Messages in this thread | | | Subject | Re: [PATCH v2]: perf/x86: store user space frame-pointer value on a sample | From | Alexey Budankov <> | Date | Mon, 21 May 2018 21:11:33 +0300 |
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Hi,
On 21.05.2018 20:23, Andy Lutomirski wrote: > >> On May 21, 2018, at 9:51 AM, Alexey Budankov <alexey.budankov@linux.intel.com> wrote: >> >> >> Hi Andy, >>> On 21.05.2018 17:14, Andy Lutomirski wrote: >>> >>>> On May 21, 2018, at 5:44 AM, Alexey Budankov <alexey.budankov@linux.intel.com> wrote: >>>> >>>> Hi Peter, >>>> >>>>> On 10.05.2018 13:14, Peter Zijlstra wrote: >>>>> On Thu, May 10, 2018 at 12:42:38PM +0300, Alexey Budankov wrote: >>>>>>> The Changelog needs to state that user_regs->bp is in fact valid and >>>>>> >>>>>> That actually was tested on binaries compiled without and with BP exposed >>>>>> and in the latter case proved the value of that change. >>>>> >>>>> Mostly works is not the same as 'always initialized', if there are entry >>>>> paths that do not store that register, then using the value might leak >>>>> values from the kernel stack, which would be bad. >>>>> >>>>> But like said, I think much of the kernel entry code was sanitized with >>>>> the PTI effort and I suspect things are in fact fine now, but lets wait >>>>> for Andy to confirm. >>>> >>>> It looks like, these days, all registers are saved on system calls, just >>>> like you anticipated. >>>> >>>> So BP register value might be stored into the Perf trace on a sample. >>>> >>>> Andy? >>> >>> Hmm, I thought I replied. Yes, they are indeed all saved, but I’m not very excited about committing to doing so forever. But storing BP should be fine. >> >> Thanks for explicit confirmation regarding BP register. >> BTW, do you see any mean to prevent possible unattended regression? >> I guess it could be some compile time assertion or regression testing. > > Write a selftest?
Hmm, that might be. It would be good to have some embedded notification when things change.
> > The whole perf user regs mechanism is buggy and fragile. I need to massively clean it up at some point.
Yep, making Perf user regs part more robust makes great sense. It is critical part of perf/core subsystem providing values of IP,SP,BP registers that are needed for user stack unwinding during Perf trace file post processing.
Thanks, Alexey
> >> >> Thanks, >> Alexey >> >>> >
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