On 19/07/2023 17:01, Masami Hiramatsu (Google) wrote: > On Wed, 19 Jul 2023 10:02:06 +0100 > Alan Maguire <alan.maguire@xxxxxxxxxx> wrote: > >> On 17/07/2023 16:23, Masami Hiramatsu (Google) wrote: >>> Hi, >>> >>> Here is the 2nd version of series to improve the BTF support on probe events. >>> The previous series is here: >>> >>> https://lore.kernel.org/linux-trace-kernel/168699521817.528797.13179901018528120324.stgit@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx/ >>> >>> In this version, I added a NULL check fix patch [1/9] (which will go to >>> fixes branch) and move BTF related API to kernel/bpf/btf.c [2/9] and add >>> a new BTF API [3/9] so that anyone can reuse it. >>> Also I decided to use '$retval' directly instead of 'retval' pseudo BTF >>> variable for field access at [5/9] because I introduced an idea to choose >>> function 'exit' event automatically if '$retval' is used [7/9]. With that >>> change, we can not use 'retval' because if a function has 'retval' >>> argument, we can not decide 'f func retval' is function exit or entry. >> >> this is fantastic work! (FWIW I ran into the retval argument issue with >> ksnoop as well; I got around it by using "return" to signify the return >> value since as a reserved word it won't clash with a variable name. >> However in the trace subsystem context retval is used extensively so >> makes sense to stick with that). > > Thanks! > >> >> 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@xxxxxxxxx/ > >> >> 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. >> 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! Alan > Thank you, > >> >> Thanks! >> >> Alan >> >> [1] >> https://lore.kernel.org/bpf/1675790102-23037-1-git-send-email-alan.maguire@xxxxxxxxxx/ >> >>> 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@xxxxxxxxxx> >>> > >
