Messages in this thread |  | | Date | Mon, 20 Nov 2017 22:46:03 +0000 (UTC) | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH v3 for 4.15 08/24] Provide cpu_opv system call |
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----- On Nov 20, 2017, at 1:49 PM, Andi Kleen andi@firstfloor.org wrote:
>> Having cpu_opv do a 4k memcpy allow it to handle scenarios where >> rseq fails to progress. > > If anybody ever gets that right. It will be really hard to just > test such a path. > > It also seems fairly theoretical to me. Do you even have a > test case where the normal path stops making forward progress?
We expect the following loop to progress, typically after a single iteration:
do { cpu = rseq_cpu_start(); ret = rseq_addv(&v, 1, cpu); attempts++; } while (ret);
Now runnig this in gdb, break on "main", run, and single-step execution with "next", the program is stuck in an infinite loop.
What solution do you have in mind to handle this kind of scenario without breaking pre-existing debuggers ?
Looking at vDSO examples of vgetcpu and vclock_gettime under gdb 7.7.1 (debian) with glibc 2.19:
sched_getcpu behavior under single-stepping per source line with "step" seems to only see the ../sysdeps/unix/sysv/linux/x86_64/sched_getcpu.S source lines, which makes it skip single-stepping of the vDSO.
sched_getcpu under "stepi": it does go through the vDSO instruction addresses. It does progress, given that there is no loop there.
clock_gettime under "step": it only sees source lines of ../sysdeps/unix/clock_gettime.c.
clock_gettime under "stepi": it's stuck in an infinite loop.
So instruction-level stepping from gdb turns clock_gettime vDSO into a never-ending loop, which is already bad. But with rseq, the situation is even worse, because it turns source line level single-stepping into infinite loops.
My understanding from https://sourceware.org/bugzilla/show_bug.cgi?id=14466 is that GDB currently simply removes the vDSO from its list of library mappings, which is probably why it skips over vDSO for the source lines single-stepping case. We cannot do that with rseq, because we _want_ the rseq critical section to be inlined into the application or library. A function call costs more than most rseq critical sections.
I plan to have the rseq user-space code provide a "__rseq_table" section so debuggers can eventually figure out that they need to skip over the rseq critical sections. However, it won't help the fact that pre-existing debugger single-stepping will start turning perfectly working programs into never-ending loops simply by having glibc use rseq for memory allocation.
Using the cpu_opv system call on rseq failure solves this problem entirely.
I would even go further and recommend to take a similar approach when lack of progress is detected in a vDSO, and invoke the equivalent system call. The current implementation of the clock_gettime() vDSO turns instruction-level single-stepping into never ending loops, which is far from being elegant.
Thanks,
Mathieu
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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