On Mon 2023-08-14 10:42:26, David Laight wrote:
From: Kees Cook
Sent: 11 August 2023 06:46
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.
...
diff --git a/kernel/printk/printk_ringbuffer.c b/kernel/printk/printk_ringbuffer.c index 2dc4d5a1f1ff..fde338606ce8 100644 --- a/kernel/printk/printk_ringbuffer.c +++ b/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);
I'd noticed that during one of my test compiles while looking at making min() less fussy.
A better fix would be: data_size = min(buf_size + 0u, len);
This looks like a magic to me. The types are:
unsigned int data_size; unsigned int buf_size; u16 len
I would naively expect that
data_size = min(buf_size, len);
would do the right job and expand @len to "unsigned int".
I do not remember why "min_t" was used. Was it an optimization? Did we miss the problem with casting "u32" down to "u16"?
I tried to read the discussion at https://lore.kernel.org/lkml/b6a49ed73aba427ca8bb433763fa94e9@AcuMS.aculab.c... but it is more about "signed" vs. "unsigned" problem. Maybe it is more complicated that I expected.
Or put an ack on my patch 3/5 to minmax.h and then min(buf_size, len) will be fine (because both arguments are unsigned).
Do you mean https://lore.kernel.org/lkml/6dc20ac7cb6f4570a0160f076e8362e3@AcuMS.aculab.c... ? It seems to be just indentation cleanup.
Best Regards, Petr
PS: I have already pushed the patch because it looked reasonable and got testing. I have to admit that I am probably in a pre-vacation hurry mode.