Re: [RFC PATCH for 4.17 02/21] rseq: Introduce restartable sequences system call (v12)

[Mathieu Desnoyers]

----- On Mar 29, 2018, at 12:24 PM, rostedt rostedt@xxxxxxxxxxx wrote:

> On Thu, 29 Mar 2018 11:39:00 -0400 (EDT)
> Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> 
>> Enforcing SIGSEGV on syscall entry when nested in a rseq critical section
>> will not be free both in terms of syscall overhead, and in terms of code
>> maintenance: we'd need to add those checks into entry.S for each architecture
>> supported, which pretty much doubles the amount of architecture-specific
>> code we need to implement for rseq. Currently, all we need is to hook in
>> signal delivery and wire up the system call numbers.
> 
> Why not have the check on syscall exit? Then we could use the ptrace
> type mechanism to only go that path when a rseq exists for the program.

Currently, anyone using ptrace on a process has pretty much given up all
hopes of performance. Processes will use rseq to gain performance, not the
opposite, so this deterioration will be unwelcome.

One thing I would find more acceptable is to only compile in this check under
a CONFIG_DEBUG_RSEQ option or something similar. This means we can then put
the check at the most convenient location without caring too much about its
performance impact.

Thoughts ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com