[PATCH 1/3] Revert "x86/retpoline: Remove .text..__x86.return_thunk section"

[David Kaplan]

This reverts commit e92626af3234708fe30f53b269d210d202b95206.

This commit broke patching of the return thunk jmp in the retpoline
sequence.

Before (broken sequence):

objdump -d -r arch/x86/lib/retpoline.o:
0000000000000000 <__x86_indirect_thunk_array>:
   ...
   a:   e9 d1 02 00 00          jmpq   2e0 <__x86_return_thunk>

live disassembly at runtime:
0xffffffff81d12a8a <+10>:    jmpq   0xffffffff81d12d60
<__x86_return_thunk>

This jmp to the default return thunk should not happen after alternatives
patching.

After reverting this:

objdump -d -r arch/x86/lib/retpoline.o:
0000000000000000 <__x86_indirect_thunk_array>:
   ...
   a:   e9 00 00 00 00          jmpq   f <__x86_indirect_thunk_array+0xf>
                        b: R_X86_64_PLT32       __x86_return_thunk-0x4

live disassembly at runtime:
 0xffffffff81d12a8a <+10>:    jmpq   0xffffffff81f0410b
<srso_alias_return_thunk>

This is correct as the jmp is written to the correct return sequence.

objtool (add_jump_destinations()) only recognizes return thunk jmps that have
relocation entries, which will not occur if the return thunk is in the
same section as the indirect thunks.

Signed-off-by: David Kaplan <david.kaplan@xxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 arch/x86/kernel/vmlinux.lds.S | 3 +++
 arch/x86/lib/retpoline.S      | 2 ++
 2 files changed, 5 insertions(+)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 9cdb1a7332c4..54a5596adaa6 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -132,7 +132,10 @@ SECTIONS
 		LOCK_TEXT
 		KPROBES_TEXT
 		SOFTIRQENTRY_TEXT
+#ifdef CONFIG_RETPOLINE
 		*(.text..__x86.indirect_thunk)
+		*(.text..__x86.return_thunk)
+#endif
 		STATIC_CALL_TEXT
 
 		ALIGN_ENTRY_TEXT_BEGIN
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index db813113e637..3da768a71cf9 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -129,6 +129,8 @@ SYM_CODE_END(__x86_indirect_jump_thunk_array)
 
 #ifdef CONFIG_RETHUNK
 
+	.section .text..__x86.return_thunk
+
 #ifdef CONFIG_CPU_SRSO
 
 /*
-- 
2.25.1