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

From: Jiri Olsa <[email-masked]>
Date: Sat, 9 Apr 2022 22:24:32 +0200

On Fri, Apr 08, 2022 at 04:19:25PM -0700, Alexei Starovoitov wrote:
> On Thu, Apr 07, 2022 at 02:52:21PM +0200, Jiri Olsa 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, u32 cnt,
> >                             unsigned long *addrs)
> > 
> > Before we iterate kallsyms we sort user provided symbols by name and
> > then use that in kalsyms iteration to find each kallsyms symbol in
> > user provided symbols.
> > 
> > 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.
> > 
> > Suggested-by: Andrii Nakryiko <[email-masked]>
> > Signed-off-by: Jiri Olsa <[email-masked]>
> > ---
> >  include/linux/kallsyms.h |  6 +++++
> >  kernel/kallsyms.c        | 48 ++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 54 insertions(+)
> > 
> > diff --git a/include/linux/kallsyms.h b/include/linux/kallsyms.h
> > index ce1bd2fbf23e..5320a5e77f61 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, u32 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;
> >  }
> >  
> > +int kallsyms_lookup_names(const char **syms, u32 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..a3738ddf9e87 100644
> > --- a/kernel/kallsyms.c
> > +++ b/kernel/kallsyms.c
> > @@ -29,6 +29,8 @@
> >  #include <linux/compiler.h>
> >  #include <linux/module.h>
> >  #include <linux/kernel.h>
> > +#include <linux/bsearch.h>
> > +#include <linux/sort.h>
> >  
> >  /*
> >   * These will be re-linked against their real values
> > @@ -572,6 +574,52 @@ int sprint_backtrace_build_id(char *buffer, unsigned long address)
> >  	return __sprint_symbol(buffer, address, -1, 1, 1);
> >  }
> >  
> > +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;
> > +	u32 cnt;
> > +	u32 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;
> > +
> > +	args->addrs[args->found++] = addr;
> > +	return args->found == args->cnt ? 1 : 0;
> > +}
> > +
> > +int kallsyms_lookup_names(const char **syms, u32 cnt, unsigned long *addrs)
> > +{
> > +	struct kallsyms_data args;
> > +
> > +	sort(syms, cnt, sizeof(*syms), symbols_cmp, NULL);
> 
> It's nice to share symbols_cmp for sort and bsearch,
> but messing technically input argument 'syms' like this will cause
> issues sooner or later.
> Lets make caller do the sort.
> Unordered input will cause issue with bsearch, of course,
> but it's a lesser evil. imo.

ok, will move it out and make some proper comment for the
function mentioning the sort requirement for syms

thanks,
jirka

> 
> > +
> > +	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 : -EINVAL;
> > +}
> > +
> >  /* To avoid using get_symbol_offset for every symbol, we carry prefix along. */
> >  struct kallsym_iter {
> >  	loff_t pos;
> > -- 
> > 2.35.1
> >