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Subject[PATCH v4 1/9] sysrq: Implement __handle_sysrq_nolock to avoid recursive locking in kdb
Date
If kdb is triggered using SysRq-g then any use of the sr command results
in the SysRq key table lock being recursively acquired, killing the debug
session. That patch resolves the problem by introducing a _nolock
alternative for __handle_sysrq.

Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
---
drivers/tty/sysrq.c | 11 ++++++++---
include/linux/sysrq.h | 1 +
kernel/debug/kdb/kdb_main.c | 9 ++++++++-
3 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index ce396ec..7b47b2d 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -505,14 +505,12 @@ static void __sysrq_put_key_op(int key, struct sysrq_key_op *op_p)
sysrq_key_table[i] = op_p;
}

-void __handle_sysrq(int key, bool check_mask)
+void __handle_sysrq_nolock(int key, bool check_mask)
{
struct sysrq_key_op *op_p;
int orig_log_level;
int i;
- unsigned long flags;

- spin_lock_irqsave(&sysrq_key_table_lock, flags);
/*
* Raise the apparent loglevel to maximum so that the sysrq header
* is shown to provide the user with positive feedback. We do not
@@ -554,6 +552,13 @@ void __handle_sysrq(int key, bool check_mask)
printk("\n");
console_loglevel = orig_log_level;
}
+}
+
+void __handle_sysrq(int key, bool check_mask)
+{
+ unsigned long flags;
+ spin_lock_irqsave(&sysrq_key_table_lock, flags);
+ __handle_sysrq_nolock(key, check_mask);
spin_unlock_irqrestore(&sysrq_key_table_lock, flags);
}

diff --git a/include/linux/sysrq.h b/include/linux/sysrq.h
index 387fa7d..1d51d64 100644
--- a/include/linux/sysrq.h
+++ b/include/linux/sysrq.h
@@ -44,6 +44,7 @@ struct sysrq_key_op {

void handle_sysrq(int key);
void __handle_sysrq(int key, bool check_mask);
+void __handle_sysrq_nolock(int key, bool check_mask);
int register_sysrq_key(int key, struct sysrq_key_op *op);
int unregister_sysrq_key(int key, struct sysrq_key_op *op);
struct sysrq_key_op *__sysrq_get_key_op(int key);
diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index 0b097c8..de439bb 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -1918,13 +1918,20 @@ static int kdb_rm(int argc, const char **argv)
* kdb_sr - This function implements the 'sr' (SYSRQ key) command
* which interfaces to the soi-disant MAGIC SYSRQ functionality.
* sr <magic-sysrq-code>
+ *
+ * Remarks:
+ * This command calls into the sysrq code without locking. The sysrq
+ * code is protected using spin_lock_irqsave meaning this command,
+ * whilst normally safe, must be used very carefully if kdb is entered
+ * using a mechanism, such as the NMI, which is immune to the interrupt
+ * mask.
*/
static int kdb_sr(int argc, const char **argv)
{
if (argc != 1)
return KDB_ARGCOUNT;
kdb_trap_printk++;
- __handle_sysrq(*argv[1], false);
+ __handle_sysrq_nolock(*argv[1], false);
kdb_trap_printk--;

return 0;
--
1.9.0


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