[PATCH 1/4] x86/entry: Work around Clang __bdos() bug

[Kees Cook <keescook@xxxxxxxxxxxx>]

After expanding bounds checking to use __builtin_dynamic_object_size(),
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:
https://github.com/ClangBuiltLinux/linux/issues/1592

Cc: Juergen Gross <jgross@xxxxxxxx>
Cc: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
Cc: Nathan Chancellor <nathan@xxxxxxxxxx>
Cc: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx
Cc: llvm@xxxxxxxxxxxxxxx
Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
---
 arch/x86/xen/enlighten_pv.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
index 0ed2e487a693..9b1a58dda935 100644
--- a/arch/x86/xen/enlighten_pv.c
+++ b/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))
-- 
2.34.1