objtool: Fix X86_FEATURE_SMAP alternative handling

For X86_FEATURE_SMAP alternatives which replace NOP with STAC or CLAC,
uaccess validation skips the NOP branch to avoid following impossible
code paths, e.g. where a STAC would be patched but a CLAC wouldn't.

However, it's not safe to assume an X86_FEATURE_SMAP alternative is
patching STAC/CLAC.  There can be other alternatives, like
static_cpu_has(), where both branches need to be validated.

Fix that by repurposing ANNOTATE_IGNORE_ALTERNATIVE for skipping either
original instructions or new ones.  This is a more generic approach
which enables the removal of the feature checking hacks and the
insn->ignore bit.

Fixes the following warnings:

  arch/x86/mm/fault.o: warning: objtool: do_user_addr_fault+0x8ec: __stack_chk_fail() missing __noreturn in .c/.h or NORETURN() in noreturns.h
  arch/x86/mm/fault.o: warning: objtool: do_user_addr_fault+0x8f1: unreachable instruction

[ mingo: Fix up conflicts with recent x86 changes. ]

Fixes: ea24213d8088 ("objtool: Add UACCESS validation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/r/de0621ca242130156a55d5d74fed86994dfa4c9c.1742852846.git.jpoimboe@kernel.org
Closes: https://lore.kernel.org/oe-kbuild-all/202503181736.zkZUBv4N-lkp@intel.com/

1 files changed, 1 insertions, 32 deletions

diff --git a/tools/objtool/arch/x86/special.c b/tools/objtool/arch/x86/special.c
index 5f46d4e7f7f8..403e587676f1 100644
--- a/tools/objtool/arch/x86/special.c
+++ b/tools/objtool/arch/x86/special.c
@@ -5,10 +5,7 @@
 #include <objtool/builtin.h>
 #include <objtool/warn.h>
 
-#define X86_FEATURE_POPCNT (4 * 32 + 23)
-#define X86_FEATURE_SMAP   (9 * 32 + 20)
-
-void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
+void arch_handle_alternative(struct special_alt *alt)
 {
 	static struct special_alt *group, *prev;
 
@@ -32,34 +29,6 @@ void arch_handle_alternative(unsigned short feature, struct special_alt *alt)
 	} else group = alt;
 
 	prev = alt;
-
-	switch (feature) {
-	case X86_FEATURE_SMAP:
-		/*
-		 * If UACCESS validation is enabled; force that alternative;
-		 * otherwise force it the other way.
-		 *
-		 * What we want to avoid is having both the original and the
-		 * alternative code flow at the same time, in that case we can
-		 * find paths that see the STAC but take the NOP instead of
-		 * CLAC and the other way around.
-		 */
-		if (opts.uaccess)
-			alt->skip_orig = true;
-		else
-			alt->skip_alt = true;
-		break;
-	case X86_FEATURE_POPCNT:
-		/*
-		 * It has been requested that we don't validate the !POPCNT
-		 * feature path which is a "very very small percentage of
-		 * machines".
-		 */
-		alt->skip_orig = true;
-		break;
-	default:
-		break;
-	}
 }
 
 bool arch_support_alt_relocation(struct special_alt *special_alt,