x86/alternative: Simplify callthunk patching

Now that paravirt call patching is implemented using alternatives, it
is possible to avoid having to patch the alternative sites by
including the altinstr_replacement calls in the call_sites list.

This means we're now stacking relative adjustments like so:

  callthunks_patch_builtin_calls():
    patches all function calls to target: func() -> func()-10
    since the CALL accounting lives in the CALL_PADDING.

    This explicitly includes .altinstr_replacement

  alt_replace_call():
    patches: x86_BUG() -> target()

    this patching is done in a relative manner, and will preserve
    the above adjustment, meaning that with calldepth patching it
    will do: x86_BUG()-10 -> target()-10

  apply_relocation():
    does code relocation, and adjusts all RIP-relative instructions
    to the new location, also in a relative manner.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/r/20250207122546.617187089@infradead.org

1 files changed, 6 insertions, 11 deletions

diff --git a/arch/x86/kernel/module.c b/arch/x86/kernel/module.c
index 837450b6e882..cb9d295e67cb 100644
--- a/arch/x86/kernel/module.c
+++ b/arch/x86/kernel/module.c
@@ -275,12 +275,7 @@ int module_finalize(const Elf_Ehdr *hdr,
 		void *rseg = (void *)returns->sh_addr;
 		apply_returns(rseg, rseg + returns->sh_size);
 	}
-	if (alt) {
-		/* patch .altinstructions */
-		void *aseg = (void *)alt->sh_addr;
-		apply_alternatives(aseg, aseg + alt->sh_size);
-	}
-	if (calls || alt) {
+	if (calls) {
 		struct callthunk_sites cs = {};
 
 		if (calls) {
@@ -288,13 +283,13 @@ int module_finalize(const Elf_Ehdr *hdr,
 			cs.call_end = (void *)calls->sh_addr + calls->sh_size;
 		}
 
-		if (alt) {
-			cs.alt_start = (void *)alt->sh_addr;
-			cs.alt_end = (void *)alt->sh_addr + alt->sh_size;
-		}
-
 		callthunks_patch_module_calls(&cs, me);
 	}
+	if (alt) {
+		/* patch .altinstructions */
+		void *aseg = (void *)alt->sh_addr;
+		apply_alternatives(aseg, aseg + alt->sh_size);
+	}
 	if (ibt_endbr) {
 		void *iseg = (void *)ibt_endbr->sh_addr;
 		apply_seal_endbr(iseg, iseg + ibt_endbr->sh_size);