24 +555,19 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
int linecount;
int colcount;
int logging, saved_loglevel = 0;
- int saved_trap_printk;
int retlen = 0;
int fnd, len;
int this_cpu, old_cpu;
- static int kdb_printf_cpu = -1;
char *cp, *cp2, *cphold = NULL, replaced_byte = ' ';
char *moreprompt = "more> ";
struct console *c = console_drivers;
unsigned long uninitialized_var(flags);

- local_irq_save(flags);
- saved_trap_printk = kdb_trap_printk;
- kdb_trap_printk = 0;
/* Serialize kdb_printf if multiple cpus try to write at once.
* But if any cpu goes recursive in kdb, just print the output,
* even if it is interleaved with any other text.
+ local_irq_save(flags);
this_cpu = smp_processor_id();
for (;;) {
old_cpu = cmpxchg(&kdb_printf_cpu, -1, this_cpu);
@@ -849,7 +845,6 @@ int vkdb_printf(enum kdb_msgsrc src, const char *fmt, va_list ap)
console_loglevel = saved_loglevel;
/* kdb_printf_cpu locked the code above. */
smp_store_release(&kdb_printf_cpu, old_cpu);
- kdb_trap_printk = saved_trap_printk;
return retlen;
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index f7a55e9ff2f7..2403800bb5f9 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -1926,7 +1926,8 @@ int vprintk_default(const char *fmt, va_list args)
int r;

- if (unlikely(kdb_trap_printk)) {
+ /* Allow to pass printk() to kdb but avoid a recursion. */
+ if (unlikely(kdb_trap_printk && kdb_printf_cpu < 0)) {
r = vkdb_printf(KDB_MSGSRC_PRINTK, fmt, args);
return r;