Re: [RFC PATCH v2] ptrace: add PTRACE_GET_SYSCALL_INFO request

[Dmitry V. Levin]

On Fri, Nov 23, 2018 at 07:01:39AM +0300, Dmitry V. Levin wrote:
> On Thu, Nov 22, 2018 at 04:19:10PM -0800, Andy Lutomirski wrote:
> > On Thu, Nov 22, 2018 at 11:15 AM Dmitry V. Levin wrote:
> > > On Thu, Nov 22, 2018 at 06:55:29AM -0800, Andy Lutomirski wrote:
> > > > On Wed, Nov 21, 2018 at 3:56 PM Dmitry V. Levin wrote:
> > > > > On Wed, Nov 21, 2018 at 02:56:57PM -0800, Andy Lutomirski wrote:
> > > > > > Please cc linux-api@xxxxxxxxxxxxxxx for future versions.
> > > > > >
> > > > > > On Wed, Nov 21, 2018 at 7:58 AM Elvira Khabirova wrote:
> > > > > > >
> > > > > > > struct ptrace_syscall_info {
> > > > > > >         __u8 op; /* 0 for entry, 1 for exit */
> > > > > >
> > > > > > Can you add proper defines, like:
> > > > > >
> > > > > > #define PTRACE_SYSCALL_ENTRY 0
> > > > > > #define PTRACE_SYSCALL_EXIT 1
> > > > > > #define PTRACE_SYSCALL_SECCOMP 2
> > > > > >
> > > > > > and make seccomp work from the start?  I'd rather we don't merge an
> > > > > > implementation that doesn't work for seccomp and then have to rework
> > > > > > it later.
> > > > >
> > > > > What's the difference between PTRACE_EVENT_SECCOMP and syscall-entry-stop
> > > > > with regards to PTRACE_GET_SYSCALL_INFO request?  At least they have the
> > > > > same entry_info to return.
> > > >
> > > > I'm not sure there's any material difference.
> > >
> > > In that case we don't really need PTRACE_SYSCALL_SECCOMP: op field
> > > describes the structure inside the union to use, not the ptrace stop.
> > 
> > Unless we think the structures might diverge in the future.
> 
> If these structures ever diverge, then a seccomp structure will be added
> to the union, and a portable userspace code will likely look this way:
> 
> #include <linux/ptrace.h>
> ...
> struct ptrace_syscall_info info;
> long rc = ptrace(PTRACE_GET_SYSCALL_INFO, pid, (void *) sizeof(info), &info);
> ...
> switch (info.op) {
> 	case PTRACE_SYSCALL_INFO_ENTRY:
> 		/* handle info.entry */
> 	case PTRACE_SYSCALL_INFO_EXIT:
> 		/* handle info.exit */
> #ifdef PTRACE_SYSCALL_INFO_SECCOMP
> 	case PTRACE_SYSCALL_INFO_SECCOMP:
> 		/* handle info.seccomp */
> #endif
> 	default:
> 		/* handle unknown info.op */
> }
> 
> In other words, it would be better if PTRACE_SYSCALL_INFO_* selector
> constants were introduced along with corresponding structures in the
> union.

However, the approach I suggested doesn't provide forward compatibility:
if userspace is compiled with kernel headers that don't define
PTRACE_SYSCALL_INFO_SECCOMP, it will break when the kernel
starts to use PTRACE_SYSCALL_INFO_SECCOMP instead of
PTRACE_SYSCALL_INFO_ENTRY for PTRACE_EVENT_SECCOMP support
in PTRACE_GET_SYSCALL_INFO.

The solution is to introduce PTRACE_SYSCALL_INFO_SECCOMP and struct
ptrace_syscall_info.seccomp along with PTRACE_EVENT_SECCOMP support
in PTRACE_GET_SYSCALL_INFO.  The initial revision of the seccomp
structure could be made the same as the entry structure, or it can
diverge from the beginning, e.g., by adding ret_data field containing
SECCOMP_RET_DATA return value stored in ptrace_message, this would save
ptracers an extra PTRACE_GETEVENTMSG call currently required to obtain it.


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