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    SubjectRe: [PATCH x86/mm 6/6] x86-64 ia32 ptrace get/putreg32 current task


    Chuck seems to have caught a bug, although the wrong one:

    On Thu, 29 Nov 2007, Chuck Ebbert wrote:
    >
    > On 11/28/2007 07:42 PM, Roland McGrath wrote:
    > > --- a/arch/x86/ia32/ptrace32.c
    > > +++ b/arch/x86/ia32/ptrace32.c
    > > ...
    > > + if (child == current)
    > > + load_gs_index(child->thread.gsindex);

    This is correct.

    But the ones that do the same thing for fs/es/ds are *not*. Those three
    registers are kernel mode registers (ds/es are the regular kernel data
    segment, fs is the per-cpu data segment), and restored on return to user
    space from the stack.

    For similar reasons, this is wrong:

    > > @@ -129,15 +137,23 @@ static int getreg32(struct task_struct *child, unsigned regno, u32 *val)
    > > switch (regno) {
    > > case offsetof(struct user32, regs.fs):
    > > *val = child->thread.fsindex;
    > > + if (child == current)
    > > + asm("movl %%fs,%0" : "=r" (*val));
    > > break;

    That %fs is the kernel per-cpu thing, not the user %fs.

    But this one is correct:

    > > case offsetof(struct user32, regs.gs):
    > > *val = child->thread.gsindex;
    > > + if (child == current)
    > > + asm("movl %%gs,%0" : "=r" (*val));
    >
    > Won't this return the kernel's GS instead of the user's?

    No, %gs is untouched by the kernel, so it contains user space version, and
    getting the value directly from %gs looks correct.

    Linus
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