vdso && cr (Was: arch_check_bp_in_kernelspace: fix the range check)

[Oleg Nesterov]

On 11/22, Amnon Shiloh wrote:
>
> Now however, that "vsyscall" was effectively replaced by vdso, it
> creates a new problem for me and probably for anyone else who uses
> some form of checkpoint/restore:

Oh, sorry, I can't help here. I can only add Cyrill and Pavel, they
seem to enjoy trying to solve the c/r problems.

> Suppose a process is checkpointed because the system needs to reboot
> for a kernel-upgrade, then restored on the new and different kernel.
> The new VDSO page may no longer match the new kernel - it could for
> example fetch data from addresses in the vsyscall page that now
> contain different things; or in case the hardware also was changed,
> it may use machine-instructions that are now illegal.

Sure. You shouldn't try to save/restore this page(s) directly. But
I do not really understand why do you need. IOW, I don't really
understand the problem, it depends on what c/r actually does.

> As I don't mind to forego the "fast" sys_time(), my obvious solution
> is to disable the vdso for traced processes that may be checkpointed.
>
> One way to do it would be by brute-force: straight after "execve"
> unmap the tracee's vdso page,

Not sure this will be always possible. For example, my (old) glibc
assumes that vsyscall() must work, I won't be surprised if some time
later it won't work without vdso. But again, I do not know.

> then manipulate the ELF tables in
> its memory so the VDSO entry is gone and the library will not go
> looking for it.

Probably it would be enough to simply erase AT_SYSINFO_EHDR note,
but again, I can be easily wrong.

> I just wonder whether you know of an easier and more standard way
> to disable the vdso in user-mode

Only the kernel parameter, afaics. vdso=0

> - ideally on a per-process basis,
> or otherwise, if it's too hard, on the whole computer.  I searched
> the web and found references to "/proc/sys/vm/vdso_enable", but I
> have no such file or "sysctl" option on my system.

sys/vm/vdso_enabled, but only if CONFIG_X86_32 for some reason. See
kernel/sysctl.c

Oleg.

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