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

[Jiri Olsa <olsajiri@gmail.com>]

On Mon, Apr 18, 2022 at 11:35:46PM +0900, Masami Hiramatsu wrote:
> 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.

yes, it was there before ;-) and you're right.. so some archs can
return different address, I did not realize that

> 
> > +
> > +	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.

ok, so how about:

  int ftrace_lookup_symbols(const char **sorted_syms, size_t cnt, unsigned long *addrs);


thanks,
jirka