From: Peter Zijlstra <peterz@infradead.org>
To: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: x86@kernel.org, jgross@suse.com, mbenes@suse.com,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 14/14] objtool,x86: Rewrite retpoline thunk calls
Date: Fri, 19 Mar 2021 09:06:44 +0100 [thread overview]
Message-ID: <YFRblERAnQu2KtZG@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20210319032955.zdx6ihhprem5owbc@treble>
On Thu, Mar 18, 2021 at 10:29:55PM -0500, Josh Poimboeuf wrote:
> On Thu, Mar 18, 2021 at 06:11:17PM +0100, Peter Zijlstra wrote:
> > When the compiler emits: "CALL __x86_indirect_thunk_\reg" for an
> > indirect call, have objtool rewrite it to:
> >
> > ALTERNATIVE "call __x86_indirect_thunk_\reg",
> > "call *%reg", ALT_NOT(X86_FEATURE_RETPOLINE)
> >
> > Additionally, in order to not emit endless identical
> > .altinst_replacement chunks, use a global symbol for them, see
> > __x86_indirect_alt_*.
> >
> > This also avoids objtool from having to do code generation.
> >
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
>
> This is better than I expected. Nice workaround for not generating
> code.
Thanks :-)
> > +.macro ALT_THUNK reg
> > +
> > + .align 1
> > +
> > +SYM_FUNC_START_NOALIGN(__x86_indirect_alt_call_\reg)
> > + ANNOTATE_RETPOLINE_SAFE
> > +1: call *%\reg
> > +2: .skip 5-(2b-1b), 0x90
> > +SYM_FUNC_END(__x86_indirect_alt_call_\reg)
> > +
> > +SYM_FUNC_START_NOALIGN(__x86_indirect_alt_jmp_\reg)
> > + ANNOTATE_RETPOLINE_SAFE
> > +1: jmp *%\reg
> > +2: .skip 5-(2b-1b), 0x90
> > +SYM_FUNC_END(__x86_indirect_alt_jmp_\reg)
>
> This mysterious code needs a comment. Shouldn't it be in
> .altinstr_replacement or something?
Comment, yes, I suppose so. And no, if we stick it in
.altinstr_replacement we'll throw them away with initmem and module
alternative patching (which will also refer to these symbols) will go
side-ways.
> Also doesn't the alternative code already insert nops?
Problem is that the {call,jmp} *%\reg thing is not fixed length. They're
2 or 3 bytes depending on which register is picked.
We could make them all 3 long and insert 0,1 nop I suppose.
Initially alternatives wouldn't re-optimize nops on patched things, it
would simply add nops on. And I had the above be:
1: INSN *%\reg
2: .nops 5-(2b-1b)
and we'd get a single right sized nop. But the .nops directive it too
new, we support binutils that don't have it :/
Hence, it now reads:
2: .skip 5-(2b-1b), 0x90
End result is that alternative NOP optimizer patch at the start of the
series that now also optimizes a bunch of cases that are unrelated and
were previously missed -- but crucially, it covers this case too :-)
Anyway, yes I could make it 3 long.
> > +int arch_rewrite_retpoline(struct objtool_file *file,
> > + struct instruction *insn,
> > + struct reloc *reloc)
> > +{
> > + struct symbol *sym;
> > + char name[32] = "";
> > +
> > + if (!strcmp(insn->sec->name, ".text.__x86.indirect_thunk"))
> > + return 0;
> > +
> > + sprintf(name, "__x86_indirect_alt_%s_%s",
> > + insn->type == INSN_JUMP_DYNAMIC ? "jmp" : "call",
> > + reloc->sym->name + 21);
> > +
> > + sym = find_symbol_by_name(file->elf, name);
> > + if (!sym) {
> > + sym = elf_create_undef_symbol(file->elf, name);
> > + if (!sym) {
> > + WARN("elf_create_undef_symbol");
> > + return -1;
> > + }
> > + }
> > +
> > + elf_add_alternative(file->elf, insn, sym,
> > + ALT_NOT(X86_FEATURE_RETPOLINE), 5, 5);
> > +
> > + return 0;
> > +}
>
> Need to propagate the error.
Oh, indeed so.
next prev parent reply other threads:[~2021-03-19 8:07 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-18 17:11 [PATCH v2 00/14] x86,objtool: Optimize !RETPOLINE Peter Zijlstra
2021-03-18 17:11 ` [PATCH v2 01/14] x86: Add insn_decode_kernel() Peter Zijlstra
2021-03-19 10:40 ` Borislav Petkov
2021-03-18 17:11 ` [PATCH v2 02/14] x86/alternatives: Optimize optimize_nops() Peter Zijlstra
2021-03-21 12:06 ` Borislav Petkov
2021-03-22 8:17 ` Peter Zijlstra
2021-03-22 11:07 ` Borislav Petkov
2021-03-18 17:11 ` [PATCH v2 03/14] x86/retpoline: Simplify retpolines Peter Zijlstra
2021-03-19 17:18 ` David Laight
2021-03-22 9:32 ` Peter Zijlstra
2021-03-22 15:41 ` David Laight
2021-03-18 17:11 ` [PATCH v2 04/14] objtool: Correctly handle retpoline thunk calls Peter Zijlstra
2021-03-18 17:11 ` [PATCH v2 05/14] objtool: Per arch retpoline naming Peter Zijlstra
2021-03-19 2:38 ` Josh Poimboeuf
2021-03-19 9:07 ` Peter Zijlstra
2021-03-18 17:11 ` [PATCH v2 06/14] objtool: Fix static_call list generation Peter Zijlstra
2021-03-22 12:44 ` Miroslav Benes
2021-03-18 17:11 ` [PATCH v2 07/14] objtool: Rework rebuild_reloc logic Peter Zijlstra
2021-03-18 17:11 ` [PATCH v2 08/14] objtool: Add elf_create_reloc() helper Peter Zijlstra
2021-03-19 1:42 ` Josh Poimboeuf
2021-03-19 9:47 ` Peter Zijlstra
2021-03-19 15:12 ` Josh Poimboeuf
2021-03-19 15:24 ` Peter Zijlstra
2021-03-19 15:37 ` Josh Poimboeuf
2021-03-18 17:11 ` [PATCH v2 09/14] objtool: Extract elf_strtab_concat() Peter Zijlstra
2021-03-19 2:10 ` Josh Poimboeuf
2021-03-19 9:52 ` Peter Zijlstra
2021-03-18 17:11 ` [PATCH v2 10/14] objtool: Extract elf_symbol_add() Peter Zijlstra
2021-03-19 2:14 ` Josh Poimboeuf
2021-03-19 9:54 ` Peter Zijlstra
2021-03-19 15:04 ` Josh Poimboeuf
2021-03-18 17:11 ` [PATCH v2 11/14] objtool: Add elf_create_undef_symbol() Peter Zijlstra
2021-03-19 2:29 ` Josh Poimboeuf
2021-03-19 7:56 ` Peter Zijlstra
2021-03-18 17:11 ` [PATCH v2 12/14] objtool: Allow archs to rewrite retpolines Peter Zijlstra
2021-03-19 2:54 ` Josh Poimboeuf
2021-03-19 11:21 ` Peter Zijlstra
2021-03-19 13:28 ` Peter Zijlstra
2021-03-18 17:11 ` [PATCH v2 13/14] objtool: Skip magical retpoline .altinstr_replacement Peter Zijlstra
2021-03-18 17:11 ` [PATCH v2 14/14] objtool,x86: Rewrite retpoline thunk calls Peter Zijlstra
2021-03-19 3:29 ` Josh Poimboeuf
2021-03-19 8:06 ` Peter Zijlstra [this message]
2021-03-19 15:30 ` Josh Poimboeuf
2021-03-19 15:56 ` Peter Zijlstra
2021-03-19 22:52 ` Josh Poimboeuf
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