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

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Date:	Mon, 16 May 2016 12:38:37 +0300
From:	Dmitry Safonov <dsafonov@...tuozzo.com>
To:	<linux-kernel@...r.kernel.org>, <mingo@...hat.com>
CC:	<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

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

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