[RFC PATCH] uprobes: copy to user-space xol page with proper cache flushing

[Victor Kamensky]

On 11 April 2014 07:56, Oleg Nesterov <oleg at redhat.com> wrote:
> First of all: I do not pretend I really understand the problems with
> icache/etc coherency and how flush_icache_range() actually works on
> alpha. Help.
>
> For those who were not cc'ed. A probed task has a special "xol" vma,
> it is used to execute the probed insn out of line.
>
> Initial implementation was x86 only, so we simply copied the probed
> insn into this vma and everything was fine, flush_icache_range() is
> nop on x86.
>
> Then we added flush_dcache_page() for powerpc,
>
>         // this is just kmap() + memcpy()
>         copy_to_page(area->page, xol_vaddr,
>                         &uprobe->arch.ixol, sizeof(uprobe->arch.ixol));
>
>         /*
>          * We probably need flush_icache_user_range() but it needs vma.
>          * This should work on supported architectures too.
>          */
>         flush_dcache_page(area->page);
>
> but this doesn't work on arm. So we need another fix.
>
> On 04/11, David Miller wrote:
>>
>> From: David Long <dave.long at linaro.org>
>> Date: Thu, 10 Apr 2014 23:45:31 -0400
>>
>> > Replace memcpy and dcache flush in generic uprobes with a call to
>> > copy_to_user_page(), which will do a proper flushing of kernel and
>> > user cache.  Also modify the inmplementation of copy_to_user_page
>> > to assume a NULL vma pointer means the user icache corresponding
>> > to this right is stale and needs to be flushed.  Note that this patch
>> > does not fix copy_to_user page for the sh, alpha, sparc, or mips
>> > architectures (which do not currently support uprobes).
>> >
>> > Signed-off-by: David A. Long <dave.long at linaro.org>
>>
>> You really need to pass the proper VMA down to the call site
>> rather than pass NULL, that's extremely ugly and totally
>> unnecesary.
>
> I agree that we should not change copy_to_user_page(), but I am not sure
> the code above should use copy_to_user_page().
>
> Because the code above really differs in my opinion from the only user of
> copy_to_user_page(), __access_remote_vm().
>
>         1. First of all, we do not know vma.
>
>            OK, we can down_read(mmap_sem) and do find_vma() of course.
>            This is a bit unfortunate, especially because the architectures
>            we currently support do not need this.

Question, maybe silly one but I don't know the answer, why can't we just do
look up for vma once and cache results in place like xol_area (along with
xol_area.vaddr) and use it all the time. IOW under what circumstances
vma for xol area can disappear change so we need constant lookup for it?
Comment in xol_area

>    /*
>    * We keep the vma's vm_start rather than a pointer to the vma
>     * itself.  The probed process or a naughty kernel module could make
>     * the vma go away, and we must handle that reasonably gracefully.
>     */
>     unsigned long         vaddr;        /* Page(s) of instruction slots */

alludes to some of those conditions, but I don't quite follow.
Should not we go after "probed process" ability to unmap xol area.
xol area is like vdso, signal page and other mapping injected by
kernel into user process address space, mmap call should ignore
those.. I wonder what would happen if process would try to unmap
vdso region.

>            But,
>
>         2. The problem is, it would be very nice to remove this vma, or
>            at least hide it somehow from find_vma/etc. This is the special
>            mapping we do not want to expose to user-space.
>
>            In fact I even have the patches which remove this vma, but they
>            do not work with compat tasks unfortunately.

I don't think it is right route. Xol area as well as vdso, signal page, etc
should be visible as regular VMAs. There are other aspects of the system
where they needed. Like core file collection - I would like to have
xol area present in my core file if traced process crashed. Like
/porc/<pid>/maps - I would like to see my memory layout through
this interface and I would like to see xol area there because I
can see xol area addresses by some other means.

>         3. Unlike __access_remote_vm() we always use current->mm, and the
>            memory range changed by the code above can only be used by
>            "current" thread.
>
>            So (perhaps) flush_icache_user_range() can even treat this case
>            as "mm->mm_users) <= 1" and avoid ipi_flush_icache_page (just in
>            case, of course I do not know if this is actually possible).
>
> So can't we do something else? Lets forget about arch/arm for the moment,
> suppose that we want to support uprobes on alpha. Can't we do _something_
> like below?

Appeal of copy_to_user_page approach is that I don't need to know
how to handle sync up of icache and dcache on that architecture, it is
already done by someone else when they programmed basic ptrace
breakpoint write behavior.

Thanks,
Victor

> And perhap we can do something like this on arm? it can do kmap(page) /
> page_address(page) itself.
>
> Oleg.
>
> --- x/arch/alpha/kernel/smp.c
> +++ x/arch/alpha/kernel/smp.c
> @@ -751,15 +751,9 @@ ipi_flush_icache_page(void *x)
>                 flush_tlb_other(mm);
>  }
>
> -void
> -flush_icache_user_range(struct vm_area_struct *vma, struct page *page,
> -                       unsigned long addr, int len)
> -{
> -       struct mm_struct *mm = vma->vm_mm;
> -
> -       if ((vma->vm_flags & VM_EXEC) == 0)
> -               return;
>
> +void __flush_icache_page_xxx(struct mm_struct *mm, struct page *page) // addr, len ?
> +{
>         preempt_disable();
>
>         if (mm == current->active_mm) {
> @@ -783,3 +777,24 @@ flush_icache_user_range(struct vm_area_s
>
>         preempt_enable();
>  }
> +
> +void flush_icache_page_xxx(struct mm_struct *mm, struct page *page)
> +{
> +       struct mm_struct *mm = current->mm;
> +
> +       down_read(&mm->mmap_sem);
> +       __flush_icache_page_xxx(mm, page);
> +       up_read(&mm->mmap_sem);
> +}
> +
> +void
> +flush_icache_user_range(struct vm_area_struct *vma, struct page *page,
> +                       unsigned long addr, int len)
> +{
> +       struct mm_struct *mm = vma->vm_mm;
> +
> +       if ((vma->vm_flags & VM_EXEC) == 0)
> +               return;
> +
> +       __flush_icache_page_xxx(mm, page);
> +}
>