Messages in this thread | | | Subject | Re: [PATCH] perf symbol: ignore $a/$d symbols for ARM modules | From | James Clark <> | Date | Thu, 28 Oct 2021 09:42:55 +0100 |
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On 28/10/2021 03:05, Lexi Shao wrote: > On 27/10/2021 23:10, James Clark wrote: >> On 27/10/2021 13:31, Lexi Shao wrote: >>> On 27/10/2021 18:24, James Clark wrote: >>>> On 27/10/2021 10:52, Lexi Shao wrote: >>>>> On ARM machine, kernel symbols from modules can be resolved to $a >>>>> instead of printing the actual symbol name. Ignore symbols starting with >>>>> "$" when building kallsyms rbtree. >>>>> >>>>> A sample stacktrace is shown as follows: >>>>> >>>>> c0f2e39c schedule_hrtimeout+0x14 ([kernel.kallsyms]) >>>>> bf4a66d8 $a+0x78 ([test_module]) >>>>> c0a4f5f4 kthread+0x15c ([kernel.kallsyms]) >>>>> c0a001f8 ret_from_fork+0x14 ([kernel.kallsyms]) >>>>> >>>>> On ARM machine, $a/$d symbols are used by the compiler to mark the >>>>> beginning of code/data part in code section. These symbols are filtered >>>>> out when linking vmlinux(see scripts/kallsyms.c ignored_prefixes), but >>>>> are left on modules. So there are $a symbols in /proc/kallsyms which >>>>> share the same addresses with the actual module symbols and confuses perf >>>>> when resolving symbols. >>>> >>>> Hi Lexi, >>>> >>>> Is it worth using or re-implementing the entire is_ignored_symbol() function >>> >from scripts/kallsyms.c? It seems like this change only fixes one occurrence, >>>> but is_ignored_symbol() has a big list of other cases. >>>> >>>> Unless those cases are different? >>>> >>>> Thanks >>>> James >>> >>> Hi James, >>> >>> I don't think it's necessary to cover all the cases listed in >>> is_ignored_symbol(). As long as the symbols are unique and not overlapping >>> with each other, they should't cause problems in resolving symbols. So most >>> of the cases in is_ignored_symbol() should be irrelevant. >> >> Is it possible to do the filtering of the module symbols somewhere else like >> in kernel/kallsyms.c? I'm not that familiar with it but it seems like >> at one point when populating kallsyms '$...' functions are filtered out, but >> when modules are loaded the symbols are not filtered because another code path is >> used? >> >> If kallsyms has some duplicate addresses in there then isn't the bug in kallsyms >> rather than perf? And another tool could get confused too. >> >> James > > Yes we can filter these symbols out when adding module symbols to the kallsyms > list. I will send another patch that modifies module.c to ignore '$' symbols > when loading module. >
That sounds great thanks.
> I think it's worth applying this patch in the sense that perf userspace tool > can be used on all 5.x kernels, and people don't have to update the kernel > to fix it.
Yes you are right. In that case I will add a review tag to this change.
Thanks James
> > Lexi > >> >>> >>> Lexi >>> >>>> >>>>> >>>>> After this patch, the module symbol name is printed: >>>>> >>>>> c0f2e39c schedule_hrtimeout+0x14 ([kernel.kallsyms]) >>>>> bf4a66d8 test_func+0x78 ([test_module]) >>>>> c0a4f5f4 kthread+0x15c ([kernel.kallsyms]) >>>>> c0a001f8 ret_from_fork+0x14 ([kernel.kallsyms]) >>>>> >>>>> Signed-off-by: Lexi Shao <shaolexi@huawei.com> >>>>> --- >>>>> tools/perf/util/symbol.c | 4 ++++ >>>>> 1 file changed, 4 insertions(+) >>>>> >>>>> diff --git a/tools/perf/util/symbol.c b/tools/perf/util/symbol.c >>>>> index 0fc9a5410739..35116aed74eb 100644 >>>>> --- a/tools/perf/util/symbol.c >>>>> +++ b/tools/perf/util/symbol.c >>>>> @@ -702,6 +702,10 @@ static int map__process_kallsym_symbol(void *arg, const char *name, >>>>> if (!symbol_type__filter(type)) >>>>> return 0; >>>>> >>>>> + /* Ignore local symbols for ARM modules */ >>>>> + if (name[0] == '$') >>>>> + return 0; >>>>> + >>>>> /* >>>>> * module symbols are not sorted so we add all >>>>> * symbols, setting length to 0, and rely on >>>>> >>>> >
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