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SubjectRe: [PATCH 0/2] ptrace_vm: ptrace for syscall emulation virtual machines
On Tue, Mar 10, 2009 at 10:44:36PM +0100, Renzo Davoli wrote:
>I have updated the PTRACE_VM patches.
>The patches have been rebased to linux-2.6.29-rc7 but apply to linux-2.6.29-rc7-git3.
>The set is composed by two patches.
>The first one is for all those architectures where PTRACE_SYSCALL is
>managed via tracehook (x86, powerpc etc).
>Given the wonderful work by Roland McGrath this patch is now
>architecture independent and straightforward simple.
>The second one is the support of PTRACE_VM for user-mode-linux.
>It provides PTRACE_VM for UML processes and uses PTRACE_VM of the hosting

I just finished reading all of them. Good work! :)

Thanks. Comments below.

>The description and motivation follows.
>Proposal: let us simplify
>and now PTRACE_BLOCKSTEP (which will require soon a PTRACE_SYSEMU_BLOCKSTEP),
>my PTRACE_SYSVM...etc. etc.
>Summary of the solution:
>Use tags in the "addr" parameter of existing
>to skip the current call (PTRACE_VM_SKIPCALL) or skip the second upcall to
>the VM/debugger after the syscall execution (PTRACE_VM_SKIPEXIT).

Why not introduce a new request for PTRACE_VM but use *tags* in 'addr'?
We are taking risks of breaking the existing code. :)

>The ptrace tag PTRACE_SYSEMU is a feature mainly used for User-Mode Linux,
>or at most for other virtual machines aiming to virtualize *all* the syscalls
>(total virtual machines).
>In fact:
>ptrace(PTRACE_SYSEMU, pid, 0, 0)
>means that the *next* system call will not be executed.
>PTRACE_SYSEMU AFAIK has been implemented only for x86_32.


>I already proposed some time ago a different tag: PTRACE_SYSVM
>(and I maintain a patch for it) where:
>ptrace(PTRACE_SYSVM, pid, XXX, 0)
>1* is the same as PTRACE_SYSCALL when XXX==0,
>2* skips the call (and stops before entering the next syscall) when
>3* skips the ptrace call after the system call if PTRACE_VM_SKIPEXIT.
> PTRACE_SYSVM has been implemented for x86_32, powerpc_32, um+x86_32.
>(x86_64 and ppc64 exist too, but are less tested).

*I think* this approach is better, since it won't break anything.

>The main difference between SYSEMU and SYSVM is that with SYSVM it is possible
>to decide if *this* system call should be executed or not (instead of the next
>SYSVM can be used also for partial virtual machines (some syscall gets
>virtualized and some others do not), like our umview.

Agreed, I like this idea, this one can finally replace SYSEMU.

>PTRACE_SYSVM above can be used instead of PTRACE_SYSEMU in user-mode linux
>and in all the others total virtual machines. In fact, provided user-mode linux
>skips *all* the syscalls it does not matter if the upcall happens just after
>(SYSEMU) or just before (SYSVM) having skipped the syscall.

My question is if there are any other usages of SYSEMU beyond UML?

>Briefly I would like to unify SYSCALL, SYSEMU and SYSVM.
>We don't need three different tags (and all their "variations",
>We could keep PTRACE_SYSCALL, using the addr parameter as in PTRACE_SYSVM.
>In this case all the code I have seen (user-mode linux, strace, umview
>and googling around) use 0 or 1 for addr (being defined unused).
>defining PTRACE_VM_SKIPCALL=4 and PTRACE_VM_SKIPEXIT=2 (i.e. by ignoring
>the lsb) everything previously coded using PTRACE_SYSCALL should continue
>to work.
>the same tags restarting after a SYSCALL.

Well, since 'addr' is said to be unused, it can have any value beyond
0 or 1, we are still having the risks of breaking existing code. :(

>This change would eventually simplify both the kernel code
>(reducing tags and exceptions) and even user-mode linux and umview.
>The skip-exit feature can be implemented in a arch-independent
>manner, while for skip_call some simple changes are needed
>(the entry assembly code should process the return value of the syscall
>tracing function call, like in arch/x86/kernel/Entry_32.S).

Anyway, we need to find a balance between unifying old stuffs and
breaking compatibility.

BTW, please always update the corresponding man pages when you change
any syscall interface. So let's Cc Michael Kerrisk.

Thank you!

Do what you love, f**k the rest! F**k the regulations!

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