Re: [PATCH v2 0/9] tracing: Improbe BTF support on probe events

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 20 Jul 2023 22:50:18 +0100
Alan Maguire <alan.maguire@oracle.com> wrote:


> >> One thing we should probably figure out is a common approach to handling
> >> ambiguous static functions that will work across ftrace and BPF.  A few
> >> edge cases that are worth figuring out:
> >>
> >> 1. a static function with the same name exists in multiple modules,
> >> either with different or identical function signatures
> >> 2. a static function has .isra.0 and other gcc suffixes applied to
> >> static functions during optimization
> >>
> >> As Alexei mentioned, we're still working on 1, so it would be good
> >> to figure out a naming scheme that works well in both ftrace and BPF
> >> contexts. There are a few hundred of these ambiguous functions. My
> >> reading of the fprobe docs seems to suggest that there is no mechanism
> >> to specify a specific module for a given symbol (as in ftrace filters),
> >> is that right?
> > 
> > Yes, it doesn't have module specificaiton at this moment. I'll considering
> > to fix this. BTW, for the same-name functions, we are discussing another
> > approach. We also need to sync this with BTF. 
> > 
> > https://lore.kernel.org/all/20230714150326.1152359-1-alessandro.carminati@gmail.com/
> > 
> >>
> >> Jiri led a session on this topic at LSF/MM/BPF ; perhaps we should
> >> carve out some time at Plumbers to discuss this?
> > 
> > Yeah, good idea.
> > 
> >>
> >> With respect to 2, pahole v1.25 will generate representations for these
> >> "."-suffixed functions in BTF via --btf_gen_optimized [1]. (BTF
> >> representation is skipped if the optimizations impact on the registers
> >> used for function arguments; if these don't match calling conventions
> >> due to optimized-out params, we don't represent the function in BTF,
> >> as the tracing expectations are violated).
> > 
> > Correct. But can't we know which argument is skipped by the optimization
> > from the DWARF? At least the function parameters will be changed.
> >
> 
> Yep; we use the expected registers to spot cases where something
> has been optimized out.

I guess DWARF knows which register is optimized out and then BTF also
knows that?
Let me update my pahole too.

> >> However the BTF function name - in line with DWARF representation -
> >> will not have the .isra suffix. So the thing to bear in mind is if
> >> you use the function name with suffix as the fprobe function name,
> >> a BTF lookup of that exact ("foo.isra.0") name will not find anything,
> >> while a lookup of "foo" will succeed. I'll add some specifics in your
> >> patch doing the lookups, but just wanted to highlight the issue at
> >> the top-level.
> > 
> > So, what about adding an index sorted list of the address and BTF entry
> > index as an expansion of the BTF? It allowed us to easily map the suffixed
> > symbol address (we can get it from kallsyms) to BTF quickly.
> > So the module will have
> > 
> > [BTF data][array length][BTF index array]
> > 
> > Index array member will be like this.
> > 
> > struct btf_index {
> > 	u32	offset;	// offset from the start text
> > 	u32	id:		// BTF type id
> > };
> > 
> > We can do binary search the function type id from the symbol address.
> > 
> 
> Yeah, I wonder if a representation that bridged between kallsyms and BTF
> might be valuable? I don't _think_ it's as much of an issue for your
> case though since you only need to do the BTF lookup once on fprobe
> setup, right? Thanks!

Yes, but I'm thinking fprobe to support some sort of 'symbol+offset' format
to specify one of the "same-name" symbols. In that case, it is important to
identify which address the BTF type is pointed.

Thank you!

> 
> Alan
> 
> 
> 
> > Thank you,
> > 
> >>
> >> Thanks!
> >>
> >> Alan
> >>
> >> [1]
> >> https://lore.kernel.org/bpf/1675790102-23037-1-git-send-email-alan.maguire@oracle.com/
> >>
> >>> Selftest test case [8/9] and document [9/9] are also updated according to
> >>> those changes.
> >>>
> >>> This series can be applied on top of "v6.5-rc2" kernel.
> >>>
> >>> You can also get this series from:
> >>>
> >>> git://git.kernel.org/pub/scm/linux/kernel/git/mhiramat/linux.git topic/fprobe-event-ext
> >>>
> >>>
> >>> Thank you,
> >>>
> >>> ---
> >>>
> >>> Masami Hiramatsu (Google) (9):
> >>>       tracing/probes: Fix to add NULL check for BTF APIs
> >>>       bpf/btf: tracing: Move finding func-proto API and getting func-param API to BTF
> >>>       bpf/btf: Add a function to search a member of a struct/union
> >>>       tracing/probes: Support BTF based data structure field access
> >>>       tracing/probes: Support BTF field access from $retval
> >>>       tracing/probes: Add string type check with BTF
> >>>       tracing/fprobe-event: Assume fprobe is a return event by $retval
> >>>       selftests/ftrace: Add BTF fields access testcases
> >>>       Documentation: tracing: Update fprobe event example with BTF field
> >>>
> >>>
> >>>  Documentation/trace/fprobetrace.rst                |   50 ++
> >>>  include/linux/btf.h                                |    7 
> >>>  kernel/bpf/btf.c                                   |   83 ++++
> >>>  kernel/trace/trace_fprobe.c                        |   58 ++-
> >>>  kernel/trace/trace_probe.c                         |  402 +++++++++++++++-----
> >>>  kernel/trace/trace_probe.h                         |   12 +
> >>>  .../ftrace/test.d/dynevent/add_remove_btfarg.tc    |   11 +
> >>>  .../ftrace/test.d/dynevent/fprobe_syntax_errors.tc |    6 
> >>>  8 files changed, 503 insertions(+), 126 deletions(-)
> >>>
> >>> --
> >>> Masami Hiramatsu (Google) <mhiramat@kernel.org>
> >>>
> > 
> > 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>