[PATCH] Add RUNTIME_DISCARD_EXIT to generic DISCARDS

H.J. Lu
2020-01-30T20:47:46Z

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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