[PATCH 1/2] x86/retpoline,kprobes: Fix position of thunk sections with CONFIG_LTO_CLANG

[Petr Pavlu]

Linker script arch/x86/kernel/vmlinux.lds.S matches the thunk sections
".text.__x86.*" from arch/x86/lib/retpoline.S as follows:

.text {
  [...]
  TEXT_TEXT
  [...]
  __indirect_thunk_start = .;
  *(.text.__x86.*)
  __indirect_thunk_end = .;
  [...]
}

Macro TEXT_TEXT references TEXT_MAIN which normally expands to only
".text". However, with CONFIG_LTO_CLANG, TEXT_MAIN becomes
".text .text.[0-9a-zA-Z_]*" which wrongly matches also the thunk
sections. The range [__indirect_thunk_start, __indirect_thunk_end] is
then empty.

A visible result is that function insn_is_indirect_jump() misbehaves and
optprobes become allowed in functions where they are not potentially
safe.

Fix the problem by using ".." as the first separator, for instance,
".text..__x86.indirect_thunk". This pattern is utilized by other
explicit section names which start with one of the standard prefixes,
such as ".text" or ".data", and that need to be individually selected in
the linker script.

Fixes: dc5723b02e52 ("kbuild: add support for Clang LTO")
Signed-off-by: Petr Pavlu <petr.pavlu@xxxxxxxx>
---
 arch/x86/kernel/vmlinux.lds.S | 2 +-
 arch/x86/lib/retpoline.S      | 4 ++--
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 03c885d3640f..a4cd04c458df 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -134,7 +134,7 @@ SECTIONS
 		SOFTIRQENTRY_TEXT
 #ifdef CONFIG_RETPOLINE
 		__indirect_thunk_start = .;
-		*(.text.__x86.*)
+		*(.text..__x86.*)
 		__indirect_thunk_end = .;
 #endif
 		STATIC_CALL_TEXT
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index 3fd066d42ec0..3bea96341d00 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -12,7 +12,7 @@
 #include <asm/percpu.h>
 #include <asm/frame.h>
 
-	.section .text.__x86.indirect_thunk
+	.section .text..__x86.indirect_thunk
 
 
 .macro POLINE reg
@@ -131,7 +131,7 @@ SYM_CODE_END(__x86_indirect_jump_thunk_array)
  */
 #ifdef CONFIG_RETHUNK
 
-	.section .text.__x86.return_thunk
+	.section .text..__x86.return_thunk
 
 /*
  * Safety details here pertain to the AMD Zen{1,2} microarchitecture:
-- 
2.35.3