objtool: Remove ANNOTATE_IGNORE_ALTERNATIVE from CLAC/STAC

ANNOTATE_IGNORE_ALTERNATIVE adds additional noise to the code generated
by CLAC/STAC alternatives, hurting readability for those whose read
uaccess-related code generation on a regular basis.

Remove the annotation specifically for the "NOP patched with CLAC/STAC"
case in favor of a manual check.

Leave the other uses of that annotation in place as they're less common
and more difficult to detect.

Suggested-by: Linus Torvalds <[email protected]>
Signed-off-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Acked-by: Linus Torvalds <[email protected]>
Link: https://lore.kernel.org/r/fc972ba4995d826fcfb8d02733a14be8d670900b.1744098446.git.jpoimboe@kernel.org

Author: jpoimboe

arch/x86/include/asm/smap.h

@@ -16,23 +16,23 @@
 #ifdef __ASSEMBLER__
 
 #define ASM_CLAC \
-	ALTERNATIVE __stringify(ANNOTATE_IGNORE_ALTERNATIVE), "clac", X86_FEATURE_SMAP
+	ALTERNATIVE "", "clac", X86_FEATURE_SMAP
 
 #define ASM_STAC \
-	ALTERNATIVE __stringify(ANNOTATE_IGNORE_ALTERNATIVE), "stac", X86_FEATURE_SMAP
+	ALTERNATIVE "", "stac", X86_FEATURE_SMAP
 
 #else /* __ASSEMBLER__ */
 
 static __always_inline void clac(void)
 {
 	/* Note: a barrier is implicit in alternative() */
-	alternative(ANNOTATE_IGNORE_ALTERNATIVE "", "clac", X86_FEATURE_SMAP);
+	alternative("", "clac", X86_FEATURE_SMAP);
 }
 
 static __always_inline void stac(void)
 {
 	/* Note: a barrier is implicit in alternative() */
-	alternative(ANNOTATE_IGNORE_ALTERNATIVE "", "stac", X86_FEATURE_SMAP);
+	alternative("", "stac", X86_FEATURE_SMAP);
 }
 
 static __always_inline unsigned long smap_save(void)
@@ -59,9 +59,9 @@ static __always_inline void smap_restore(unsigned long flags)
 
 /* These macros can be used in asm() statements */
 #define ASM_CLAC \
-	ALTERNATIVE(ANNOTATE_IGNORE_ALTERNATIVE "", "clac", X86_FEATURE_SMAP)
+	ALTERNATIVE("", "clac", X86_FEATURE_SMAP)
 #define ASM_STAC \
-	ALTERNATIVE(ANNOTATE_IGNORE_ALTERNATIVE "", "stac", X86_FEATURE_SMAP)
+	ALTERNATIVE("", "stac", X86_FEATURE_SMAP)
 
 #define ASM_CLAC_UNSAFE \
 	ALTERNATIVE("", ANNOTATE_IGNORE_ALTERNATIVE "clac", X86_FEATURE_SMAP)

tools/objtool/check.c

@@ -3505,6 +3505,34 @@ static struct instruction *next_insn_to_validate(struct objtool_file *file,
 	return next_insn_same_sec(file, alt_group->orig_group->last_insn);
 }
 
+static bool skip_alt_group(struct instruction *insn)
+{
+	struct instruction *alt_insn = insn->alts ? insn->alts->insn : NULL;
+
+	/* ANNOTATE_IGNORE_ALTERNATIVE */
+	if (insn->alt_group && insn->alt_group->ignore)
+		return true;
+
+	/*
+	 * For NOP patched with CLAC/STAC, only follow the latter to avoid
+	 * impossible code paths combining patched CLAC with unpatched STAC
+	 * or vice versa.
+	 *
+	 * ANNOTATE_IGNORE_ALTERNATIVE could have been used here, but Linus
+	 * requested not to do that to avoid hurting .s file readability
+	 * around CLAC/STAC alternative sites.
+	 */
+
+	if (!alt_insn)
+		return false;
+
+	/* Don't override ASM_{CLAC,STAC}_UNSAFE */
+	if (alt_insn->alt_group && alt_insn->alt_group->ignore)
+		return false;
+
+	return alt_insn->type == INSN_CLAC || alt_insn->type == INSN_STAC;
+}
+
 /*
  * Follow the branch starting at the given instruction, and recursively follow
  * any other branches (jumps).  Meanwhile, track the frame pointer state at
@@ -3625,7 +3653,7 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
 			}
 		}
 
-		if (insn->alt_group && insn->alt_group->ignore)
+		if (skip_alt_group(insn))
 			return 0;
 
 		if (handle_insn_ops(insn, next_insn, &state))