Re: [PATCH 3/3] um: Implement a x86_64 vDSO

From: Andy Lutomirski
Date: Wed Jul 20 2011 - 13:54:14 EST

On 07/18/2011 09:07 AM, Richard Weinberger wrote:
Until now UML had no x86_64 vDSO.
So glibc always used the vsyscall page for gettimeday()
and friends.
Calls to gettimeday() returned falsely the host time and
confused some programs.

This patch adds a vDSO which turns all __vdso_* calls into
a system call so that UML can trap them.

As glibc still uses the vsyscall page for static binaries this
patch improves the situation only for dynamic binaries.

Signed-off-by: Richard Weinberger<richard@xxxxxx>

+ asm("syscall" : "=a" (ret) :
+ "0" (__NR_clock_gettime), "D" (clock), "S" (ts) : "memory");

The x86-64 ABI and glibc think that rcx, and r11 are clobbered. glibc also thinks that cc is clobbered. I personally think that rdx and r8-r15 ought to be clobbered under the principle that syscall should look like a function call.

It's hard to imagine this causing a problem in a function this trivial, though.

This applies to the other two syscall instructions as well.


P.S. If you really care, with the changes in tip/x86/vdso, it ought to be possible for UML to trap vsyscalls as well. This would need a change to do_emulate_vsyscall to honor TIF_SYSCALL_EMU with intelligent semantics. I know nothing about ptrace, so I'm not volunteering :)
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