Re: [PATCH 3/6] kallsyms/bpf: Set module buildid in bpf_address_lookup()

[Petr Mladek <pmladek@suse.com>]

On Wed 2025-11-05 18:53:23, Alexei Starovoitov wrote:
> On Wed, Nov 5, 2025 at 6:24 AM Petr Mladek <pmladek@suse.com> wrote:
> >
> > Make bpf_address_lookup() compatible with module_address_lookup()
> > and clear the pointer to @modbuildid together with @modname.
> >
> > It is not strictly needed because __sprint_symbol() reads @modbuildid
> > only when @modname is set. But better be on the safe side and make
> > the API more safe.
> >
> > Fixes: 9294523e3768 ("module: add printk formats to add module build ID to stacktraces")
> > Signed-off-by: Petr Mladek <pmladek@suse.com>
> > ---
> >  include/linux/filter.h | 15 +++++++++++----
> >  kernel/kallsyms.c      |  4 ++--
> >  2 files changed, 13 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/filter.h b/include/linux/filter.h
> > index f5c859b8131a..b7b95840250a 100644
> > --- a/include/linux/filter.h
> > +++ b/include/linux/filter.h
> > @@ -1362,12 +1362,18 @@ struct bpf_prog *bpf_prog_ksym_find(unsigned long addr);
> >
> >  static inline int
> >  bpf_address_lookup(unsigned long addr, unsigned long *size,
> > -                  unsigned long *off, char **modname, char *sym)
> > +                  unsigned long *off, char **modname,
> > +                  const unsigned char **modbuildid, char *sym)
> >  {
> >         int ret = __bpf_address_lookup(addr, size, off, sym);
> >
> > -       if (ret && modname)
> > -               *modname = NULL;
> > +       if (ret) {
> > +               if (modname)
> > +                       *modname = NULL;
> > +               if (modbuildid)
> > +                       *modbuildid = NULL;
> > +       }
> > +
> >         return ret;
> >  }
> >
> > @@ -1433,7 +1439,8 @@ static inline struct bpf_prog *bpf_prog_ksym_find(unsigned long addr)
> >
> >  static inline int
> >  bpf_address_lookup(unsigned long addr, unsigned long *size,
> > -                  unsigned long *off, char **modname, char *sym)
> > +                  unsigned long *off, char **modname,
> > +                  const unsigned char **modbuildid, char *sym)
> >  {
> >         return 0;
> >  }
> > diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
> > index 9455e3bb07fc..efb12b077220 100644
> > --- a/kernel/kallsyms.c
> > +++ b/kernel/kallsyms.c
> > @@ -374,8 +374,8 @@ static int kallsyms_lookup_buildid(unsigned long addr,
> >         ret = module_address_lookup(addr, symbolsize, offset,
> >                                     modname, modbuildid, namebuf);
> >         if (!ret)
> > -               ret = bpf_address_lookup(addr, symbolsize,
> > -                                        offset, modname, namebuf);
> > +               ret = bpf_address_lookup(addr, symbolsize, offset,
> > +                                        modname, modbuildid, namebuf);
> 
> The initial bpf_address_lookup() 8 years ago was trying
> to copy paste args and style of kallsyms_lookup().
> It was odd back then. This change is doubling down on the wrong thing.
> It's really odd to pass a pointer into bpf_address_lookup()
> so it zero initializes it.
> bpf ksyms are in the core kernel. They're never in modules.
> Just call __bpf_address_lookup() here and remove the wrapper.

I agree that it is ugly. It would make sense to initialize the
pointers in kallsyms_lookup_buildid and call there
__bpf_address_lookup() variant. Something like:

static int kallsyms_lookup_buildid(unsigned long addr,
			unsigned long *symbolsize,
			unsigned long *offset, char **modname,
			const unsigned char **modbuildid, char *namebuf)
{
	int ret;

	if (modname)
		*modname = NULL;
	if (modbuildid)
		*modbuildid = NULL;
	namebuf[0] = 0;
[...]
	if (!ret)
		ret = __bpf_address_lookup(addr, symbolsize, offset, namebuf);

}

As a result bpf_address_lookup() won't have any caller.
And __bpf_address_lookup() would have two callers.

It would make sense to remove bpf_address_lookup() and
rename __bpf_address_lookup() to bpf_address_lookup().

How does that sound?
Would you prefer to do this in one patch or in two steps, please?

Best Regards,
Petr