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SubjectRe: VDSO unmap and remap support for additional architectures
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:

Could arm64 and ppc use this approach? These arch_xyz hooks are gross.

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.


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