linux-hardening - Re: [PATCH] tty: mxser: Remove __counted_by from mxser

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Message-ID: <c80e41e6-793e-4311-8e15-f5eda91e723e@embeddedor.com>
Date: Wed, 12 Jun 2024 14:04:31 -0600
From: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To: Jiri Slaby <jirislaby@...nel.org>, Bill Wendling <morbo@...gle.com>
Cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
 Nathan Chancellor <nathan@...nel.org>, Kees Cook <keescook@...omium.org>,
 "Gustavo A. R. Silva" <gustavoars@...nel.org>,
 Justin Stitt <justinstitt@...gle.com>, linux-serial@...r.kernel.org,
 imx@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
 linux-hardening@...r.kernel.org, llvm@...ts.linux.dev,
 patches@...ts.linux.dev, stable@...r.kernel.org
Subject: Re: [PATCH] tty: mxser: Remove __counted_by from mxser_board.ports[]

On 6/3/24 02:26, Gustavo A. R. Silva wrote:
> 
>> FWIW undoing:
>> commit 7391ee16950e772076d321792d9fbf030f921345
>> Author: Peter Hurley <peter@...leysoftware.com>
>> Date:   Sat Jun 15 09:36:07 2013 -0400
>>
>>      tty: Simplify flip buffer list with 0-sized sentinel
>>
>>
>> would do the job, IMO.
> 
> So, not even _sentinel_ is actually needed? Awesome!
> 

It seems that a clean revert is not possible at this point, as
the original patch is more than a decade old. If _sentinel_ is
not needed, and based on the original patch, would the following
suffice?

diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index 79f0ff94ce00..1b77019cc510 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -135,10 +135,7 @@ void tty_buffer_free_all(struct tty_port *port)
         llist_for_each_entry_safe(p, next, llist, free)
                 kfree(p);

-       tty_buffer_reset(&buf->sentinel, 0);
-       buf->head = &buf->sentinel;
-       buf->tail = &buf->sentinel;
-
+       buf->tail = NULL;
         still_used = atomic_xchg(&buf->mem_used, 0);
         WARN(still_used != freed, "we still have not freed %d bytes!",
                         still_used - freed);
@@ -578,9 +575,8 @@ void tty_buffer_init(struct tty_port *port)
         struct tty_bufhead *buf = &port->buf;

         mutex_init(&buf->lock);
-       tty_buffer_reset(&buf->sentinel, 0);
-       buf->head = &buf->sentinel;
-       buf->tail = &buf->sentinel;
+       buf->head = NULL;
+       buf->tail = NULL;
         init_llist_head(&buf->free);
         atomic_set(&buf->mem_used, 0);
         atomic_set(&buf->priority, 0);
diff --git a/include/linux/tty_buffer.h b/include/linux/tty_buffer.h
index 31125e3be3c5..75fb041e43fe 100644
--- a/include/linux/tty_buffer.h
+++ b/include/linux/tty_buffer.h
@@ -37,7 +37,6 @@ struct tty_bufhead {
         struct work_struct work;
         struct mutex       lock;
         atomic_t           priority;
-       struct tty_buffer sentinel;
         struct llist_head free;         /* Free queue head */
         atomic_t           mem_used;    /* In-use buffers excluding free list */
         int                mem_limit;

Thanks
--
Gustavo