linux-kernel - x86/insn-eval: negative return value?

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Date:   Wed, 22 Nov 2017 12:45:17 -0800
From:   Kees Cook <keescook@...gle.com>
To:     Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Borislav Petkov <bp@...e.de>
Subject: x86/insn-eval: negative return value?

Hi,

While doing some Clang test builds, this was reported:

arch/x86/lib/insn-eval.c:780:10: warning: implicit conversion from
'int' to 'char' changes value from 132 to -124 [-Wconstant-conversion]
                return INSN_CODE_SEG_PARAMS(4, 8);
                ~~~~~~ ^~~~~~~~~~~~~~~~~~~~~~~~~~
./arch/x86/include/asm/insn-eval.h:16:57: note: expanded from macro
'INSN_CODE_SEG_PARAMS'
#define INSN_CODE_SEG_PARAMS(oper_sz, addr_sz) (oper_sz | (addr_sz << 4))
                                                ~~~~~~~~^~~~~~~~~~~~~~~~

Is this really expected to wrap negative on IA-32e 64-bit mode case?

Thanks!

-Kees

-- 
Kees Cook
Pixel Security