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    SubjectRe: [PATCH v7 12/26] x86/insn-eval: Add utility functions to get segment descriptor base address and limit
    On Fri, May 05, 2017 at 11:17:10AM -0700, Ricardo Neri wrote:
    > With segmentation, the base address of the segment descriptor is needed
    > to compute a linear address. The segment descriptor used in the address
    > computation depends on either any segment override prefixes in the
    > instruction or the default segment determined by the registers involved
    > in the address computation. Thus, both the instruction as well as the
    > register (specified as the offset from the base of pt_regs) are given as
    > inputs, along with a boolean variable to select between override and
    > default.

    ...

    > diff --git a/arch/x86/lib/insn-eval.c b/arch/x86/lib/insn-eval.c
    > index f46cb31..c77ed80 100644
    > --- a/arch/x86/lib/insn-eval.c
    > +++ b/arch/x86/lib/insn-eval.c
    > @@ -476,6 +476,133 @@ static struct desc_struct *get_desc(unsigned short sel)
    > }
    >
    > /**
    > + * insn_get_seg_base() - Obtain base address of segment descriptor.
    > + * @regs: Structure with register values as seen when entering kernel mode
    > + * @insn: Instruction structure with selector override prefixes
    > + * @regoff: Operand offset, in pt_regs, of which the selector is needed
    > + *
    > + * Obtain the base address of the segment descriptor as indicated by either
    > + * any segment override prefixes contained in insn or the default segment
    > + * applicable to the register indicated by regoff. regoff is specified as the
    > + * offset in bytes from the base of pt_regs.
    > + *
    > + * Return: In protected mode, base address of the segment. Zero in for long
    > + * mode, except when FS or GS are used. In virtual-8086 mode, the segment
    > + * selector shifted 4 positions to the right. -1L in case of
    > + * error.
    > + */
    > +unsigned long insn_get_seg_base(struct pt_regs *regs, struct insn *insn,
    > + int regoff)
    > +{
    > + struct desc_struct *desc;
    > + unsigned short sel;
    > + enum segment_register seg_reg;
    > +
    > + seg_reg = resolve_seg_register(insn, regs, regoff);
    > + if (seg_reg == SEG_REG_INVAL)
    > + return -1L;
    > +
    > + sel = get_segment_selector(regs, seg_reg);
    > + if ((short)sel < 0)

    I guess it would be better if that function returned a signed short so
    you don't have to cast it here. (You're casting it to an unsigned long
    below anyway.)

    > + return -1L;
    > +
    > + if (v8086_mode(regs))
    > + /*
    > + * Base is simply the segment selector shifted 4
    > + * positions to the right.
    > + */
    > + return (unsigned long)(sel << 4);
    > +

    ...

    > +static unsigned long get_seg_limit(struct pt_regs *regs, struct insn *insn,
    > + int regoff)
    > +{
    > + struct desc_struct *desc;
    > + unsigned short sel;
    > + unsigned long limit;
    > + enum segment_register seg_reg;
    > +
    > + seg_reg = resolve_seg_register(insn, regs, regoff);
    > + if (seg_reg == SEG_REG_INVAL)
    > + return 0;
    > +
    > + sel = get_segment_selector(regs, seg_reg);
    > + if ((short)sel < 0)

    Ditto.

    > + return 0;
    > +
    > + if (user_64bit_mode(regs) || v8086_mode(regs))
    > + return -1L;
    > +
    > + if (!sel)
    > + return 0;
    > +
    > + desc = get_desc(sel);
    > + if (!desc)
    > + return 0;
    > +
    > + /*
    > + * If the granularity bit is set, the limit is given in multiples
    > + * of 4096. When the granularity bit is set, the least 12 significant

    the 12 least significant bits

    > + * bits are not tested when checking the segment limits. In practice,
    > + * this means that the segment ends in (limit << 12) + 0xfff.
    > + */
    > + limit = get_desc_limit(desc);
    > + if (desc->g)
    > + limit <<= 12 | 0x7;

    That 0x7 doesn't look like 0xfff - it shifts limit by 15 instead. You
    can simply write it like you mean it:

    limit = (limit << 12) + 0xfff;


    --
    Regards/Gruss,
    Boris.

    SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
    --

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