linux-kernel - Re: [PATCH] perf symbol: ignore $a/$d symbols for ARM modules

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Date:   Wed, 27 Oct 2021 11:23:57 +0100
From:   James Clark <james.clark@....com>
To:     Lexi Shao <shaolexi@...wei.com>, linux-perf-users@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     acme@...nel.org, mark.rutland@....com, peterz@...radead.org,
        mingo@...hat.com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, qiuxi1@...wei.com,
        nixiaoming@...wei.com, wangbing6@...wei.com
Subject: Re: [PATCH] perf symbol: ignore $a/$d symbols for ARM modules



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

> 
> 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@...wei.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
>