Re: [PATCHv2 bpf-next 1/4] kallsyms: Add kallsyms_lookup_names function

[Masami Hiramatsu <mhiramat@kernel.org>]

On Mon, 18 Apr 2022 14:48:31 +0200
Jiri Olsa <jolsa@kernel.org> wrote:

> Adding kallsyms_lookup_names function that resolves array of symbols
> with single pass over kallsyms.
> 
> The user provides array of string pointers with count and pointer to
> allocated array for resolved values.
> 
>   int kallsyms_lookup_names(const char **syms, size_t cnt,
>                             unsigned long *addrs)

What about renaming the 'syms' argument to 'sorted_syms' so that user
is easily notice what is required?
Or renaming the function as kallsyms_lookup_sorted_names()?


> 
> It iterates all kalsyms symbols and tries to loop up each in provided
> symbols array with bsearch. The symbols array needs to be sorted by
> name for this reason.
> 
> We also check each symbol to pass ftrace_location, because this API
> will be used for fprobe symbols resolving. This can be optional in
> future if there's a need.
> 
> We need kallsyms_on_each_symbol function, so enabling it and also
> the new function for CONFIG_FPROBE option.
> 
> Suggested-by: Andrii Nakryiko <andrii@kernel.org>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>
> ---
>  include/linux/kallsyms.h |  6 ++++
>  kernel/kallsyms.c        | 70 +++++++++++++++++++++++++++++++++++++++-
>  2 files changed, 75 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/kallsyms.h b/include/linux/kallsyms.h
> index ce1bd2fbf23e..7c82fa7445d4 100644
> --- a/include/linux/kallsyms.h
> +++ b/include/linux/kallsyms.h
> @@ -72,6 +72,7 @@ int kallsyms_on_each_symbol(int (*fn)(void *, const char *, struct module *,
>  #ifdef CONFIG_KALLSYMS
>  /* Lookup the address for a symbol. Returns 0 if not found. */
>  unsigned long kallsyms_lookup_name(const char *name);
> +int kallsyms_lookup_names(const char **syms, size_t cnt, unsigned long *addrs);
>  
>  extern int kallsyms_lookup_size_offset(unsigned long addr,
>  				  unsigned long *symbolsize,
> @@ -103,6 +104,11 @@ static inline unsigned long kallsyms_lookup_name(const char *name)
>  	return 0;
>  }
>  
> +static inline int kallsyms_lookup_names(const char **syms, size_t cnt, unsigned long *addrs)
> +{
> +	return -ERANGE;
> +}
> +
>  static inline int kallsyms_lookup_size_offset(unsigned long addr,
>  					      unsigned long *symbolsize,
>  					      unsigned long *offset)
> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> index 79f2eb617a62..ef940b25f3fc 100644
> --- a/kernel/kallsyms.c
> +++ b/kernel/kallsyms.c
> @@ -29,6 +29,7 @@
>  #include <linux/compiler.h>
>  #include <linux/module.h>
>  #include <linux/kernel.h>
> +#include <linux/bsearch.h>
>  
>  /*
>   * These will be re-linked against their real values
> @@ -228,7 +229,7 @@ unsigned long kallsyms_lookup_name(const char *name)
>  	return module_kallsyms_lookup_name(name);
>  }
>  
> -#ifdef CONFIG_LIVEPATCH
> +#if defined(CONFIG_LIVEPATCH) || defined(CONFIG_FPROBE)
>  /*
>   * Iterate over all symbols in vmlinux.  For symbols from modules use
>   * module_kallsyms_on_each_symbol instead.
> @@ -572,6 +573,73 @@ int sprint_backtrace_build_id(char *buffer, unsigned long address)
>  	return __sprint_symbol(buffer, address, -1, 1, 1);
>  }
>  
> +#ifdef CONFIG_FPROBE
> +static int symbols_cmp(const void *a, const void *b)
> +{
> +	const char **str_a = (const char **) a;
> +	const char **str_b = (const char **) b;
> +
> +	return strcmp(*str_a, *str_b);
> +}
> +
> +struct kallsyms_data {
> +	unsigned long *addrs;
> +	const char **syms;
> +	size_t cnt;
> +	size_t found;
> +};
> +
> +static int kallsyms_callback(void *data, const char *name,
> +			     struct module *mod, unsigned long addr)
> +{
> +	struct kallsyms_data *args = data;
> +
> +	if (!bsearch(&name, args->syms, args->cnt, sizeof(*args->syms), symbols_cmp))
> +		return 0;
> +
> +	addr = ftrace_location(addr);
> +	if (!addr)
> +		return 0;

Ooops, wait. Did you do this last version? I missed this point.
This changes the meanings of the kernel function.

> +
> +	args->addrs[args->found++] = addr;
> +	return args->found == args->cnt ? 1 : 0;
> +}
> +
> +/**
> + * kallsyms_lookup_names - Lookup addresses for array of symbols

More correctly "Lookup 'ftraced' addresses for array of sorted symbols", right?

I'm not sure, we can call it as a 'kallsyms' API, since this is using
kallsyms but doesn't return symbol address, but ftrace address.
I think this name misleads user to expect returning symbol address.

> + *
> + * @syms: array of symbols pointers symbols to resolve, must be
> + * alphabetically sorted
> + * @cnt: number of symbols/addresses in @syms/@addrs arrays
> + * @addrs: array for storing resulting addresses
> + *
> + * This function looks up addresses for array of symbols provided in
> + * @syms array (must be alphabetically sorted) and stores them in
> + * @addrs array, which needs to be big enough to store at least @cnt
> + * addresses.

Hmm, sorry I changed my mind. I rather like to expose kallsyms_on_each_symbol()
and provide this API from fprobe or ftrace, because this returns ftrace address
and thus this is only used from fprobe.

Thank you,

> + *
> + * This function returns 0 if all provided symbols are found,
> + * -ESRCH otherwise.
> + */
> +int kallsyms_lookup_names(const char **syms, size_t cnt, unsigned long *addrs)
> +{
> +	struct kallsyms_data args;
> +
> +	args.addrs = addrs;
> +	args.syms = syms;
> +	args.cnt = cnt;
> +	args.found = 0;
> +	kallsyms_on_each_symbol(kallsyms_callback, &args);
> +
> +	return args.found == args.cnt ? 0 : -ESRCH;
> +}
> +#else
> +int kallsyms_lookup_names(const char **syms, size_t cnt, unsigned long *addrs)
> +{
> +	return -ERANGE;
> +}
> +#endif /* CONFIG_FPROBE */
> +
>  /* To avoid using get_symbol_offset for every symbol, we carry prefix along. */
>  struct kallsym_iter {
>  	loff_t pos;
> -- 
> 2.35.1
> 


-- 
Masami Hiramatsu <mhiramat@kernel.org>