Re: [PATCH printk v2 1/2] printk_ringbuffer: Fix get_data() size sanity check

[Petr Mladek <pmladek@suse.com>]

On Thu 2026-03-26 14:44:01, John Ogness wrote:
> Commit cc3bad11de6e ("printk_ringbuffer: Fix check of valid data
> size when blk_lpos overflows") added sanity checking to get_data()
> to avoid returning data of illegal sizes (too large or too small).
> It uses the helper function data_check_size() for the check.
> However, data_check_size() expects the size of the data, not the
> size of the data block. get_data() is providing the size of the
> data block.  This means that if the data size (text_buf_size) is
> at or near the maximum legal size:
> 
> sizeof(prb_data_block) + text_buf_size == DATA_SIZE(data_ring) / 2
> 
> data_check_size() will report failure because it adds
> sizeof(prb_data_block) to the provided size. The sanity check in
> get_data() is counting the data block header twice. The result is
> that the reader fails to read the legal record.
> 
> Since get_data() subtracts the data block header size before returning,
> move the sanity check to after the subtraction.
> 
> Luckily printk() is not vulnerable to this problem because
> truncate_msg() limits printk-messages to 1/4 of the ringbuffer.
> Indeed, by adjusting the printk_ringbuffer KUnit test, which does not
> use printk() and its truncate_msg() check, it is easy to see that the
> reader fails and the WARN_ON is triggered.
> 
> Fixes: cc3bad11de6e ("printk_ringbuffer: Fix check of valid data size when blk_lpos overflows")
> Signed-off-by: John Ogness <john.ogness@linutronix.de>

Great catch. Looks good to me.

Reviewed-by: Petr Mladek <pmladek@suse.com>
Tested-by: Petr Mladek <pmladek@suse.com>

Best Regards,
Petr