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SubjectRe: [RFC][PATCH 7/8] x86/mm/vsyscall: consider vsyscall page part of user address space
On Sat, Sep 8, 2018 at 2:28 AM Dave Hansen <dave.hansen@linux.intel.com> wrote:
> The vsyscall page is weird. It is in what is traditionally part of the
> kernel address space. But, it has user permissions and we handle faults
> on it like we would on a user page: interrupts on.
>
> Right now, we handle vsyscall emulation in the "bad_area" code, which
> is used for both user-address-space and kernel-address-space faults. Move
> the handling to the user-address-space code *only* and ensure we get there
> by "excluding" the vsyscall page from the kernel address space via a check
> in fault_in_kernel_space().
[...]
> static int fault_in_kernel_space(unsigned long address)
> {
> + /*
> + * The vsyscall page is at an address above TASK_SIZE_MAX,
> + * but is not considered part of the kernel address space.
> + */
> + if (is_vsyscall_vaddr(address))
> + return false;

I think something should check for "#ifdef CONFIG_X86_64"? 32-bit
doesn't have a vsyscall page, right? And this code probably shouldn't
veer off into the userspace-area fault handling code for addresses in
the range 0xff600000-0xff600fff... what is in that region on 32-bit?
Modules or something like that?
Maybe change is_vsyscall_vaddr() so that it always returns false on
32-bit, or put both the definition of is_vsyscall_vaddr() and this
code behind #ifdef guards.
And, in a separate patch, maybe also #ifdef-guard the definition of
VSYSCALL_ADDR in vsyscall.h? Nothing good is going to result from
making a garbage VSYSCALL_ADDR available to 32-bit code.

> +#ifdef CONFIG_X86_64
> + /*
> + * Instruction fetch faults in the vsyscall page might need
> + * emulation. The vsyscall page is at a high address
> + * (>PAGE_OFFSET), but is considered to be part of the user
> + * address space.
> + *
> + * The vsyscall page does not have a "real" VMA, so do this
> + * emulation before we go searching for VMAse

"VMAse"? Is that a typo?

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