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From: Josh Poimboeuf <jpoimboe@redhat.com>
To: Nick Desaulniers <ndesaulniers@google.com>
Cc: "Borislav Petkov" <bp@alien8.de>,
	"Fāng-ruì Sòng" <maskray@google.com>,
	"Andy Lutomirski" <luto@kernel.org>,
	"Thomas Gleixner" <tglx@linutronix.de>,
	"Ingo Molnar" <mingo@redhat.com>, "Arnd Bergmann" <arnd@arndb.de>,
	"X86 ML" <x86@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
	"Nathan Chancellor" <natechancellor@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	clang-built-linux <clang-built-linux@googlegroups.com>
Subject: Re: [PATCH v3] x86/entry: emit a symbol for register restoring thunk
Date: Mon, 11 Jan 2021 19:59:52 -0600	[thread overview]
Message-ID: <20210112015952.jdystnwkvuxsrwa2@treble> (raw)
In-Reply-To: <CAKwvOdmGS97e4Rj_oW+RnkYAMjycTFQiiPJAfCvKTdxgv2KfEA@mail.gmail.com>

On Mon, Jan 11, 2021 at 05:13:16PM -0800, Nick Desaulniers wrote:
> On Mon, Jan 11, 2021 at 5:00 PM Borislav Petkov <bp@alien8.de> wrote:
> >
> > On Mon, Jan 11, 2021 at 04:41:52PM -0800, Fāng-ruì Sòng wrote:
> > > To be fair: we cannot use
> >
> > Who's "we"?
> >
> > > .L-prefixed local because of the objtool limitation.
> >
> > What objtool limitation? I thought clang's assembler removes .text which
> > objtool uses. It worked fine with GNU as so far.
> 
> I don't think we need to completely stop using .L prefixes in the
> kernel, just this one location since tracking the control flow seems a
> little tricky for objtool. Maybe Josh can clarify more if needed?

Right.  In the vast majority of cases, .L symbols are totally fine.

The limitation now being imposed by objtool (due to these assembler
changes) is that all code must be contained in an ELF symbol.  And .L
symbols don't create such symbols.

So basically, you can use an .L symbol *inside* a function or a code
segment, you just can't use the .L symbol to contain the code using a
SYM_*_START/END annotation pair.

It only affects a tiny fraction of all .L usage.  Just a handful of code
sites I think.

-- 
Josh


  reply	other threads:[~2021-01-12  2:01 UTC|newest]

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-12-23 23:21 [PATCH] x86/entry: use STB_GLOBAL for register restoring thunk Nick Desaulniers
2020-12-24  4:55 ` Josh Poimboeuf
2021-01-06  0:43   ` [PATCH v2] " Nick Desaulniers
2021-01-06  1:58     ` Josh Poimboeuf
2021-01-11 20:38       ` [PATCH v3] x86/entry: emit a symbol " Nick Desaulniers
2021-01-11 20:58         ` Fangrui Song
2021-01-11 22:09           ` Josh Poimboeuf
2021-01-11 22:16             ` Fāng-ruì Sòng
2021-01-11 22:12         ` Josh Poimboeuf
2021-01-12  0:38         ` Borislav Petkov
2021-01-12  0:41           ` Fāng-ruì Sòng
2021-01-12  1:00             ` Borislav Petkov
2021-01-12  1:13               ` Nick Desaulniers
2021-01-12  1:59                 ` Josh Poimboeuf [this message]
2021-01-12 11:54                   ` Borislav Petkov
2021-01-12 19:46                     ` [PATCH v4] " Nick Desaulniers
2021-01-12 21:01                       ` Mark Brown
2021-01-13 16:59                         ` Josh Poimboeuf
2021-01-13 17:46                           ` [PATCH] Documentation: asm-annotation: clarify .L local symbol names Nick Desaulniers
2021-01-13 19:56                             ` Mark Brown
2021-01-13 17:56                           ` [PATCH v4] x86/entry: emit a symbol for register restoring thunk Nick Desaulniers
2021-01-14 10:39                             ` Borislav Petkov
2021-01-14 11:36                               ` Peter Zijlstra
2021-01-14 13:28                                 ` [PATCH] x86/entry: Remove put_ret_addr_in_rdi THUNK macro argument Borislav Petkov
2021-01-14 15:53                                   ` Peter Zijlstra
2021-01-19 10:12                                 ` [tip: x86/entry] " tip-bot2 for Borislav Petkov
2021-01-14 15:14                               ` [PATCH v4] x86/entry: emit a symbol for register restoring thunk Josh Poimboeuf
2021-01-14 11:49                         ` [Linux-kernel-mentees] Fwd: " Lukas Bulwahn
2021-01-14 14:21                           ` Aditya
2021-01-14 14:35                             ` Lukas Bulwahn
2021-01-13 11:52                       ` [tip: x86/entry] x86/entry: Emit " tip-bot2 for Nick Desaulniers
2021-01-19 10:12                       ` tip-bot2 for Nick Desaulniers
2021-01-12 11:47                 ` [PATCH v3] x86/entry: emit " Borislav Petkov

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