[lttng-dev] Compat syscall instrumentation and return from execve issue

[Andy Lutomirski]

On Mon, Nov 9, 2015 at 11:43 AM, Steven Rostedt <rostedt at goodmis.org> wrote:
> On Mon, 9 Nov 2015 11:29:10 -0800
> Andy Lutomirski <luto at kernel.org> wrote:
>
>> > I'm thinking the TS_EXECVE flag would be the least intrusive. Add a
>> > comment that it is used by tracepoints to map between compat and
>> > non-compat syscalls when execve switches the flag. This would not need
>> > to touch any of the logic of the hotpaths within the systemcalls
>> > themselves.
>>
>> Let's make it really simple: add an 'unsigned int arch' to
>> syscall_return_slowpath.  As of last week, Linus' tree sends all compat
>> returns, without exception (except brand new children, depending on your
>> point of view), through that path, and the caller always knows the
>> architecture.
>>
>> But keep in mind that any games you play here are going to get
>> completely and utterly screwed up if anyone is playing with ptrace to
>> change syscall numbers.  You'd also going to have problems with syscall
>> restart, sigreturn, etc, so it would be nice to have an argument that
>> the putative solution solves the problem for real instead of just adding
>> complexity to paper it over.
>>
>> Meanwhile, I'm trying to remove all of the magic from the handling of
>> execve, and I'm half-way there.  Let's please not add more, especially
>> if that magic needs to touch asm code.
>
> The solution I suggested wouldn't touch any asm code. The only change
> would be to reserve the TS_EXECVE flag. Actually, come to think of it,
> we could have Mathieu's TS_ORIG_COMPAT flag, and still only have the
> tracepoint syscall set it, such that the matching tracepoint syscall
> exit would know that the initial call was COMPAT or not.

Someone needs to clear TS_EXECVE, though.

>
> The goal is only to make sure that the system call exit tracepoint
> matches the system call enter tracepoint.
>
> The system call enter would set or clear the TS_ORIG_COMPAT if the
> TS_COMPAT is set when entering the system call, and it would check that
> flag when exiting the system call.

This seems a bit odd, though, since we aren't very good about
preserving the syscall nr or the args through syscall processing.  In
any event, in the new improved x86 syscall code, we know what arch we
are just by following the control flow, so no flags should be needed.
Hence my suggestion of just adding an "unsigned int arch" to the
return slowpath.

--Andy