linux-kernel - Re: [PATCHv8 resend 1/2] x86/vdso: add mremap hook to vm_special

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Date:	Mon, 16 May 2016 12:54:29 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dmitry Safonov <dsafonov@...tuozzo.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...hat.com,
	luto@...capital.net, tglx@...utronix.de, hpa@...or.com,
	x86@...nel.org, akpm@...ux-foundation.org, linux-mm@...ck.org,
	0x7f454c46@...il.com
Subject: Re: [PATCHv8 resend 1/2] x86/vdso: add mremap hook to
 vm_special_mapping


* Dmitry Safonov <dsafonov@...tuozzo.com> wrote:

> On 05/10/2016 04:29 PM, Dmitry Safonov wrote:
> >Add possibility for userspace 32-bit applications to move
> >vdso mapping. Previously, when userspace app called
> >mremap for vdso, in return path it would land on previous
> >address of vdso page, resulting in segmentation violation.
> >Now it lands fine and returns to userspace with remapped vdso.
> >This will also fix context.vdso pointer for 64-bit, which does not
> >affect the user of vdso after mremap by now, but this may change.
> >
> >As suggested by Andy, return EINVAL for mremap that splits vdso image.
> >
> >Renamed and moved text_mapping structure declaration inside
> >map_vdso, as it used only there and now it complement
> >vvar_mapping variable.
> >
> >There is still problem for remapping vdso in glibc applications:
> >linker relocates addresses for syscalls on vdso page, so
> >you need to relink with the new addresses. Or the next syscall
> >through glibc may fail:
> >  Program received signal SIGSEGV, Segmentation fault.
> >  #0  0xf7fd9b80 in __kernel_vsyscall ()
> >  #1  0xf7ec8238 in _exit () from /usr/lib32/libc.so.6
> >
> >Signed-off-by: Dmitry Safonov <dsafonov@...tuozzo.com>
> >Acked-by: Andy Lutomirski <luto@...nel.org>
> >---
> >v8: add WARN_ON_ONCE on current->mm != new_vma->vm_mm
> >v7: build fix
> >v6: moved vdso_image_32 check and fixup code into vdso_fix_landing function
> >    with ifdefs around
> >v5: as Andy suggested, add a check that new_vma->vm_mm and current->mm are
> >    the same, also check not only in_ia32_syscall() but image == &vdso_image_32
> >v4: drop __maybe_unused & use image from mm->context instead vdso_image_32
> >v3: as Andy suggested, return EINVAL in case of splitting vdso blob on mremap;
> >    used is_ia32_task instead of ifdefs
> >v2: added __maybe_unused for pt_regs in vdso_mremap
> 
> Ping?

There's no 0/2 boilerplate explaining the background of the changes - why do you 
want to mremap() the vDSO?

Thanks,

	Ingo