Am Mittwoch 20 Juli 2011, 19:54:02 schrieb Andy Lutomirski:
> 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<rich...@nod.at>
> >
> > + 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.
Hmm, I thought my implementation is fine because it follows
arch/x86/vdso/vclock_gettime.c (Linus' tree).
notrace static long vdso_fallback_gettime(long clock, struct timespec *ts)
{
long ret;
asm("syscall" : "=a" (ret) :
"0" (__NR_clock_gettime),"D" (clock), "S" (ts) : "memory");
return ret;
}
Am I missing something or needs vdso_fallback_gettime() also some updates?
> 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.
>
> --Andy
>
> 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 :)
Ok, I'll have at look at it.
Thanks,
//richard
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