From: Len Baker <len.baker@gmx.com>
To: Hans de Goede <hdegoede@redhat.com>, Kees Cook <keescook@chromium.org>
Cc: Len Baker <len.baker@gmx.com>,
Henrique de Moraes Holschuh <hmh@hmh.eng.br>,
Mark Gross <mgross@linux.intel.com>,
"Gustavo A. R. Silva" <gustavoars@kernel.org>,
ibm-acpi-devel@lists.sourceforge.net,
platform-driver-x86@vger.kernel.org,
linux-hardening@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] platform/x86: thinkpad_acpi: Prefer struct_size over open coded arithmetic
Date: Sat, 25 Sep 2021 12:37:41 +0200 [thread overview]
Message-ID: <20210925075920.GC1660@titan> (raw)
In-Reply-To: <ba427967-cb1b-58a8-ec93-bd5ae89f58f8@redhat.com>
Hi,
On Tue, Sep 21, 2021 at 03:46:23PM +0200, Hans de Goede wrote:
> On 9/20/21 7:58 AM, Kees Cook wrote:
> > On Sat, Sep 18, 2021 at 05:05:00PM +0200, Len Baker wrote:
> >>
> >> static struct attribute_set *create_attr_set(unsigned int max_members,
> >> @@ -1020,13 +1020,11 @@ static struct attribute_set *create_attr_set(unsigned int max_members,
> >> return NULL;
> >>
> >> /* Allocates space for implicit NULL at the end too */
> >> - sobj = kzalloc(sizeof(struct attribute_set_obj) +
> >> - max_members * sizeof(struct attribute *),
> >> - GFP_KERNEL);
> >> + sobj = kzalloc(struct_size(sobj, a, max_members + 1), GFP_KERNEL);
> >
> > Whoa, this needs a lot more detail in the changelog if this is actually
> > correct. The original code doesn't seem to match the comment? (Where is
> > the +1?) So is this also a bug-fix?
>
> Kees, at first I thought you were spot-on with this comment, but the
> truth is more subtle. struct attribute_set_obj was:
>
> struct attribute_set_obj {
> struct attribute_set s;
> struct attribute *a;
> } __attribute__((packed));
>
> Another way of looking at this, which makes things more clear is as:
>
> struct attribute_set_obj {
> struct attribute_set s;
> struct attribute *a[1];
> } __attribute__((packed));
>
> So the sizeof(struct attribute_set_obj) in the original kzalloc call
> included room for 1 "extra" pointer which is reserved for the terminating
> NULL pointer.
>
> Changing the struct to:
>
> struct attribute_set_obj {
> struct attribute_set s;
> struct attribute *a[];
> } __attribute__((packed));
>
> Is equivalent to changing it to:
>
> struct attribute_set_obj {
> struct attribute_set s;
> struct attribute *a[0];
> } __attribute__((packed));
>
> So the change in the struct declaration reduces the sizeof(struct attribute_set_obj)
> by the size of 1 pointer, making the +1 necessary.
>
> So AFAICT there is actually no functional change here.
Hans, thanks for the explanation. Yes, this is the reason I added the "plus 1".
Not only based on the comment :)
Regards,
Len
prev parent reply other threads:[~2021-09-25 10:38 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-18 15:05 [PATCH] platform/x86: thinkpad_acpi: Prefer struct_size over open coded arithmetic Len Baker
2021-09-20 5:58 ` Kees Cook
2021-09-21 13:46 ` Hans de Goede
2021-09-21 15:15 ` Greg KH
2021-09-21 15:38 ` Hans de Goede
2021-09-21 15:45 ` Greg KH
2021-09-25 10:40 ` Len Baker
2021-09-25 11:07 ` Greg KH
2021-09-25 13:33 ` Len Baker
2021-09-25 10:37 ` Len Baker [this message]
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