From: Andrew Cooper <andrew.cooper3@citrix.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: <x86@kernel.org>, <jpoimboe@redhat.com>,
<linux-kernel@vger.kernel.org>, <alexei.starovoitov@gmail.com>,
<ndesaulniers@google.com>,
Andrew Cooper <andrew.cooper3@citrix.com>
Subject: Re: [PATCH 4/9] x86/alternative: Implement .retpoline_sites support
Date: Wed, 13 Oct 2021 18:11:11 +0100 [thread overview]
Message-ID: <2c707dd6-796e-2de6-bce9-d0242f010513@citrix.com> (raw)
In-Reply-To: <YWb3TdmyPK7GwBP4@hirez.programming.kicks-ass.net>
On 13/10/2021 16:12, Peter Zijlstra wrote:
> On Wed, Oct 13, 2021 at 03:38:27PM +0100, Andrew Cooper wrote:
>> On 13/10/2021 13:22, Peter Zijlstra wrote:
>>> +/*
>>> + * Rewrite the compiler generated retpoline thunk calls.
>>> + *
>>> + * For spectre_v2=off (!X86_FEATURE_RETPOLINE), rewrite them into immediate
>>> + * indirect instructions, avoiding the extra indirection.
>>> + *
>>> + * For example, convert:
>>> + *
>>> + * CALL __x86_indirect_thunk_\reg
>>> + *
>>> + * into:
>>> + *
>>> + * CALL *%\reg
>>> + *
>>> + */
>>> +static int patch_retpoline(void *addr, struct insn *insn, u8 *bytes)
>>> +{
>>> + void (*target)(void);
>>> + int reg, i = 0;
>>> +
>>> + if (cpu_feature_enabled(X86_FEATURE_RETPOLINE))
>>> + return -1;
>>> +
>>> + target = addr + insn->length + insn->immediate.value;
>>> + reg = (target - &__x86_indirect_thunk_rax) /
>>> + (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);
>> This is equal measures beautiful and terrifying.
> Thanks! :-)
>
>> Something around here really wants to BUG_ON(reg == 4), because
>> literally nothing good can come from selecting %rsp.
> Ack, I had to add rsp to get the offsets right, but indeed, if anything
> ever selects that we're in trouble.
Actually, all you need is space for the RSP thunk, not an actual RSP
thunk, and it's probably a wise move not to write one out.
You can fill it with 0xcc's, and make sure not to make it an exported
symbol.
>
>> Also, it might be a good idea (previous patch perhaps) to have some
>> linker assertions to confirm that the symbols are laid out safely to do
>> this calculation.
> I was hoping that since all this is in .S it would be immune from crazy
> things like a compiler and do as told. But I suppose carzy stuff like
> LTO (or worse BOLT) can totaly wreck this still (then BOLT won't care
> about linker script assertions either).
>
> I'll see if I can come up with something.
Another cross check could be something like:
unsigned long reg_to_thunk[] = {
&__x86_indirec_thunk_rax,
...
};
because then BUG_ON(target != reg_to_thunk[reg]) will catch any errors
from layout issues.
Using 0 for rsp could then subsume the individual check.
~Andrew
next prev parent reply other threads:[~2021-10-13 17:18 UTC|newest]
Thread overview: 52+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-13 12:22 [PATCH 0/9] x86: Rewrite the retpoline rewrite logic Peter Zijlstra
2021-10-13 12:22 ` [PATCH 1/9] objtool,x86: Replace alternatives with .retpoline_sites Peter Zijlstra
2021-10-13 13:29 ` Borislav Petkov
2021-10-13 20:11 ` Josh Poimboeuf
2021-10-14 15:43 ` Peter Zijlstra
2021-10-13 12:22 ` [PATCH 2/9] x86/retpoline: Remove unused replacement symbols Peter Zijlstra
2021-10-13 12:22 ` [PATCH 3/9] x86/asm: Fix register order Peter Zijlstra
2021-10-13 20:15 ` Josh Poimboeuf
2021-10-13 12:22 ` [PATCH 4/9] x86/alternative: Implement .retpoline_sites support Peter Zijlstra
2021-10-13 14:38 ` Andrew Cooper
2021-10-13 15:12 ` Peter Zijlstra
2021-10-13 17:11 ` Andrew Cooper [this message]
2021-10-14 10:05 ` Peter Zijlstra
2021-10-13 20:39 ` Josh Poimboeuf
2021-10-13 21:20 ` Peter Zijlstra
2021-10-13 21:49 ` Josh Poimboeuf
2021-10-13 21:52 ` Josh Poimboeuf
2021-10-13 22:10 ` Peter Zijlstra
2021-10-13 22:47 ` Andrew Cooper
2021-10-13 20:52 ` Josh Poimboeuf
2021-10-13 21:00 ` Peter Zijlstra
2021-10-19 11:37 ` Peter Zijlstra
2021-10-19 16:46 ` Josh Poimboeuf
2021-10-19 16:49 ` Josh Poimboeuf
2021-10-20 8:25 ` Peter Zijlstra
2021-10-20 8:30 ` Peter Zijlstra
2021-10-13 21:11 ` Josh Poimboeuf
2021-10-13 21:43 ` Peter Zijlstra
2021-10-13 22:05 ` Josh Poimboeuf
2021-10-13 22:14 ` Peter Zijlstra
2021-10-15 14:24 ` Borislav Petkov
2021-10-15 16:56 ` Peter Zijlstra
2021-10-18 23:06 ` Alexander Lobakin
2021-10-19 0:25 ` Alexander Lobakin
2021-10-19 9:47 ` Alexander Lobakin
2021-10-19 10:16 ` Peter Zijlstra
2021-10-19 15:37 ` Sami Tolvanen
2021-10-19 18:00 ` Alexander Lobakin
2021-10-19 9:40 ` Peter Zijlstra
2021-10-19 10:02 ` Peter Zijlstra
2021-10-13 12:22 ` [PATCH 5/9] x86/alternative: Handle Jcc __x86_indirect_thunk_\reg Peter Zijlstra
2021-10-13 20:11 ` Nick Desaulniers
2021-10-13 21:08 ` Peter Zijlstra
2021-10-13 12:22 ` [PATCH 6/9] x86/alternative: Try inline spectre_v2=retpoline,amd Peter Zijlstra
2021-10-13 12:22 ` [PATCH 7/9] x86/alternative: Add debug prints to apply_retpolines() Peter Zijlstra
2021-10-13 12:22 ` [PATCH 8/9] x86,bugs: Unconditionally allow spectre_v2=retpoline,amd Peter Zijlstra
2021-10-13 12:22 ` [PATCH 9/9] bpf,x86: Respect X86_FEATURE_RETPOLINE* Peter Zijlstra
2021-10-13 21:06 ` Josh Poimboeuf
2021-10-13 21:54 ` Peter Zijlstra
2021-10-14 9:46 ` Peter Zijlstra
2021-10-14 9:48 ` Peter Zijlstra
2021-10-20 7:34 ` Peter Zijlstra
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=2c707dd6-796e-2de6-bce9-d0242f010513@citrix.com \
--to=andrew.cooper3@citrix.com \
--cc=alexei.starovoitov@gmail.com \
--cc=jpoimboe@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=ndesaulniers@google.com \
--cc=peterz@infradead.org \
--cc=x86@kernel.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).