[RFC PATCH v4 4/9] printk-rb: initialize new descriptors as invalid

John Ogness
2019-08-07T22:27:20Z

Initialize never-used descriptors as permanently invalid so there
is no risk of the descriptor unexpectedly being determined as
valid due to dataring head overflowing/wrapping.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
---
kernel/printk/dataring.c | 42 +++++++++++++++++++++++++++-----------
kernel/printk/dataring.h | 12 +++++++++++
kernel/printk/ringbuffer.c | 5 +++++
kernel/printk/ringbuffer.h | 7 ++++++-
4 files changed, 53 insertions(+), 13 deletions(-)

diff --git a/kernel/printk/dataring.c b/kernel/printk/dataring.c
index 6642e085a05d..345c46dba5bb 100644
--- a/kernel/printk/dataring.c
+++ b/kernel/printk/dataring.c
@@ -316,8 +316,8 @@ static unsigned long _dataring_pop(struct dataring *dr,
* If dB reads from gA, then dD reads from fH.
* If dB reads from gA, then dE reads from fE.
*
- * Note that if dB reads from gA, then dC cannot read from fC.
- * Note that if dB reads from gA, then dD cannot read from fD.
+ * Note that if dB reads from gA, then dC cannot read from fC->nA.
+ * Note that if dB reads from gA, then dD cannot read from fC->nB.
*
* Relies on:
*
@@ -489,6 +489,30 @@ static bool get_new_lpos(struct dataring *dr, unsigned int size,
}
}

+/**
+ * dataring_desc_init() - Initialize a descriptor to be permanently invalid.
+ *
+ * @desc: The descriptor to initialize.
+ *
+ * Setting a descriptor to be permanently invalid means that there is no risk
+ * of the descriptor later unexpectedly being determined as valid due to
+ * overflowing/wrapping of @head_lpos.
+ */
+void dataring_desc_init(struct dr_desc *desc)
+{
+ /*
+ * An unaligned @begin_lpos can never point to a data block and
+ * having the same value for @begin_lpos and @next_lpos is also
+ * invalid.
+ */
+
+ /* nA: */
+ WRITE_ONCE(desc->begin_lpos, 1);
+
+ /* nB: */
+ WRITE_ONCE(desc->next_lpos, 1);
+}
+
/**
* dataring_push() - Reserve a data block in the data array.
*
@@ -568,20 +592,14 @@ char *dataring_push(struct dataring *dr, unsigned int size,

if (!ret) {
/*
+ * fC:
+ *
* Force @desc permanently invalid to minimize risk
* of the descriptor later unexpectedly being
* determined as valid due to overflowing/wrapping of
- * @head_lpos. An unaligned @begin_lpos can never
- * point to a data block and having the same value
- * for @begin_lpos and @next_lpos is also invalid.
+ * @head_lpos.
*/
-
- /* fC: */
- WRITE_ONCE(desc->begin_lpos, 1);
-
- /* fD: */
- WRITE_ONCE(desc->next_lpos, 1);
-
+ dataring_desc_init(desc);
return NULL;
}
/* fE: */
diff --git a/kernel/printk/dataring.h b/kernel/printk/dataring.h
index 346a455a335a..413ee95f4dd6 100644
--- a/kernel/printk/dataring.h
+++ b/kernel/printk/dataring.h
@@ -43,6 +43,17 @@ struct dr_desc {
unsigned long next_lpos;
};

+/*
+ * Initialize a descriptor to be permanently invalid so there is no risk
+ * of the descriptor later unexpectedly being determined as valid due to
+ * overflowing/wrapping of @head_lpos.
+ */
+#define __DR_DESC_INITIALIZER \
+ { \
+ .begin_lpos = 1, \
+ .next_lpos = 1, \
+ }
+
/**
* struct dataring - A data ringbuffer with support for entry IDs.
*
@@ -90,6 +101,7 @@ void dataring_datablock_setid(struct dataring *dr, struct dr_desc *desc,
struct dr_datablock *dataring_getdatablock(struct dataring *dr,
struct dr_desc *desc, int *size);
bool dataring_datablock_isvalid(struct dataring *dr, struct dr_desc *desc);
+void dataring_desc_init(struct dr_desc *desc);
void dataring_desc_copy(struct dr_desc *dst, struct dr_desc *src);

#endif /* _KERNEL_PRINTK_DATARING_H */
diff --git a/kernel/printk/ringbuffer.c b/kernel/printk/ringbuffer.c
index b9fc13597b10..9be841761ea2 100644
--- a/kernel/printk/ringbuffer.c
+++ b/kernel/printk/ringbuffer.c
@@ -345,6 +345,11 @@ static bool assign_desc(struct prb_reserved_entry *e)
if (i < DESCS_COUNT(rb)) {
d = &rb->descs[i];
atomic_long_set(&d->id, i);
+ /*
+ * Initialize the new descriptor such that
+ * it is permanently invalid.
+ */
+ dataring_desc_init(&d->desc);
break;
}
}
diff --git a/kernel/printk/ringbuffer.h b/kernel/printk/ringbuffer.h
index 9fe54a09fbc2..462b4d3a3ee2 100644
--- a/kernel/printk/ringbuffer.h
+++ b/kernel/printk/ringbuffer.h
@@ -178,7 +178,12 @@ struct dr_desc *prb_getdesc(unsigned long id, void *arg);
char _##name##_data[(1 << ((avgdatabits) + (descbits))) + \
sizeof(long)] \
__aligned(__alignof__(long)); \
-struct prb_desc _##name##_descs[1 << (descbits)]; \
+struct prb_desc _##name##_descs[1 << (descbits)] = { \
+ { \
+ .id = ATOMIC_LONG_INIT(0), \
+ .desc = __DR_DESC_INITIALIZER, \
+ }, \
+ }; \
struct printk_ringbuffer name = { \
.desc_count_bits = descbits, \
.descs = &_##name##_descs[0], \
--
2.20.1