Re: [v3 PATCH 04/10] x86/insn-kernel: Add a function to obtain register offset in ModRM

Ricardo Neri
2017-01-26T06:07:16Z

Hi Masami,

On Thu, 2017-01-26 at 11:11 +0900, Masami Hiramatsu wrote:
> On Wed, 25 Jan 2017 12:23:47 -0800
> Ricardo Neri <ricardo.neri-calderon@linux.intel.com> wrote:
>
> > The function insn_get_reg_offset requires a type to indicate whether
> > the returned offset is that given by by the ModRM or the SIB byte.
> > Callers of this function would need the definition of the type struct.
> > This is not needed. Instead, auxiliary functions can be defined for
> > this purpose.
> >
> > When the operand is a register, the emulation code for User-Mode
> > Instruction Prevention needs to know the offset of the register indicated
> > in the r/m part of the ModRM byte. Thus, start by adding an auxiliary
> > function for this purpose.
>
> Hmm, why wouldn't you just rename it to insn_get_reg_offset() and export it?

Do you mean exporting the structure that I mention above? The problem
that I am trying to solve is that callers sometimes want to know the
offset of the register encoded in the SiB or the ModRM bytes. I could
use something

insn_get_reg_offset(insn, regs, INSN_TYPE_MODRM)
insn_get_reg_offset(insn, regs, INSN_TYPE_SIB)

Instead, I opted for

insn_get_reg_offset_rm(insn, regs)
insn_get_reg_offset_sib(insn, regs)

to avoid exposing an enum with the INSN_TYPE_MODRM, INSN_TYPE_SIB.

If you feel that the former makes more sense, I can change the
implementation.

Thanks and BR,
Ricardo
>
> Thank you,
>
> >
> > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > Cc: Adam Buchbinder <adam.buchbinder@gmail.com>
> > Cc: Colin Ian King <colin.king@canonical.com>
> > Cc: Lorenzo Stoakes <lstoakes@gmail.com>
> > Cc: Qiaowei Ren <qiaowei.ren@intel.com>
> > Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
> > Cc: Masami Hiramatsu <mhiramat@kernel.org>
> > Cc: Adrian Hunter <adrian.hunter@intel.com>
> > Cc: Kees Cook <keescook@chromium.org>
> > Cc: Thomas Garnier <thgarnie@google.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Borislav Petkov <bp@suse.de>
> > Cc: Dmitry Vyukov <dvyukov@google.com>
> > Cc: Ravi V. Shankar <ravi.v.shankar@intel.com>
> > Cc: x86@kernel.org
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
> > ---
> > arch/x86/include/asm/insn-kernel.h | 1 +
> > arch/x86/lib/insn-kernel.c | 5 +++++
> > 2 files changed, 6 insertions(+)
> >
> > diff --git a/arch/x86/include/asm/insn-kernel.h b/arch/x86/include/asm/insn-kernel.h
> > index aef416a..3f34649 100644
> > --- a/arch/x86/include/asm/insn-kernel.h
> > +++ b/arch/x86/include/asm/insn-kernel.h
> > @@ -12,5 +12,6 @@
> > #include <asm/ptrace.h>
> >
> > void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs);
> > +int insn_get_reg_offset_rm(struct insn *insn, struct pt_regs *regs);
> >
> > #endif /* _ASM_X86_INSN_KERNEL_H */
> > diff --git a/arch/x86/lib/insn-kernel.c b/arch/x86/lib/insn-kernel.c
> > index 8072abe..267cab4 100644
> > --- a/arch/x86/lib/insn-kernel.c
> > +++ b/arch/x86/lib/insn-kernel.c
> > @@ -95,6 +95,11 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
> > return regoff[regno];
> > }
> >
> > +int insn_get_reg_offset_rm(struct insn *insn, struct pt_regs *regs)
> > +{
> > + return get_reg_offset(insn, regs, REG_TYPE_RM);
> > +}
> > +
> > /*
> > * return the address being referenced be instruction
> > * for rm=3 returning the content of the rm reg
> > --
> > 2.9.3
> >
>
>