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[ Upstream commit9361ebfbb7] gsmtty_write() does not prevent the user to use the full fifo size of 4096 bytes as allocated in gsm_dlci_alloc(). However, gsmtty_write_room() tries to limit the return value by 'TX_SIZE' and returns a negative value if the fifo has more than 'TX_SIZE' bytes stored. This is obviously wrong as 'TX_SIZE' is defined as 512. Define 'TX_SIZE' to the fifo size and use it accordingly for allocation to keep the current behavior. Return the correct remaining size of the fifo in gsmtty_write_room() via kfifo_avail(). Fixes:e1eaea46bb("tty: n_gsm line discipline") Cc: stable@vger.kernel.org Signed-off-by: Daniel Starke <daniel.starke@siemens.com> Link: https://lore.kernel.org/r/20220504081733.3494-3-daniel.starke@siemens.com Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org> Signed-off-by: Sasha Levin <sashal@kernel.org>