Re: [v6 PATCH 05/21] x86/insn-eval: Add utility functions to get register offsets

[Ricardo Neri]

On Wed, 2017-04-12 at 18:28 +0200, Borislav Petkov wrote:
> On Tue, Mar 07, 2017 at 04:32:38PM -0800, Ricardo Neri wrote:
> > The function insn_get_reg_offset takes as argument an enumeration that
> 
> Please end function names with parentheses.

Will do! 
> 
> And do you mean get_reg_offset(), per chance?

Yes, I meant that. This was a copy/paste error.
> 
> > indicates the type of offset that is returned: the R/M part of the ModRM
> > byte, the index of the SIB byte or the base of the SIB byte.
> 
> Err, you mean, it returns the offset to the register the argument
> specifies.

Yes. I will reword.
> 
> > Callers of
> > this function would need the definition of such enumeration. This is not
> > needed. Instead, helper functions can be defined for this purpose can be
> > added.
> 
> "Instead, add helpers... "

I will reword.
> 
> > These functions are useful in cases when, for instance, the caller
> > needs to decide whether the operand is a register or a memory location by
> > looking at the mod part of the ModRM byte.
> > 
> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Cc: Adam Buchbinder <adam.buchbinder@xxxxxxxxx>
> > Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
> > Cc: Lorenzo Stoakes <lstoakes@xxxxxxxxx>
> > Cc: Qiaowei Ren <qiaowei.ren@xxxxxxxxx>
> > Cc: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
> > Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > Cc: Adrian Hunter <adrian.hunter@xxxxxxxxx>
> > Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> > Cc: Thomas Garnier <thgarnie@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Borislav Petkov <bp@xxxxxxx>
> > Cc: Dmitry Vyukov <dvyukov@xxxxxxxxxx>
> > Cc: Ravi V. Shankar <ravi.v.shankar@xxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > ---
> >  arch/x86/include/asm/insn-eval.h |  3 +++
> >  arch/x86/lib/insn-eval.c         | 51 ++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 54 insertions(+)
> > 
> > diff --git a/arch/x86/include/asm/insn-eval.h b/arch/x86/include/asm/insn-eval.h
> > index 5cab1b1..754211b 100644
> > --- a/arch/x86/include/asm/insn-eval.h
> > +++ b/arch/x86/include/asm/insn-eval.h
> > @@ -12,5 +12,8 @@
> >  #include <asm/ptrace.h>
> >  
> >  void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs);
> > +int insn_get_reg_offset_modrm_rm(struct insn *insn, struct pt_regs *regs);
> > +int insn_get_reg_offset_sib_base(struct insn *insn, struct pt_regs *regs);
> > +int insn_get_reg_offset_sib_base(struct insn *insn, struct pt_regs *regs);
> 
> Forgotten to edit the copy-paste?
> 
> Which means, nothing really needs insn_get_reg_offset_sib_index() and
> you can get rid of it?

Yes, I can get rid of it.
> 
> >  #endif /* _ASM_X86_INSN_EVAL_H */
> > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
> > index 23cf010..78df1c9 100644
> > --- a/arch/x86/lib/insn-eval.c
> > +++ b/arch/x86/lib/insn-eval.c
> > @@ -98,6 +98,57 @@ static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
> >  	return regoff[regno];
> >  }
> >  
> > +/**
> > + * insn_get_reg_offset_modrm_rm - Obtain register in r/m part of ModRM byte
> > + * @insn:	Instruction structure containing the ModRM byte
> > + * @regs:	Set of registers indicated by the ModRM byte
> 
> That's simply struct pt_regs - not a set of registers indicated by
> ModRM?!?

I will reword it to say "A struct pt_regs containing register values
indicated by the ModRM byte".
> 
> > + * Obtain the register indicated by the r/m part of the ModRM byte. The
> > + * register is obtained as an offset from the base of pt_regs. In specific
> > + * cases, the returned value can be -EDOM to indicate that the particular value
> > + * of ModRM does not refer to a register.
> 
> Put that sentence under the "Return: " paragraph below so that it is
> immediately obvious what the retvals are.

Will do.
> 
> > + *
> > + * Return: Register indicated by r/m, as an offset within struct pt_regs
> > + */
> > +int insn_get_reg_offset_modrm_rm(struct insn *insn, struct pt_regs *regs)
> 
> That name is too long: insn_get_modrm_rm_off() should be enough.
> 
> > +{
> > +	return get_reg_offset(insn, regs, REG_TYPE_RM);
> > +}
> > +
> > +/**
> > + * insn_get_reg_offset_sib_base - Obtain register in base part of SiB byte
> > + * @insn:	Instruction structure containing the SiB byte
> > + * @regs:	Set of registers indicated by the SiB byte
> > + *
> > + * Obtain the register indicated by the base part of the SiB byte. The
> > + * register is obtained as an offset from the base of pt_regs. In specific
> > + * cases, the returned value can be -EDOM to indicate that the particular value
> > + * of SiB does not refer to a register.
> > + *
> > + * Return: Register indicated by SiB's base, as an offset within struct pt_regs

Will make the spelling consistent.
> 
> Let's stick to a single spelling: SIB, all caps.
> 
> > + */
> > +int insn_get_reg_offset_sib_base(struct insn *insn, struct pt_regs *regs)
> 
> insn_get_sib_base_off()
> 
> Ditto for the rest of the comments on insn_get_reg_offset_modrm_rm() above.
> 
> > +{
> > +	return get_reg_offset(insn, regs, REG_TYPE_BASE);
> > +}
> > +
> > +/**
> > + * insn_get_reg_offset_sib_index - Obtain register in index part of SiB byte
> > + * @insn:	Instruction structure containing the SiB byte
> > + * @regs:	Set of registers indicated by the SiB byte
> > + *
> > + * Obtain the register indicated by the index part of the SiB byte. The
> > + * register is obtained as an offset from the index of pt_regs. In specific
> > + * cases, the returned value can be -EDOM to indicate that the particular value
> > + * of SiB does not refer to a register.
> > + *
> > + * Return: Register indicated by SiB's base, as an offset within struct pt_regs
> > + */
> > +int insn_get_reg_offset_sib_index(struct insn *insn, struct pt_regs *regs)
> 
> insn_get_sib_idx_off()
> 
> And again, if this function is unused, don't add it.

Masami Hiramatsu had originally requested to add the two functions. I
suppose the unneeded functions could be added if/when needed.

Thanks and BR,
Ricardo