linux-kernel - Re: [PATCHv2 02/10] kernel: Provide READ_ONCE and ASSIGN

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

Message-ID: <5474D9EE.6070305@de.ibm.com>
Date:	Tue, 25 Nov 2014 20:35:10 +0100
From:	Christian Borntraeger <borntraeger@...ibm.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>, mingo@...nel.org,
	torvalds@...ux-foundation.org,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Alexei Starovoitov <alexei.starovoitov@...il.com>,
	David Howells <dhowells@...hat.com>,
	Russell King <linux@....linux.org.uk>
Subject: Re: [PATCHv2 02/10] kernel: Provide READ_ONCE and ASSIGN_ONCE

Am 25.11.2014 um 16:59 schrieb Paul E. McKenney:
> On Tue, Nov 25, 2014 at 01:38:29PM +0100, Christian Borntraeger wrote:
>> ACCESS_ONCE does not work reliably on non-scalar types. For
>> example gcc 4.6 and 4.7 might remove the volatile tag for such
>> accesses during the SRA (scalar replacement of aggregates) step
>> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=58145)
>>
>> Let's provide READ_ONCE/ASSIGN_ONCE that will do all accesses via
>> scalar types.
>>
>> Signed-off-by: Christian Borntraeger <borntraeger@...ibm.com>
>> ---
>>  include/linux/compiler.h | 34 ++++++++++++++++++++++++++++++++++
>>  1 file changed, 34 insertions(+)
>>
>> diff --git a/include/linux/compiler.h b/include/linux/compiler.h
>> index d5ad7b1..0ff01f2 100644
>> --- a/include/linux/compiler.h
>> +++ b/include/linux/compiler.h
>> @@ -186,6 +186,40 @@ void ftrace_likely_update(struct ftrace_branch_data *f, int val, int expect);
>>  # define __UNIQUE_ID(prefix) __PASTE(__PASTE(__UNIQUE_ID_, prefix), __LINE__)
>>  #endif
>>
>> +#include <linux/types.h>
>> +
>> +static __always_inline void __assign_once_size(volatile void *p, void *res, int size)
>> +{
>> +	switch (size) {
>> +	case 1: *(volatile u8 *)p = *(u8 *)res; break;
>> +	case 2: *(volatile u16 *)p = *(u16 *)res; break;
>> +	case 4: *(volatile u32 *)p = *(u32 *)res; break;
>> +#ifdef CONFIG_64BIT
>> +	case 8: *(volatile u64 *)p = *(u64 *)res; break;
> 
> We really need something like this to catch invalid sizes:
> 
> 	default: invoke_nonexistent_function();
> 
> Of course, a BUILD_BUG_ON() would give a nicer error message.
> 
> Without this, in my testing, the following compiles without error, generating
> no code:
> 
> 	struct foo {
> 		int field[10];
> 	} f, f1;
> 
> 	f1 = READ_ONCE(f);
> 
> There is probably some better way to do this.

Yes, I was trying to do something for default, but we are in compiler.h and BUILD_BUG_ON etc are not defined. including other header files gave me some trouble, but doing it only inside the ifdef !assembly might work out (as I did with linux/types.h). 

The thing is this case was actually detected see the pmd_t compile error for m68k and sparc. Defining an extern function named read_once_called_for_large_object and then let the linker do the real work might work out as well.


--
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/