linux-kernel - Re: [PATCH/RFC 7/7] kernel: Force ACCESS

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAADnVQ+8d0GC5OwQTdJKkmG8=vshUVqK1pPou1GCGa_iuU2v-g@mail.gmail.com>
Date:	Mon, 24 Nov 2014 14:58:08 -0800
From:	Alexei Starovoitov <alexei.starovoitov@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	David Howells <dhowells@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
	linux-mips <linux-mips@...ux-mips.org>,
	linux-x86_64@...r.kernel.org,
	linux-s390 <linux-s390@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH/RFC 7/7] kernel: Force ACCESS_ONCE to work only on scalar types

On Mon, Nov 24, 2014 at 12:34 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Nov 24, 2014 at 12:04 PM, David Howells <dhowells@...hat.com> wrote:
>>
>> Reserve ACCESS_ONCE() for reading and add an ASSIGN_ONCE() or something like
>> that for writing?
>
> I wouldn't mind that. We've had situations where reading and writing
> isn't really similar - like alpha where reading a byte is atomic, but
> writing one isn't.
>
> Then we could also make it have the "get_user()/put_user()" kind of
> semantics - .and then use the same "sizeopf()" tricks that we use for
> get_user/put_user.
>
> That would actually work around the gcc bug a completely different way:
>
>   #define ACCESS_ONCE(p) \
>       ({ typeof(*p) __val; __read_once_size(p, &__val, sizeof(__val)); __val; })
>
> and then we can do things like this:
>
>   static __always_inline void __read_once_size(volatile void *p, void
> *res, int size)
>   {
>        switch (size) {
>        case 1: *(u8 *)res = *(volatile u8 *)p; break;
>        case 2: *(u16 *)res = *(volatile u16 *)p; break;
>        case 4: *(u32 *)res = *(volatile u32 *)p; break;
> #ifdef CONFIG_64BIT
>        case 8: *(u64 *)res = *(volatile u64 *)p; break;
> #endif
>        }
>   }
>
> and same for ASSIGN_ONCE(val, p).
>
> That also hopefully avoids the whole "oops, gcc has a bug", because
> the actual volatile access is always done using a scalar type, even if
> the type of "__val" may in fact be a structure.

I've changed gcc pr58145-1.c reproducer to use
__read_once_size() approach above, but it
didn't help to workaround the bug.
gcc 'sra' pass still lost 'volatile' modifier.

modified reproducer:
struct S { unsigned int data; };
void bar(int val)
{
  struct S _s = { .data = val };
  *(volatile struct S *) 0x880000UL = ACCESS_ONCE(&_s);
}
assembler code looks as expected, but
internal gcc state after 'sra' pass is
missing volatile...
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/