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SubjectRe: [PATCH v10 3.3-rc2 1/9] uprobes: Install and remove breakpoints.
(2012/02/09 15:37), Srikar Dronamraju wrote:
>>
>> No, that is a meaningless operation.
>> As I originally said,
>>
>>> insn_get_length(insn);
>>> if (insn->rex_prefix.nbytes) {
>>> cursor = uprobe->insn + insn_offset_rex_prefix(insn);
>>> *cursor &= 0xfe; /* Clearing REX.B bit */
>>> }
>>
>
> I am confused by why we need to call insn_get_length(insn) before
> checking insn->rex_prefix.nbytes? Is it needed.

Ah, certainly, no, if the insn is already decoded.

> Denys and Masami, can you please confirm if below is fine.
>
>
> #ifdef CONFIG_X86_64
> /*
> * If uprobe->insn doesn't use rip-relative addressing, return
> * immediately. Otherwise, rewrite the instruction so that it accesses
> * its memory operand indirectly through a scratch register. Set
> * uprobe->arch_info.fixups and uprobe->arch_info.rip_rela_target_address
> * accordingly. (The contents of the scratch register will be saved
> * before we single-step the modified instruction, and restored
> * afterward.)
> *
> * We do this because a rip-relative instruction can access only a
> * relatively small area (+/- 2 GB from the instruction), and the XOL
> * area typically lies beyond that area. At least for instructions
> * that store to memory, we can't execute the original instruction
> * and "fix things up" later, because the misdirected store could be
> * disastrous.
> *
> * Some useful facts about rip-relative instructions:
> * - There's always a modrm byte.
> * - There's never a SIB byte.
> * - The displacement is always 4 bytes.
> */
> static void handle_riprel_insn(struct mm_struct *mm, struct uprobe *uprobe,
> struct insn *insn)
> {

> u8 *cursor;
> u8 reg;
>
> if (mm->context.ia32_compat)
> return;
>
> uprobe->arch_info.rip_rela_target_address = 0x0;
> if (!insn_rip_relative(insn))
> return;

Here, I think it is better to add a comment that
insn_rip_relative() decodes until modrm. :)

>
> /* Clear REX.b bit (extension of MODRM.rm field):
> * we want to encode rax/rcx, not r8/r9.
> */
> if (insn->rex_prefix.nbytes) {
> cursor = uprobe->insn + insn_offset_rex_prefix(insn);
> *cursor &= 0xfe;
> }
>
> /*
> * Point cursor at the modrm byte. The next 4 bytes are the
> * displacement. Beyond the displacement, for some instructions,
> * is the immediate operand.
> */
> cursor = uprobe->insn + insn_offset_modrm(insn);
> insn_get_length(insn);
>
> /*
> * Convert from rip-relative addressing to indirect addressing
> * via a scratch register. Change the r/m field from 0x5 (%rip)
> * to 0x0 (%rax) or 0x1 (%rcx), and squeeze out the offset field.
> */
> reg = MODRM_REG(insn);
> if (reg == 0) {
> /*
> * The register operand (if any) is either the A register
> * (%rax, %eax, etc.) or (if the 0x4 bit is set in the
> * REX prefix) %r8. In any case, we know the C register
> * is NOT the register operand, so we use %rcx (register
> * #1) for the scratch register.
> */
> uprobe->arch_info.fixups = UPROBES_FIX_RIP_CX;
> /* Change modrm from 00 000 101 to 00 000 001. */
> *cursor = 0x1;
> } else {
> /* Use %rax (register #0) for the scratch register. */
> uprobe->arch_info.fixups = UPROBES_FIX_RIP_AX;
> /* Change modrm from 00 xxx 101 to 00 xxx 000 */
> *cursor = (reg << 3);
> }
>
> /* Target address = address of next instruction + (signed) offset */
> uprobe->arch_info.rip_rela_target_address = (long)insn->length
> + insn->displacement.value;
> /* Displacement field is gone; slide immediate field (if any) over. */
> if (insn->immediate.nbytes) {
> cursor++;
> memmove(cursor, cursor + insn->displacement.nbytes,
> insn->immediate.nbytes);
> }
> return;
> }

Confirmed, this looks good to me ;)

Thanks!


--
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com


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