printk: ringbuffer: Fix truncating buffer size min_t cast

If an output buffer size exceeded U16_MAX, the min_t(u16, ...) cast in
copy_data() was causing writes to truncate. This manifested as output
bytes being skipped, seen as %NUL bytes in pstore dumps when the available
record size was larger than 65536. Fix the cast to no longer truncate
the calculation.

Cc: Petr Mladek <[email protected]>
Cc: Sergey Senozhatsky <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: John Ogness <[email protected]>
Reported-by: Vijay Balakrishna <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]/
Fixes: b6cf8b3 ("printk: add lockless ringbuffer")
Cc: [email protected]
Signed-off-by: Kees Cook <[email protected]>
Tested-by: Vijay Balakrishna <[email protected]>
Tested-by: Guilherme G. Piccoli <[email protected]> # Steam Deck
Reviewed-by: Tyler Hicks (Microsoft) <[email protected]>
Tested-by: Tyler Hicks (Microsoft) <[email protected]>
Reviewed-by: John Ogness <[email protected]>
Reviewed-by: Sergey Senozhatsky <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Signed-off-by: Petr Mladek <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: kees

kernel/printk/printk_ringbuffer.c

@@ -1735,7 +1735,7 @@ static bool copy_data(struct prb_data_ring *data_ring,
 	if (!buf || !buf_size)
 		return true;
 
-	data_size = min_t(u16, buf_size, len);
+	data_size = min_t(unsigned int, buf_size, len);
 
 	memcpy(&buf[0], data, data_size); /* LMM(copy_data:A) */
 	return true;