[PATCH] Add RUNTIME_DISCARD_EXIT to generic DISCARDS

From: "H.J. Lu" <hjl.tools@gmail.com>
To: Kees Cook <keescook@chromium.org>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Thomas Lendacky <Thomas.Lendacky@amd.com>,
	Sami Tolvanen <samitolvanen@google.com>,
	Heiko Carstens <heiko.carstens@de.ibm.com>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
	"the arch/x86 maintainers" <x86@kernel.org>,
	Yu-cheng Yu <yu-cheng.yu@intel.com>
Subject: [PATCH] Add RUNTIME_DISCARD_EXIT to generic DISCARDS
Date: Thu, 30 Jan 2020 12:47:07 -0800	[thread overview]
Message-ID: <CAMe9rOqYh3QEdT16C8TOCBhYqfzsYJ6re0x+MDjpad9_59krZw@mail.gmail.com> (raw)
In-Reply-To: <CAMe9rOp1SJvsjSMtDFi4HWKPpu2eePCDiedTPAndUEL5-HSU1w@mail.gmail.com>

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On Thu, Jan 30, 2020 at 12:06 PM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Thu, Jan 30, 2020 at 11:58 AM Kees Cook <keescook@chromium.org> wrote:
> >
> > On Thu, Jan 30, 2020 at 11:45:15AM -0800, H.J. Lu wrote:
> > > On Thu, Jan 30, 2020 at 11:40 AM Kees Cook <keescook@chromium.org> wrote:
> > > >
> > > > On Thu, Jan 30, 2020 at 10:00:48AM -0800, H.J. Lu wrote:
> > > > > Since .exit.text and .exit.data sections are discarded at runtime, we
> > > > > should undefine EXIT_TEXT and EXIT_DATA to exclude .exit.text and
> > > > > .exit.data sections from default discarded sections.
> > > >
> > > > This is just a correctness fix, yes? The EXIT_TEXT and EXIT_DATA were
> > > > already included before the /DISCARD/ section here, so there's no
> > > > behavioral change with this patch, correct?
> > >
> > > That is correct.  I was confused by EXIT_TEXT and EXIT_DATA in generic
> > > DISCARDS.   My patch just makes it more explicit.
> >
> > Okay, so to that end and because this isn't arch-specific, I'd like to
> > see this be a behavioral flag, and then the generic DISCARDS macro can
> > be adjusted. This lets all architectures implement this without having
> > to scatter undef/define lines in each arch.
> >
> > Something like this:
> >
> > diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> > index e00f41aa8ec4..f242d3b4814d 100644
> > --- a/include/asm-generic/vmlinux.lds.h
> > +++ b/include/asm-generic/vmlinux.lds.h
> > @@ -894,11 +894,17 @@
> >   * section definitions so that such archs put those in earlier section
> >   * definitions.
> >   */
> > -#define DISCARDS                                                       \
> > -       /DISCARD/ : {                                                   \
> > +#ifdef RUNTIME_DISCARD_EXIT
> > +#define EXIT_DISCARDS
> > +#else
> > +#define EXIT_DISCARDS                                                  \
> >         EXIT_TEXT                                                       \
> >         EXIT_DATA                                                       \
> > -       EXIT_CALL                                                       \
> > +       EXIT_CALL
> > +#endif

We should keep EXIT_CALL in DISCARDS.  Only .exit.text and .exit.data
sections are discarded at runtime.

> > +#define DISCARDS                                                       \
> > +       /DISCARD/ : {                                                   \
> > +       EXIT_DISCARDS                                                   \
> >         *(.discard)                                                     \
> >         *(.discard.*)                                                   \
> >         *(.modinfo)                                                     \
> >
> > Then x86 and all other architectures that do this can just use
> > #define RUNTIME_DISCARD_EXIT
> > at the top (like EMITS_PT_NOTE, etc).
> >
>
> It should work.

Here is the patch to add RUNTIME_DISCARD_EXIT to generic DISCARDS.

-- 
H.J.

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From bde2821f5e01a5f49b227c6fb8ba6195c26381a9 Mon Sep 17 00:00:00 2001
From: "H.J. Lu" <hjl.tools@gmail.com>
Date: Thu, 30 Jan 2020 12:31:22 -0800
Subject: [PATCH] Add RUNTIME_DISCARD_EXIT to generic DISCARDS

In x86 kernel, .exit.text and .exit.data sections are discarded at
runtime not by linker.  Add RUNTIME_DISCARD_EXIT to generic DISCARDS
and define it in x86 kernel linker script to keep them.
---
 arch/x86/kernel/vmlinux.lds.S     |  1 +
 include/asm-generic/vmlinux.lds.h | 10 ++++++++--
 2 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index e3296aa028fe..7206e1ac23dd 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -21,6 +21,7 @@
 #define LOAD_OFFSET __START_KERNEL_map
 #endif
 
+#define RUNTIME_DISCARD_EXIT
 #define EMITS_PT_NOTE
 #define RO_EXCEPTION_TABLE_ALIGN	16
 
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index e00f41aa8ec4..6b943fb8c5fd 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -894,10 +894,16 @@
  * section definitions so that such archs put those in earlier section
  * definitions.
  */
+#ifdef RUNTIME_DISCARD_EXIT
+#define EXIT_DISCARDS
+#else
+#define EXIT_DISCARDS							\
+	EXIT_TEXT							\
+	EXIT_DATA
+#endif
 #define DISCARDS							\
 	/DISCARD/ : {							\
-	EXIT_TEXT							\
-	EXIT_DATA							\
+	EXIT_DISCARDS							\
 	EXIT_CALL							\
 	*(.discard)							\
 	*(.discard.*)							\
-- 
2.24.1

