Re: [Qemu-devel] [PATCH] User mode: Handle x86_64 vsyscall

[Laurent Desnogues <laurent.desnogues@gmail.com>]

On Sun, Oct 18, 2009 at 5:09 AM, Jamie Lokier <jamie@shareable.org> wrote:
[...]
>
> Please don't do that.  Some code traces instructions through the
> vsyscall/vdso page, and will be surprised if a syscall instruction
> does not do what's expected based on the registers at that point.
>
> Also I don't know if anyone's done this, but I have played with the
> idea of an optimising x86->x86 JIT translator (similar to valgrind or
> qemu's TCG) which would include the vdso instruction sequence in it's
> traces, just because it didn't treat that any differently from other
> userspace code.  Making the syscall instruction behave differently due
> to EIP would break that sort of thing.
>
> There's no performance penalty in setting a few registers prior to
> using the syscall instruction normally, so please do that.

My proposed patch intercepts vsyscall as soon as the PC is
in the [VSYSCALL_START, VSYSCALL_END[ range, so all
instructions in that range won't be translated.  Doing it
differently will cause problems due to the virtual address.

> On x86_64, the vsyscall page has fixed address (see
> linux/arch/x86/kernel/vsyscall_64.c), but the vdso usually has
> variable address.
>
> On x86_32, the vdso has randomised address unless configurd to be a
> fixed address.  On older kernels it was a fixed address and some
> binary programs assume they can call that.

So QEMU can't do things properly and some binaries will
fail, right?


Laurent