diff options
| author | Borislav Petkov (AMD) <bp@alien8.de> | 2023-10-20 14:17:14 +0300 |
|---|---|---|
| committer | Borislav Petkov (AMD) <bp@alien8.de> | 2023-10-20 14:17:14 +0300 |
| commit | 9d9c22cc444af01ce254872b729af26864c43a3a (patch) | |
| tree | e62cbb20929fec3e0d67938250a34c115b48d286 /arch/x86/lib | |
| parent | 2d7ce49f58dc95495b3e22e45d2be7de909b2c63 (diff) | |
| download | linux-9d9c22cc444af01ce254872b729af26864c43a3a.tar.xz | |
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: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231010171020.462211-1-david.kaplan@amd.com
Diffstat (limited to 'arch/x86/lib')
| -rw-r--r-- | arch/x86/lib/retpoline.S | 15 |
1 files changed, 15 insertions, 0 deletions
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S index d410abacbf88..a48077c5ca61 100644 --- a/arch/x86/lib/retpoline.S +++ b/arch/x86/lib/retpoline.S @@ -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 |