From: Linus Torvalds <torvalds@linux-foundation.org>
To: Joe Perches <joe@perches.com>
Cc: Waiman Long <longman@redhat.com>,
Andrew Morton <akpm@linux-foundation.org>,
David Howells <dhowells@redhat.com>,
Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>,
James Morris <jmorris@namei.org>,
"Serge E. Hallyn" <serge@hallyn.com>,
Linux-MM <linux-mm@kvack.org>,
keyrings@vger.kernel.org,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Matthew Wilcox <willy@infradead.org>,
David Rientjes <rientjes@google.com>
Subject: Re: [PATCH v2] mm: Add kvfree_sensitive() for freeing sensitive data objects
Date: Tue, 7 Apr 2020 13:16:47 -0700 [thread overview]
Message-ID: <CAHk-=wg_mkSc-pH8ntGHR=no9DOLRQyxdtU20p55DrM1su6QzA@mail.gmail.com> (raw)
In-Reply-To: <c2c8adf48be7cb18bbdf0aef7d21e2defe3d2183.camel@perches.com>
On Mon, Apr 6, 2020 at 12:40 PM Joe Perches <joe@perches.com> wrote:
>
> 2.1.44 changed kfree(void *) to kfree(const void *) but
> I didn't find a particular reason why.
Because "free()" should always have been const (and volatile, for that
matter, but the kernel doesn't care since we eschew volatile data
structures).
It's a bug in the C library standard.
Think of it this way: free() doesn't really change the data, it kills
the lifetime of it. You can't access it afterwards - you can neither
read it nor write it validly. That is a completely different - and
independent - operation from writing to it.
And more importantly, it's perfectly fine to have a const data
structure (or a volatile one) that you free. The allocation may have
done something like this:
struct mystruct {
const struct dictionary *dictionary;
...
};
and it was allocated and initialized before it was assigned to that
"dictionary" pointer. That's _good_ code.
So it wasn't const before the allocation, but it turned const
afterwards, and freeing it doesn't change that, it just kills the
lifetime entirely.
So "free()" should take a const pointer without complaining, and saying
free(mystruct->dictionary);
free(mystruct);
is a sensible an correct thing to do. Warning about - or requiring
that dictionary pointer to be cast to be freed - is fundamentally
wrong.
We're not bound by the fact that the C standard library got their
rules wrong, so we can fix it in the kernel.
Linus
next prev parent reply other threads:[~2020-04-07 20:17 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-06 18:58 [PATCH v2] mm: Add kvfree_sensitive() for freeing sensitive data objects Waiman Long
2020-04-06 19:38 ` Joe Perches
2020-04-07 2:16 ` Waiman Long
2020-04-07 6:41 ` Joe Perches
2020-04-07 20:16 ` Linus Torvalds [this message]
2020-04-07 20:26 ` Linus Torvalds
2020-04-07 21:14 ` David Howells
2020-04-07 21:22 ` Linus Torvalds
2020-04-07 22:24 ` Matthew Wilcox
2020-04-07 22:54 ` David Howells
2020-04-07 23:50 ` Linus Torvalds
2020-04-06 20:00 ` Matthew Wilcox
2020-04-07 20:07 ` Waiman Long
2020-04-07 20:01 Waiman Long
2020-04-07 20:02 ` Waiman Long
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