Re: [PATCH] powerpc/vdso: Separate vvar vma from vdso

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 26/03/2021 à 20:17, Dmitry Safonov a écrit :
> Since commit 511157ab641e ("powerpc/vdso: Move vdso datapage up front")
> VVAR page is in front of the VDSO area. In result it breaks CRIU
> (Checkpoint Restore In Userspace) [1], where CRIU expects that "[vdso]"
> from /proc/../maps points at ELF/vdso image, rather than at VVAR data page.
> Laurent made a patch to keep CRIU working (by reading aux vector).
> But I think it still makes sence to separate two mappings into different
> VMAs. It will also make ppc64 less "special" for userspace and as
> a side-bonus will make VVAR page un-writable by debugger (which previously
> would COW page and can be unexpected).
> 
> I opportunistically Cc stable on it: I understand that usually such
> stuff isn't a stable material, but that will allow us in CRIU have
> one workaround less that is needed just for one release (v5.11) on
> one platform (ppc64), which we otherwise have to maintain.
> I wouldn't go as far as to say that the commit 511157ab641e is ABI
> regression as no other userspace got broken, but I'd really appreciate
> if it gets backported to v5.11 after v5.12 is released, so as not
> to complicate already non-simple CRIU-vdso code. Thanks!
> 
> Cc: Andrei Vagin <avagin@gmail.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
> Cc: Laurent Dufour <ldufour@linux.ibm.com>
> Cc: Michael Ellerman <mpe@ellerman.id.au>
> Cc: Paul Mackerras <paulus@samba.org>
> Cc: linuxppc-dev@lists.ozlabs.org
> Cc: stable@vger.kernel.org # v5.11
> [1]: https://github.com/checkpoint-restore/criu/issues/1417
> Signed-off-by: Dmitry Safonov <dima@arista.com>
> Tested-by: Christophe Leroy <christophe.leroy@csgroup.eu>
> ---
>   arch/powerpc/include/asm/mmu_context.h |  2 +-
>   arch/powerpc/kernel/vdso.c             | 54 +++++++++++++++++++-------
>   2 files changed, 40 insertions(+), 16 deletions(-)
> 

> @@ -133,7 +135,13 @@ static int __arch_setup_additional_pages(struct linux_binprm *bprm, int uses_int
>   	 * install_special_mapping or the perf counter mmap tracking code
>   	 * will fail to recognise it as a vDSO.
>   	 */
> -	mm->context.vdso = (void __user *)vdso_base + PAGE_SIZE;
> +	mm->context.vdso = (void __user *)vdso_base + vvar_size;
> +
> +	vma = _install_special_mapping(mm, vdso_base, vvar_size,
> +				       VM_READ | VM_MAYREAD | VM_IO |
> +				       VM_DONTDUMP | VM_PFNMAP, &vvar_spec);
> +	if (IS_ERR(vma))
> +		return PTR_ERR(vma);
>   
>   	/*
>   	 * our vma flags don't have VM_WRITE so by default, the process isn't


IIUC, VM_PFNMAP is for when we have a vvar_fault handler.
Allthough we will soon have one for handle TIME_NS, at the moment powerpc doesn't have that handler.
Isn't it dangerous to set VM_PFNMAP then ?

Christophe