[PATCH 5.4 v2 1/6] x86, vmlinux.lds: Add RUNTIME_DISCARD_EXIT to generic DISCARDS

[Tom Saeger]

From: "H.J. Lu" <hjl.tools@xxxxxxxxx>

commit 84d5f77fc2ee4e010c2c037750e32f06e55224b0 upstream.

In the x86 kernel, .exit.text and .exit.data sections are discarded at
runtime, not by the linker. Add RUNTIME_DISCARD_EXIT to generic DISCARDS
and define it in the x86 kernel linker script to keep them.

The sections are added before the DISCARD directive so document here
only the situation explicitly as this change doesn't have any effect on
the generated kernel. Also, other architectures like ARM64 will use it
too so generalize the approach with the RUNTIME_DISCARD_EXIT define.

 [ bp: Massage and extend commit message. ]

Signed-off-by: H.J. Lu <hjl.tools@xxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200326193021.255002-1-hjl.tools@xxxxxxxxx
Signed-off-by: Tom Saeger <tom.saeger@xxxxxxxxxx>
---
 arch/x86/kernel/vmlinux.lds.S     |  1 +
 include/asm-generic/vmlinux.lds.h | 11 +++++++++--
 2 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 1afe211d7a7c..0ae3cd9a25ea 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
 #include <asm-generic/vmlinux.lds.h>
 #include <asm/asm-offsets.h>
 #include <asm/thread_info.h>
diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index c3bcac22c389..2d45d98773e2 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -900,10 +900,17 @@
  * 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.*)							\

-- 
2.39.1