Re: [PATCH bpf-next 1/2] bpf: Add a bpf_kallsyms_lookup helper

[Florent Revest <revest@chromium.org>]

On Sat, 2020-11-28 at 17:07 -0800, Alexei Starovoitov wrote:
> On Thu, Nov 26, 2020 at 05:57:47PM +0100, Florent Revest wrote:
> > This helper exposes the kallsyms_lookup function to eBPF tracing
> > programs. This can be used to retrieve the name of the symbol at an
> > address. For example, when hooking into nf_register_net_hook, one
> > can
> > audit the name of the registered netfilter hook and potentially
> > also
> > the name of the module in which the symbol is located.
> > 
> > Signed-off-by: Florent Revest <revest@google.com>
> > ---
> >  include/uapi/linux/bpf.h       | 16 +++++++++++++
> >  kernel/trace/bpf_trace.c       | 41
> > ++++++++++++++++++++++++++++++++++
> >  tools/include/uapi/linux/bpf.h | 16 +++++++++++++
> >  3 files changed, 73 insertions(+)
> > 
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index c3458ec1f30a..670998635eac 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -3817,6 +3817,21 @@ union bpf_attr {
> >   *		The **hash_algo** is returned on success,
> >   *		**-EOPNOTSUP** if IMA is disabled or **-EINVAL** if
> >   *		invalid arguments are passed.
> > + *
> > + * long bpf_kallsyms_lookup(u64 address, char *symbol, u32
> > symbol_size, char *module, u32 module_size)
> > + *	Description
> > + *		Uses kallsyms to write the name of the symbol at
> > *address*
> > + *		into *symbol* of size *symbol_sz*. This is guaranteed
> > to be
> > + *		zero terminated.
> > + *		If the symbol is in a module, up to *module_size* bytes
> > of
> > + *		the module name is written in *module*. This is also
> > + *		guaranteed to be zero-terminated. Note: a module name
> > + *		is always shorter than 64 bytes.
> > + *	Return
> > + *		On success, the strictly positive length of the full
> > symbol
> > + *		name, If this is greater than *symbol_size*, the
> > written
> > + *		symbol is truncated.
> > + *		On error, a negative value.
> 
> Looks like debug-only helper.
> I cannot think of a way to use in production code.
> What program suppose to do with that string?
> Do string compare? BPF side doesn't have a good way to do string
> manipulations.
> If you really need to print a symbolic name for a given address
> I'd rather extend bpf_trace_printk() to support %pS

We actually use this helper for auditing, not debugging.
We don't want to parse /proc/kallsyms from userspace because we have no
guarantee that the module will still be loaded by the time the event
reaches userspace (this is also faster in kernelspace).