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/

3 files changed, 21 insertions, 14 deletions

diff --git a/arch/x86/include/asm/arch_hweight.h b/arch/x86/include/asm/arch_hweight.h
index b5982b94bdba..cbc6157f0b4b 100644
--- a/arch/x86/include/asm/arch_hweight.h
+++ b/arch/x86/include/asm/arch_hweight.h
@@ -16,7 +16,8 @@ static __always_inline unsigned int __arch_hweight32(unsigned int w)
 {
 	unsigned int res;
 
-	asm_inline (ALTERNATIVE("call __sw_hweight32",
+	asm_inline (ALTERNATIVE(ANNOTATE_IGNORE_ALTERNATIVE
+				"call __sw_hweight32",
 				"popcntl %[val], %[cnt]", X86_FEATURE_POPCNT)
 			 : [cnt] "=" REG_OUT (res), ASM_CALL_CONSTRAINT
 			 : [val] REG_IN (w));
@@ -45,7 +46,8 @@ static __always_inline unsigned long __arch_hweight64(__u64 w)
 {
 	unsigned long res;
 
-	asm_inline (ALTERNATIVE("call __sw_hweight64",
+	asm_inline (ALTERNATIVE(ANNOTATE_IGNORE_ALTERNATIVE
+				"call __sw_hweight64",
 				"popcntq %[val], %[cnt]", X86_FEATURE_POPCNT)
 			 : [cnt] "=" REG_OUT (res), ASM_CALL_CONSTRAINT
 			 : [val] REG_IN (w));
diff --git a/arch/x86/include/asm/smap.h b/arch/x86/include/asm/smap.h
index daea94c2993c..55a5e656e4b9 100644
--- a/arch/x86/include/asm/smap.h
+++ b/arch/x86/include/asm/smap.h
@@ -16,23 +16,23 @@
 #ifdef __ASSEMBLER__
 
 #define ASM_CLAC \
-	ALTERNATIVE "", "clac", X86_FEATURE_SMAP
+	ALTERNATIVE __stringify(ANNOTATE_IGNORE_ALTERNATIVE), "clac", X86_FEATURE_SMAP
 
 #define ASM_STAC \
-	ALTERNATIVE "", "stac", X86_FEATURE_SMAP
+	ALTERNATIVE __stringify(ANNOTATE_IGNORE_ALTERNATIVE), "stac", X86_FEATURE_SMAP
 
 #else /* __ASSEMBLER__ */
 
 static __always_inline void clac(void)
 {
 	/* Note: a barrier is implicit in alternative() */
-	alternative("", "clac", X86_FEATURE_SMAP);
+	alternative(ANNOTATE_IGNORE_ALTERNATIVE "", "clac", X86_FEATURE_SMAP);
 }
 
 static __always_inline void stac(void)
 {
 	/* Note: a barrier is implicit in alternative() */
-	alternative("", "stac", X86_FEATURE_SMAP);
+	alternative(ANNOTATE_IGNORE_ALTERNATIVE "", "stac", X86_FEATURE_SMAP);
 }
 
 static __always_inline unsigned long smap_save(void)
@@ -40,7 +40,8 @@ static __always_inline unsigned long smap_save(void)
 	unsigned long flags;
 
 	asm volatile ("# smap_save\n\t"
-		      ALTERNATIVE("", "pushf; pop %0; " "clac" "\n\t",
+		      ALTERNATIVE(ANNOTATE_IGNORE_ALTERNATIVE
+				  "", "pushf; pop %0; clac",
 				  X86_FEATURE_SMAP)
 		      : "=rm" (flags) : : "memory", "cc");
 
@@ -50,16 +51,22 @@ static __always_inline unsigned long smap_save(void)
 static __always_inline void smap_restore(unsigned long flags)
 {
 	asm volatile ("# smap_restore\n\t"
-		      ALTERNATIVE("", "push %0; popf\n\t",
+		      ALTERNATIVE(ANNOTATE_IGNORE_ALTERNATIVE
+				  "", "push %0; popf",
 				  X86_FEATURE_SMAP)
 		      : : "g" (flags) : "memory", "cc");
 }
 
 /* These macros can be used in asm() statements */
 #define ASM_CLAC \
-	ALTERNATIVE("", "clac", X86_FEATURE_SMAP)
+	ALTERNATIVE(ANNOTATE_IGNORE_ALTERNATIVE "", "clac", X86_FEATURE_SMAP)
 #define ASM_STAC \
-	ALTERNATIVE("", "stac", X86_FEATURE_SMAP)
+	ALTERNATIVE(ANNOTATE_IGNORE_ALTERNATIVE "", "stac", X86_FEATURE_SMAP)
+
+#define ASM_CLAC_UNSAFE \
+	ALTERNATIVE("", ANNOTATE_IGNORE_ALTERNATIVE "clac", X86_FEATURE_SMAP)
+#define ASM_STAC_UNSAFE \
+	ALTERNATIVE("", ANNOTATE_IGNORE_ALTERNATIVE "stac", X86_FEATURE_SMAP)
 
 #endif /* __ASSEMBLER__ */
 
diff --git a/arch/x86/include/asm/xen/hypercall.h b/arch/x86/include/asm/xen/hypercall.h
index 97771b9d33af..59a62c3780a2 100644
--- a/arch/x86/include/asm/xen/hypercall.h
+++ b/arch/x86/include/asm/xen/hypercall.h
@@ -231,14 +231,12 @@ static __always_inline void __xen_stac(void)
 	 * Suppress objtool seeing the STAC/CLAC and getting confused about it
 	 * calling random code with AC=1.
 	 */
-	asm volatile(ANNOTATE_IGNORE_ALTERNATIVE
-		     ASM_STAC ::: "memory", "flags");
+	asm volatile(ASM_STAC_UNSAFE ::: "memory", "flags");
 }
 
 static __always_inline void __xen_clac(void)
 {
-	asm volatile(ANNOTATE_IGNORE_ALTERNATIVE
-		     ASM_CLAC ::: "memory", "flags");
+	asm volatile(ASM_CLAC_UNSAFE ::: "memory", "flags");
 }
 
 static inline long