Re: Why do kprobes and uprobes singlestep?

[Daniel Xu]

On Tue, Mar 02, 2021 at 06:18:23PM -0800, Alexei Starovoitov wrote:
> On Tue, Mar 2, 2021 at 5:46 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> >
> >
> > > On Mar 2, 2021, at 5:22 PM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
> > >
> > > On Tue, Mar 2, 2021 at 1:02 PM Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > >>
> > >>
> > >>>> On Mar 2, 2021, at 12:24 PM, Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:
> > >>>
> > >>> On Tue, Mar 2, 2021 at 10:38 AM Andy Lutomirski <luto@xxxxxxxxxx> wrote:
> > >>>>
> > >>>> Is there something like a uprobe test suite?  How maintained /
> > >>>> actively used is uprobe?
> > >>>
> > >>> uprobe+bpf is heavily used in production.
> > >>> selftests/bpf has only one test for it though.
> > >>>
> > >>> Why are you asking?
> > >>
> > >> Because the integration with the x86 entry code is a mess, and I want to know whether to mark it BROKEN or how to make sure the any cleanups actually work.
> > >
> > > Any test case to repro the issue you found?
> > > Is it a bug or just messy code?
> >
> > Just messy code.
> >
> > > Nowadays a good chunk of popular applications (python, mysql, etc) has
> > > USDTs in them.
> > > Issues reported with bcc:
> > > https://github.com/iovisor/bcc/issues?q=is%3Aissue+USDT
> > > Similar thing with bpftrace.
> > > Both standard USDT and semaphore based are used in the wild.
> > > uprobe for containers has been a long standing feature request.
> > > If you can improve uprobe performance that would be awesome.
> > > That's another thing that people report often. We optimized it a bit.
> > > More can be done.
> >
> >
> > Wait... USDT is much easier to implement well.  Are we talking just USDT or are we talking about general uprobes in which almost any instruction can get probed?  If the only users that care about uprobes are doing USDT, we could vastly simplify the implementation and probably make it faster, too.
> 
> USDTs are driving the majority of uprobe usage.

I'd say 50/50 in my experience. Larger userspace applications using bpf
for production monitoring tend to use USDT for stability and ABI reasons
(hard for bpf to read C++ classes). Bare uprobes (ie not USDT) are used
quite often for ad-hoc production debugging.

> If they can get faster it will increase their adoption even more.
> There are certainly cases of normal uprobes.
> They are at the start of the function 99% of the time.
> Like the following:
> "uprobe:/lib64/libc.so:malloc(u64 size):size:size,_ret",
> "uprobe:/lib64/libc.so:free(void *ptr)::ptr",
> is common despite its overhead.
> 
> Here is the most interesting and practical usage of uprobes:
> https://github.com/iovisor/bcc/blob/master/tools/sslsniff.py
> and the manpage for the tool:
> https://github.com/iovisor/bcc/blob/master/tools/sslsniff_example.txt
> 
> uprobe in the middle of the function is very rare.
> If the kernel starts rejecting uprobes on some weird instructions
> I suspect no one will complain.

I think it would be great if the kernel could reject mid-instruction
uprobes. Unlike with kprobes, you can place uprobes on immediate
operands which can cause silent data corruption. See
https://github.com/iovisor/bpftrace/pull/803#issuecomment-507693933
for a funny example.

To prevent accidental (and silent) data corruption, bpftrace uses a
disassembler to ensure uprobes are placed on instruction boundaries.

<...>

Daniel