x86/boot/tboot: Avoid Wstringop-overread-warning

gcc-11 warns about using string operations on pointers that are
defined at compile time as offsets from a NULL pointer. Unfortunately
that also happens on the result of fix_to_virt(), which is a
compile-time constant for a constant input:

  arch/x86/kernel/tboot.c: In function 'tboot_probe':
  arch/x86/kernel/tboot.c:70:13: error: '__builtin_memcmp_eq' specified bound 16 exceeds source size 0 [-Werror=stringop-overread]
     70 |         if (memcmp(&tboot_uuid, &tboot->uuid, sizeof(tboot->uuid))) {
        |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

I hope this can get addressed in gcc-11 before the release.

As a workaround, split up the tboot_probe() function in two halves
to separate the pointer generation from the usage. This is a bit
ugly, and hopefully gcc understands that the code is actually correct
before it learns to peek into the noinline function.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Martin Sebor <msebor@gmail.com>
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99578
Link: https://lore.kernel.org/r/20210322160253.4032422-3-arnd@kernel.org

arch/x86/kernel/tboot.c

@@ -49,6 +49,30 @@ bool tboot_enabled(void)
 	return tboot != NULL;
 }
 
+/* noinline to prevent gcc from warning about dereferencing constant fixaddr */
+static noinline __init bool check_tboot_version(void)
+{
+	if (memcmp(&tboot_uuid, &tboot->uuid, sizeof(tboot->uuid))) {
+		pr_warn("tboot at 0x%llx is invalid\n", boot_params.tboot_addr);
+		return false;
+	}
+
+	if (tboot->version < 5) {
+		pr_warn("tboot version is invalid: %u\n", tboot->version);
+		return false;
+	}
+
+	pr_info("found shared page at phys addr 0x%llx:\n",
+		boot_params.tboot_addr);
+	pr_debug("version: %d\n", tboot->version);
+	pr_debug("log_addr: 0x%08x\n", tboot->log_addr);
+	pr_debug("shutdown_entry: 0x%x\n", tboot->shutdown_entry);
+	pr_debug("tboot_base: 0x%08x\n", tboot->tboot_base);
+	pr_debug("tboot_size: 0x%x\n", tboot->tboot_size);
+
+	return true;
+}
+
 void __init tboot_probe(void)
 {
 	/* Look for valid page-aligned address for shared page. */
@@ -66,25 +90,9 @@ void __init tboot_probe(void)
 
 	/* Map and check for tboot UUID. */
 	set_fixmap(FIX_TBOOT_BASE, boot_params.tboot_addr);
-	tboot = (struct tboot *)fix_to_virt(FIX_TBOOT_BASE);
-	if (memcmp(&tboot_uuid, &tboot->uuid, sizeof(tboot->uuid))) {
-		pr_warn("tboot at 0x%llx is invalid\n", boot_params.tboot_addr);
+	tboot = (void *)fix_to_virt(FIX_TBOOT_BASE);
+	if (!check_tboot_version())
 		tboot = NULL;
-		return;
-	}
-	if (tboot->version < 5) {
-		pr_warn("tboot version is invalid: %u\n", tboot->version);
-		tboot = NULL;
-		return;
-	}
-
-	pr_info("found shared page at phys addr 0x%llx:\n",
-		boot_params.tboot_addr);
-	pr_debug("version: %d\n", tboot->version);
-	pr_debug("log_addr: 0x%08x\n", tboot->log_addr);
-	pr_debug("shutdown_entry: 0x%x\n", tboot->shutdown_entry);
-	pr_debug("tboot_base: 0x%08x\n", tboot->tboot_base);
-	pr_debug("tboot_size: 0x%x\n", tboot->tboot_size);
 }
 
 static pgd_t *tboot_pg_dir;