From: Alexandre Chartre <alexandre.chartre@oracle.com>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>,
x86@kernel.org, linux-kernel@vger.kernel.org,
jthierry@redhat.com, tglx@linutronix.de
Subject: Re: [PATCH V2 9/9] x86/speculation: Remove all ANNOTATE_NOSPEC_ALTERNATIVE directives
Date: Tue, 7 Apr 2020 19:01:43 +0200 [thread overview]
Message-ID: <06605b1c-c9ae-fd18-bcb1-e4a48f2cb60e@oracle.com> (raw)
In-Reply-To: <20200407162838.5hlh6oom4oa45ugt@treble>
On 4/7/20 6:28 PM, Josh Poimboeuf wrote:
> On Tue, Apr 07, 2020 at 06:18:51PM +0200, Alexandre Chartre wrote:
>>
>> On 4/7/20 4:32 PM, Alexandre Chartre wrote:
>>>
>>> On 4/7/20 3:34 PM, Josh Poimboeuf wrote:
>>>> On Tue, Apr 07, 2020 at 03:28:37PM +0200, Peter Zijlstra wrote:
>>>>> Josh, we should probably have objtool verify it doesn't emit ORC entries
>>>>> in alternative ranges.
>>>>
>>>> Agreed, it might be as simple as checking for insn->alt_group in the
>>>> INSN_STACK check or in update_insn_state().
>>>>
>>>
>>> We could do that only for the "objtool orc generate" command. That way
>>> "objtool check" would still check the alternative, but "objtool orc generate"
>>> will just use the first half of the alternative (like it does today with
>>> ANNOTATE_NOSPEC_ALTERNATIVE). We can even keep all ANNOTATE_NOSPEC_ALTERNATIVE
>>> but only use them for "objtool orc generate".
>>>
>>
>> I have checked and objtool doesn't emit ORC entries for alternative:
>> decode_instructions() doesn't mark such section with sec->text = true
>> so create_orc_sections() doesn't emit corresponding ORC entries.
>>
>> So I think we can remove the ANNOTATE_NOSPEC_ALTERNATIVE directives,
>> this will allow objtool to check the instructions but it still won't
>> emit ORC entries (same behavior as today). In the future, if ORC
>> eventually supports alternative we will be ready to have objtool emit
>> ORC entries.
>
> What's the benefit of removing ANNOTATE_NOSPEC_ALTERNATIVE if there's no
> ORC support to go along with it?
To have the code validated by objtool like any other alternative code
(which is not tagged with ANNOTATE_NOSPEC_ALTERNATIVE).
> Also I want to avoid adding "ORC alternatives". ORC is nice and simple
> and we should keep it that way as much as possible.
>
> Again, we should warn on stack changes inside alternatives, and then
> look at converting RSB and retpolines to use static branches so they
> have deterministic stacks.
>
objtool doesn't currently warn on stack changes inside alternatives.
The RSB/retpoline alternatives have warning because objtool doesn't
support retpoline ret and intra-function calls. If you have an alternative
doing stack changes that objtool understand (like push/pop, add/remove
to sp) then you won't have a warning.
I think that's the case with smap_save:
static __always_inline unsigned long smap_save(void)
{
unsigned long flags;
asm volatile (ALTERNATIVE("", "pushf; pop %0; " __ASM_CLAC,
X86_FEATURE_SMAP)
: "=rm" (flags) : : "memory", "cc");
return flags;
}
The alternative does change the stack but objtool won't complain
because it handles the pushf and pop instruction.
alex.
next prev parent reply other threads:[~2020-04-07 16:57 UTC|newest]
Thread overview: 40+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-07 7:31 [PATCH V2 0/9] objtool changes to remove all ANNOTATE_NOSPEC_ALTERNATIVE Alexandre Chartre
2020-04-07 7:31 ` [PATCH V2 1/9] objtool: Introduce HINT_RET_OFFSET Alexandre Chartre
2020-04-07 12:53 ` Peter Zijlstra
2020-04-07 13:17 ` Alexandre Chartre
2020-04-07 7:31 ` [PATCH V2 2/9] objtool: UNWIND_HINT_RET_OFFSET should not check registers Alexandre Chartre
2020-05-01 18:22 ` [tip: objtool/core] " tip-bot2 for Alexandre Chartre
2020-04-07 7:31 ` [PATCH V2 3/9] objtool: is_fentry_call() crashes if call has no destination Alexandre Chartre
2020-04-07 7:31 ` [PATCH V2 4/9] objtool: Allow branches within the same alternative Alexandre Chartre
2020-04-07 7:31 ` [PATCH V2 5/9] objtool: Add support for intra-function calls Alexandre Chartre
2020-04-07 13:07 ` Peter Zijlstra
2020-04-07 13:28 ` Alexandre Chartre
2020-04-08 14:06 ` Alexandre Chartre
2020-04-08 14:19 ` Julien Thierry
2020-04-08 16:03 ` Alexandre Chartre
2020-04-08 16:04 ` Julien Thierry
2020-04-08 17:06 ` Alexandre Chartre
2020-04-08 17:07 ` Julien Thierry
2020-04-07 7:31 ` [PATCH V2 6/9] x86/speculation: Change __FILL_RETURN_BUFFER to work with objtool Alexandre Chartre
2020-04-07 13:27 ` Josh Poimboeuf
2020-04-07 7:31 ` [PATCH V2 7/9] x86/speculation: Annotate intra-function calls Alexandre Chartre
2020-04-07 7:31 ` [PATCH V2 8/9] x86/speculation: Add unwind hint to trampoline return Alexandre Chartre
2020-04-07 7:31 ` [PATCH V2 9/9] x86/speculation: Remove all ANNOTATE_NOSPEC_ALTERNATIVE directives Alexandre Chartre
2020-04-07 13:28 ` Peter Zijlstra
2020-04-07 13:34 ` Josh Poimboeuf
2020-04-07 14:32 ` Alexandre Chartre
2020-04-07 16:18 ` Alexandre Chartre
2020-04-07 16:28 ` Josh Poimboeuf
2020-04-07 17:01 ` Alexandre Chartre [this message]
2020-04-07 17:26 ` Peter Zijlstra
2020-04-07 17:27 ` Peter Zijlstra
2020-04-08 21:35 ` Peter Zijlstra
2020-04-09 8:18 ` Alexandre Chartre
2020-04-09 10:34 ` Peter Zijlstra
2020-04-09 10:40 ` Peter Zijlstra
2020-04-07 16:41 ` Peter Zijlstra
2020-04-07 17:04 ` Alexandre Chartre
2020-04-07 13:52 ` Peter Zijlstra
2020-04-07 13:59 ` Peter Zijlstra
2020-04-07 13:35 ` [PATCH V2 0/9] objtool changes to remove all ANNOTATE_NOSPEC_ALTERNATIVE Josh Poimboeuf
2020-04-07 14:02 ` Alexandre Chartre
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