Re: [PATCH] mm, gup: introduce concept of "foreign" get_user_pages()
From: Srikar Dronamraju
Date: Mon Jan 25 2016 - 08:18:13 EST
> One of Vlastimil's comments made me go dig back in to the uprobes
> code's use of get_user_pages(). I decided to change both of them
> to be "foreign" accesses.
> This also fixes the nommu breakage that Vlastimil noted last time.
> Srikar, I'd appreciate if you can have a look at the uprobes.c
> modifications, especially the comment. I don't think this will
> change any behavior, but I want to make sure the comment is
> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> For protection keys, we need to understand whether protections
> should be enforced in software or not. In general, we enforce
> protections when working on our own task, but not when on others.
> We call these "current" and "foreign" operations.
> This patch introduces a new get_user_pages() variant:
> We modify the vanilla get_user_pages() so it can no longer be
> used on mm/tasks other than 'current/current->mm', which is by
> far the most common way it is called. Using it makes a few of
> the call sites look a bit nicer.
> In other words, get_user_pages_foreign() is a replacement for
> when get_user_pages() is called on non-current tsk/mm.
> This also switches get_user_pages_(un)locked() over to be like
> get_user_pages() and not take a tsk/mm. There is no
> get_user_pages_foreign_(un)locked(). If someone wants that
> behavior they just have to use "__" variant and pass in
> FOLL_FOREIGN explicitly.
> The uprobes is_trap_at_addr() location holds mmap_sem and
> calls get_user_pages(current->mm) on an instruction address. This
> makes it a pretty unique gup caller. Being an instruction access
> and also really originating from the kernel (vs. the app), I opted
> to consider this a 'foreign' access where protection keys will not
> be enforced.
Changes for uprobes.c looks good to me.
Acked-by: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
> Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx>
> Cc: Srikar Dronamraju <srikar@xxxxxxxxxxxxxxxxxx>
> Cc: vbabka@xxxxxxx
> Cc: jack@xxxxxxx
> diff -puN kernel/events/uprobes.c~get_current_user_pages kernel/events/uprobes.c
> --- a/kernel/events/uprobes.c~get_current_user_pages 2016-01-22 08:43:42.602473969 -0800
> +++ b/kernel/events/uprobes.c 2016-01-22 09:36:14.203845894 -0800
> @@ -299,7 +299,7 @@ int uprobe_write_opcode(struct mm_struct
> /* Read the page with vaddr into memory */
> - ret = get_user_pages(NULL, mm, vaddr, 1, 0, 1, &old_page, &vma);
> + ret = get_user_pages_foreign(NULL, mm, vaddr, 1, 0, 1, &old_page, &vma);
> if (ret <= 0)
> return ret;
> @@ -1700,7 +1700,13 @@ static int is_trap_at_addr(struct mm_str
> if (likely(result == 0))
> goto out;
> - result = get_user_pages(NULL, mm, vaddr, 1, 0, 1, &page, NULL);
> + /*
> + * The NULL 'tsk' here ensures that any faults that occur here
> + * will not be accounted to the task. 'mm' *is* current->mm,
> + * but we treat this as a 'foreign' access since it is
> + * essentially a kernel access to the memory.
> + */
> + result = get_user_pages_foreign(NULL, mm, vaddr, 1, 0, 1, &page, NULL);
> if (result < 0)
> return result;
Thanks and Regards