Re: [PATCH v4 17/21] mm/mmap: Drop arch_unmap() call from all archs

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* LEROY Christophe <christophe.leroy2@cs-soprasteria.com> [240710 17:02]:
> 
> 
> Le 10/07/2024 à 21:22, Liam R. Howlett a écrit :
> > From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
> > 
> > The arch_unmap call was previously moved above the rbtree modifications
> > in commit 5a28fc94c914 ("x86/mpx, mm/core: Fix recursive munmap()
> > corruption").  The move was motivated by an issue with calling
> > arch_unmap() after the rbtree was modified.
> > 
> > Since the above commit, mpx was dropped from the kernel in 45fc24e89b7c
> > ("x86/mpx: remove MPX from arch/x86"), so the motivation for calling
> > arch_unmap() prior to modifying the vma tree no longer exists
> > (regardless of rbtree or maple tree implementations).
> > 
> > Furthermore, the powerpc implementation is also no longer needed as per
> > [1] and [2].  So the arch_unmap() function can be completely removed.
> 
> I'm not sure to understand. Is it replaced by something else ?
> We wanted to get rid of arch_unmap() but it was supposed to be replaced 
> by some core function because the functionnality itself is still 
> required and indeed all the discussion around [2] demonstrated that not 
> only powerpc but at least arm and probably others needed to properly 
> clean-up reference to VDSO mappings on unmapping.
> 
> So as mentioned by Michael you can't just drop that without replacing it 
> by something else. We need the VDSO signal handling to properly fallback 
> on stack-based trampoline when the VDSO trampoline gets mapped out.

I'll address this after the part I missed..

> 
> Or did I miss something ?
> 

I think I missed something in regards to what you need in ppc.

From what I understand, other platforms still map and use the vdso
(context.vdso is set), but unmap_arch() does nothing.  It is only the
powerpc version that clears the vdso pointer if it is unmapped.

git grep -w arch_unmap shows:
arch/powerpc/include/asm/mmu_context.h
arch/x86/include/asm/mmu_context.h
include/asm-generic/mm_hooks.h
mm/mmap.c

The generic and x86 versions are empty.

From the patch set you referenced, we see changes related to the files
modified, but I don't think any of them did anything with unmap_arch().

arm: a0d2fcd62ac2 ("vdso/ARM: Make union vdso_data_store available for all architectures")
arm64: d0fba04847ae ("arm64: vdso: Use generic union vdso_data_store")
mips: d697a9997a0d ("MIPS: vdso: Use generic union vdso_data_store")
s390: cb3444cfdb48 ("s390/vdso: Use generic union vdso_data_store")
riscv: eba755314fa7 ("riscv: vdso: Use generic union vdso_data_store")

ia64 is dead
nds32 is dead
hexagon has a bunch of vdso work in the logs as well.

There is also a6c19dfe3994 ("arm64,ia64,ppc,s390,sh,tile,um,x86,mm: remove default gate area")

I do not see sparc changing away from what the patches were doing, but
again, the arch_unmap() seems to do nothing there as well.

So, what I was looking to do is to avoid a call to arch specific
functions that does nothing but set the vdso pointer to NULL for
powerpc.

The thread referenced in the git bug [1] seems to indicate this is for
CRIU unmapping/restoring a task, but CRIU now just moves the vdso
mapping (or just works on ppc at this point?).  Since [2] hasn't landed,
isn't this still broken for CRIU on powerpc as it is?

So, are we keeping the unmap_arch() function around, which has errors
that were never fixed, for a single application that utilizes a newer
method of moving the vdso anyways?

On the note of CRIU, it seems it cannot handle tasks which don't have
the vdso mapped anymore [3], so setting it to NULL is probably a bad
plan even for that one application?


So, I think this just leaves the fallback when the VDSO is unmapped..
Well, it seems what people have been doing is unmap the vdso to stop
these functions from working [4]. At least this is what some users are
doing.  The ability to replace this vma with a guard vma leads me to
believe that other archs don't fall back at all - please correct me if
I'm wrong!

I also cannot find any reference to other archs clearing the
context.vdso (aside from failures in __setup_additional_pages).

But maybe I don't fully understand how this works?

Thanks,
Liam


[1] https://lore.kernel.org/lkml/87d0lht1c0.fsf@concordia.ellerman.id.au/
[2] https://lore.kernel.org/lkml/9c2b2826-4083-fc9c-5a4d-c101858dd560@linux.vnet.ibm.com/
[3] https://github.com/checkpoint-restore/criu/issues/488
[4] https://github.com/insanitybit/void-ship

Thanks,
Liam