x86/entry: Work around Clang __bdos() bug

Clang produces a false positive when building with CONFIG_FORTIFY_SOURCE=y
and CONFIG_UBSAN_BOUNDS=y when operating on an array with a dynamic
offset. Work around this by using a direct assignment of an empty
instance. Avoids this warning:

../include/linux/fortify-string.h:309:4: warning: call to __write_overflow_field declared with 'warn
ing' attribute: detected write beyond size of field (1st parameter); maybe use struct_group()? [-Wat
tribute-warning]
                        __write_overflow_field(p_size_field, size);
                        ^

which was isolated to the memset() call in xen_load_idt().

Note that this looks very much like another bug that was worked around:
ClangBuiltLinux#1592

Cc: Juergen Gross <[email protected]>
Cc: Boris Ostrovsky <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: [email protected]
Cc: "H. Peter Anvin" <[email protected]>
Cc: [email protected]
Reviewed-by: Boris Ostrovsky <[email protected]>
Link: https://lore.kernel.org/lkml/[email protected]
Signed-off-by: Kees Cook <[email protected]>

Author: kees

arch/x86/xen/enlighten_pv.c

@@ -765,6 +765,7 @@ static void xen_load_idt(const struct desc_ptr *desc)
 {
 	static DEFINE_SPINLOCK(lock);
 	static struct trap_info traps[257];
+	static const struct trap_info zero = { };
 	unsigned out;
 
 	trace_xen_cpu_load_idt(desc);
@@ -774,7 +775,7 @@ static void xen_load_idt(const struct desc_ptr *desc)
 	memcpy(this_cpu_ptr(&idt_desc), desc, sizeof(idt_desc));
 
 	out = xen_convert_trap_info(desc, traps, false);
-	memset(&traps[out], 0, sizeof(traps[0]));
+	traps[out] = zero;
 
 	xen_mc_flush();
 	if (HYPERVISOR_set_trap_table(traps))