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

From: Jiri Olsa <[email-masked]>
Date: Sat, 16 Apr 2022 00:39:08 +0200

On Fri, Apr 15, 2022 at 09:47:27AM +0900, Masami Hiramatsu wrote:
> Hi Jiri,
> 
> Sorry for replying later.
> 
> On Tue, 12 Apr 2022 22:46:15 +0200
> Jiri Olsa <[email-masked]> wrote:
> 
> > 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.
> > > 
> > 
> > Masami,
> > this logic bubbles up to the register_fprobe_syms, because user
> > provides symbols as its argument. Can we still force this assumption
> > to the 'syms' array, like with the comment change below?
> > 
> > FYI the bpf side does not use register_fprobe_syms, it uses
> > register_fprobe_ips, because it always needs ips as search
> > base for cookie values
> 
> Hmm, in that case fprobe can call sort() in the register function.
> That will be much easier and safer. The bpf case, the input array will
> be generated by the bpftool (not by manual), so it can ensure the 
> syms is sorted. But we don't know how fprobe user passes syms array.
> Then register_fprobe_syms() will always requires sort(). I don't like
> such redundant requirements.

ok, I'll add it to register_fprobe_syms

thanks,
jirka