Messages in this thread | | | Date | Tue, 23 Aug 2011 20:41:03 +0100 | From | Al Viro <> | Subject | Re: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386) |
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On Tue, Aug 23, 2011 at 12:24:22PM -0700, Linus Torvalds wrote: > On Tue, Aug 23, 2011 at 12:18 PM, H. Peter Anvin <hpa@zytor.com> wrote: > > > > We could drop that information in a metaregister. ?It's not backward > > compatible, but at least it will be obvious when that information is > > available and not. > > Well, seriously, UML already looks at the word at "ip-2" for other > reasons. So it isn't like there is any point in adding more support to > just give you that information in another form.
That is done only for task singlestepped in the guest: /* * This closes a way to execute a system call on the host. If * you set a breakpoint on a system call instruction and singlestep * from it, the tracing thread used to PTRACE_SINGLESTEP the process * rather than PTRACE_SYSCALL it, allowing the system call to execute * on the host. The tracing thread will check this flag and * PTRACE_SYSCALL if necessary. */ if (current->ptrace & PT_DTRACE) current->thread.singlestep_syscall = is_syscall(PT_REGS_IP(¤t->thread.regs)); with PT_DTRACE set by uml user_enable_single_step()
And it's not cheap - doing that on each syscall will be unpleasant... Frankly, I'd rather stopped telling the uml userland about vdso in such setups. And anything that plays with SYSCALL outside of vdso... <shrug> we already have a "don't run it native on 32bit", adding "don't run it on 32bit uml on amd64 host" is not too serious. At least for now...
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