linux-kernel - Re: [PATCH] staging: rtl8712: remove unnecessary alignment of pxmitpriv->pxmitbuf

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Date: Mon, 27 May 2024 10:06:22 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Nam Cao <namcao@...utronix.de>
Cc: Larry Finger <Larry.Finger@...inger.net>,
	Florian Schilhabel <florian.c.schilhabel@...glemail.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-staging@...ts.linux.dev, linux-kernel@...r.kernel.org,
	Nikita Zhandarovich <n.zhandarovich@...tech.ru>,
	syzbot+83763e624cfec6b462cb@...kaller.appspotmail.com
Subject: Re: [PATCH] staging: rtl8712: remove unnecessary alignment of
 pxmitpriv->pxmitbuf

On Sat, May 25, 2024 at 09:32:29AM +0200, Nam Cao wrote:
> This driver wants pxmitpriv->pxmitbuf to be 4-byte aligned. This is ensured
> by allocating 4 more bytes than required with kmalloc(), then do the
> p = p + 4 - (p & 3) trick to make sure the pointer is 4-byte aligned.
> 
> This is unnecessary. Pointers from kmalloc() are already at least
> 8-byte-aligned.
> 
> Remove this alignment trick to simplify the code, and also to stop wasting
> 4 extra bytes of dynamic memory allocator.
> 
> This also gets rid of a (false) warning from kmemleak. This 4-byte-aligned
> buffer is used to store pointers from kmalloc(). For 64-bit platforms,
> pointer size is 8 bytes and kmemleak only scans for pointers in 8-byte
> blocks, thus it misses the pointers stored in this 4-byte-aligned buffer
> and thinks that these pointers have been leaked. This is just a false
> warning, not a real problem. But still, it would be nice to get rid of
> these warnings.

Are you sure it's a false positive?  I've always wondered what happens
when you do:

	p = kmalloc();
	kfree((char *)p + 4);

regards,
dan carpenter