Re: [syzbot] [iomap?] kernel BUG in folio_end_read (2)

John Ogness
2025-11-06T11:36:24Z

On 2025-11-05, John Ogness <john.ogness@linutronix.de> wrote:
>> Another question is whether this is the only problem caused the patch.
>
> This comparison is quite special. It caught my attention while combing
> through the code.

The reason that this comparison is special is because it is the only one
that does not take wrapping into account. I did it that way originally
because it is AND with a wrap check. But this is an ugly special
case. It should use the same wrap check as the other 3 cases in
nbcon.c. If it had, the bug would not have happened.

I always considered these wrap checks to be non-obvious and
error-prone. So what if we create a nice helper function to simplify and
unify the wrap checks? Something like this:

diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c
index 839f504db6d30..8499ee642c31d 100644
--- a/kernel/printk/printk_ringbuffer.c
+++ b/kernel/printk/printk_ringbuffer.c
@@ -390,6 +390,17 @@ static unsigned int to_blk_size(unsigned int size)
return size;
}

+/*
+ * Check if @lpos1 is before @lpos2. This takes ringbuffer wrapping
+ * into account. If @lpos1 is more than a full wrap before @lpos2,
+ * it is considered to be after @lpos2.
+ */
+static bool lpos1_before_lpos2(struct prb_data_ring *data_ring,
+ unsigned long lpos1, unsigned long lpos2)
+{
+ return lpos2 - lpos1 - 1 < DATA_SIZE(data_ring);
+}
+
/*
* Sanity checker for reserve size. The ringbuffer code assumes that a data
* block does not exceed the maximum possible size that could fit within the
@@ -577,7 +588,7 @@ static bool data_make_reusable(struct printk_ringbuffer *rb,
unsigned long id;

/* Loop until @lpos_begin has advanced to or beyond @lpos_end. */
- while ((lpos_end - lpos_begin) - 1 < DATA_SIZE(data_ring)) {
+ while (lpos1_before_lpos2(data_ring, lpos_begin, lpos_end)) {
blk = to_block(data_ring, lpos_begin);

/*
@@ -668,7 +679,7 @@ static bool data_push_tail(struct printk_ringbuffer *rb, unsigned long lpos)
* sees the new tail lpos, any descriptor states that transitioned to
* the reusable state must already be visible.
*/
- while ((lpos - tail_lpos) - 1 < DATA_SIZE(data_ring)) {
+ while (lpos1_before_lpos2(data_ring, tail_lpos, lpos)) {
/*
* Make all descriptors reusable that are associated with
* data blocks before @lpos.
@@ -1149,7 +1160,7 @@ static char *data_realloc(struct printk_ringbuffer *rb, unsigned int size,
next_lpos = get_next_lpos(data_ring, blk_lpos->begin, size);

/* If the data block does not increase, there is nothing to do. */
- if (head_lpos - next_lpos < DATA_SIZE(data_ring)) {
+ if (!lpos1_before_lpos2(data_ring, head_lpos, next_lpos)) {
if (wrapped)
blk = to_block(data_ring, 0);
else
@@ -1262,7 +1273,7 @@ static const char *get_data(struct prb_data_ring *data_ring,

/* Regular data block: @begin less than @next and in same wrap. */
if (!is_blk_wrapped(data_ring, blk_lpos->begin, blk_lpos->next) &&
- blk_lpos->begin < blk_lpos->next) {
+ lpos1_before_lpos2(data_ring, blk_lpos->begin, blk_lpos->next)) {
db = to_block(data_ring, blk_lpos->begin);
*data_size = blk_lpos->next - blk_lpos->begin;

This change also fixes the issue. Thoughts?

John