Commit 9d9c22cc authored by Borislav Petkov (AMD)'s avatar Borislav Petkov (AMD)
Browse files

x86/retpoline: Document some thunk handling aspects 



After a lot of experimenting (see thread Link points to) document for
now the issues and requirements for future improvements to the thunk
handling and potential issuing of a diagnostic when the default thunk
hasn't been patched out.

This documentation is only temporary and that close before the merge
window it is only a placeholder for those future improvements.

Suggested-by: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarBorislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231010171020.462211-1-david.kaplan@amd.com
parent 2d7ce49f

arch/x86/lib/retpoline.S

+15 −0
Original line number Diff line number Diff line
@@ -129,6 +129,13 @@ SYM_CODE_END(__x86_indirect_jump_thunk_array) 


#ifdef CONFIG_RETHUNK



/*

 * Be careful here: that label cannot really be removed because in

 * some configurations and toolchains, the JMP __x86_return_thunk the

 * compiler issues is either a short one or the compiler doesn't use

 * relocations for same-section JMPs and that breaks the returns

 * detection logic in apply_returns() and in objtool.

 */

	.section .text..__x86.return_thunk



#ifdef CONFIG_CPU_SRSO
@@ -361,6 +368,14 @@ SYM_FUNC_END(call_depth_return_thunk) 
 * This code is only used during kernel boot or module init.  All

 * 'JMP __x86_return_thunk' sites are changed to something else by

 * apply_returns().

 *

 * This should be converted eventually to call a warning function which

 * should scream loudly when the default return thunk is called after

 * alternatives have been applied.

 *

 * That warning function cannot BUG() because the bug splat cannot be

 * displayed in all possible configurations, leading to users not really

 * knowing why the machine froze.

 */

SYM_CODE_START(__x86_return_thunk)

	UNWIND_HINT_FUNC