Re: VDSO unmap and remap support for additional architectures

[Christopher Covington]

Hi Andy,

On 04/28/2016 02:53 PM, Andy Lutomirski wrote:
> On 04/28/2016 08:18 AM, Christopher Covington wrote:
>> Please take a look at the following prototype of sharing the PowerPC
>> VDSO unmap and remap code with other architectures. I've only hooked
>> up arm64 to begin with. If folks think this is a reasonable approach I
>> can work on 32 bit ARM as well. Not hearing back from an earlier
>> request for guidance [1], I simply dove in and started hacking.
>> Laurent's test case [2][3] is a compelling illustration of whether VDSO
>> remap works or not on a given architecture.
> 
> I think there's a much nicer way:
> 
> https://lkml.kernel.org/r/1461584223-9418-1-git-send-email-dsafonov@xxxxxxxxxxxxx
> 
> Could arm64 and ppc use this approach?  These arch_xyz hooks are gross.

Thanks for the pointer. Any thoughts on how to keep essentially
identical definitions of vdso_mremap from proliferating into every
architecture and variant?

> Also, at some point, possibly quite soon, x86 will want a way for
> user code to ask the kernel to map a specific vdso variant at a specific
> address. Could we perhaps add a new pair of syscalls:
> 
> struct vdso_info {
>     unsigned long space_needed_before;
>     unsigned long space_needed_after;
>     unsigned long alignment;
> };
> 
> long vdso_get_info(unsigned int vdso_type, struct vdso_info *info);
> 
> long vdso_remap(unsigned int vdso_type, unsigned long addr, unsigned int flags);
> 
> #define VDSO_X86_I386 0
> #define VDSO_X86_64 1
> #define VDSO_X86_X32 2
> // etc.
> 
> vdso_remap will map the vdso of the chosen type such at
> AT_SYSINFO_EHDR lines up with addr. It will use up to
> space_needed_before bytes before that address and space_needed_after
> after than address. It will also unmap the old vdso (or maybe only do
> that if some flag is set).
> 
> On x86, mremap is *not* sufficient for everything that's needed,
> because some programs will need to change the vdso type.

I don't I understand. Why can't people just exec() the ELF type that
corresponds to the VDSO they want?

Thanks,
Cov

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