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 <torvalds@linux-foundation.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/fc972ba4995d826fcfb8d02733a14be8d670900b.1744098446.git.jpoimboe@kernel.org

diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
index 55a5e656e4b9ea861825a35a6f6fbc14817511ef..4f84d421d1cf294d4fab7ede318db95c3fc55b71 100644 (file)
--- a/arch/x86/include/asm/smap.h
+++ b/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)

diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 69f94bc4749985cf53eef2eea3c240fee104b0bd..b649049b6a11b28f00af412b512d8c3066d34e8f 100644 (file)
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -3505,6 +3505,34 @@ next_orig:
        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))