From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
x86@kernel.org, linux-kernel@vger.kernel.org,
alexei.starovoitov@gmail.com, ndesaulniers@google.com
Subject: Re: [PATCH v2 08/14] x86/retpoline: Create a retpoline thunk array
Date: Wed, 20 Oct 2021 12:43:52 -0700 [thread overview]
Message-ID: <20211020194352.7id6nkvtpenmt3p2@treble> (raw)
In-Reply-To: <20211020192229.GP174703@worktop.programming.kicks-ass.net>
On Wed, Oct 20, 2021 at 09:22:29PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 20, 2021 at 10:09:56AM -0700, Josh Poimboeuf wrote:
> > On Wed, Oct 20, 2021 at 05:46:39PM +0100, Andrew Cooper wrote:
> > > On 20/10/2021 16:57, Josh Poimboeuf wrote:
> > > > On Wed, Oct 20, 2021 at 12:44:50PM +0200, Peter Zijlstra wrote:
> > > >> Stick all the retpolines in a single symbol and have the individual
> > > >> thunks as inner labels, this should guarantee thunk order and layout.
> > > > How so?
> > > >
> > > > Just wondering what the purpose of the array is. It doesn't seem to be
> > > > referenced anywhere.
> > >
> > > The array property is what makes:
> > >
> > > > + reg = (target - &__x86_indirect_thunk_rax) /
> > > > + (&__x86_indirect_thunk_rcx - &__x86_indirect_thunk_rax);
> > >
> > > safe in the next path.
> >
> > The thunks were already 32-byte aligned. I don't see how slapping a few
> > unused symbols around them does anything.
>
> Previously there were 16 (or rather 15 without rsp) separate symbols and
> a toolchain might reasonably expect it could displace them however it
> liked, with disregard for the relative position.
>
> However, now they're part of a larger symbol. Any change to their
> relative position would disrupt this larger _array symbol and thus not
> be sound.
>
> This is I think the same reasoning used for data symbols. On their own
> there is no guarantee about their relative position wrt to one aonther,
> but we're still able to do arrays because an array as a whole is a
> single larger symbol.
Makes sense, I think (and good fodder for the commit log).
--
Josh
next prev parent reply other threads:[~2021-10-20 19:44 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-10-20 10:44 [PATCH v2 00/14] x86: Rewrite the retpoline rewrite logic Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 01/14] objtool: Tag retpoline thunk symbols Peter Zijlstra
2021-10-20 15:17 ` Josh Poimboeuf
2021-10-26 7:55 ` Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 02/14] objtool: Explicitly avoid self modifying code in .altinstr_replacement Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 03/14] objtool: Shrink struct instruction Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 04/14] objtool,x86: Replace alternatives with .retpoline_sites Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 05/14] x86/retpoline: Remove unused replacement symbols Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 06/14] x86/asm: Fix register order Peter Zijlstra
2021-10-22 19:27 ` David Laight
2021-10-25 14:09 ` Borislav Petkov
2021-10-20 10:44 ` [PATCH v2 07/14] x86/asm: Fixup odd GEN-for-each-reg.h usage Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 08/14] x86/retpoline: Create a retpoline thunk array Peter Zijlstra
2021-10-20 15:57 ` Josh Poimboeuf
2021-10-20 16:46 ` Andrew Cooper
2021-10-20 17:09 ` Josh Poimboeuf
2021-10-20 19:22 ` Peter Zijlstra
2021-10-20 19:43 ` Josh Poimboeuf [this message]
2021-10-20 19:34 ` Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 09/14] x86/alternative: Implement .retpoline_sites support Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 10/14] x86/alternative: Handle Jcc __x86_indirect_thunk_\reg Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 11/14] x86/alternative: Try inline spectre_v2=retpoline,amd Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 12/14] x86/alternative: Add debug prints to apply_retpolines() Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 13/14] x86,bugs: Unconditionally allow spectre_v2=retpoline,amd Peter Zijlstra
2021-10-20 10:44 ` [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE* Peter Zijlstra
2021-10-20 11:09 ` Peter Zijlstra
2021-10-20 16:56 ` Josh Poimboeuf
2021-10-20 19:23 ` Peter Zijlstra
2021-10-21 0:05 ` Alexei Starovoitov
2021-10-21 8:47 ` Peter Zijlstra
2021-10-21 18:03 ` Alexei Starovoitov
2021-10-21 22:37 ` Peter Zijlstra
2021-10-21 23:24 ` Alexei Starovoitov
2021-10-21 23:38 ` Josh Poimboeuf
2021-10-21 23:42 ` Alexei Starovoitov
2021-10-22 11:31 ` Peter Zijlstra
2021-10-22 15:22 ` Alexei Starovoitov
2021-10-25 13:44 ` Maciej Fijalkowski
2021-10-25 12:42 ` Peter Zijlstra
2021-10-21 23:51 ` Zvi Effron
2021-10-22 8:33 ` Peter Zijlstra
2021-10-22 21:06 ` Zvi Effron
2021-10-21 0:07 ` Alexei Starovoitov
2021-10-21 0:18 ` Josh Poimboeuf
2021-10-21 8:53 ` Peter Zijlstra
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