Re: [PATCH v5 bpf-next 3/5] libbpf: add auto-attach for uprobes based on section name

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Sun, 2022-04-03 at 21:46 -0700, Andrii Nakryiko wrote:
> On Sun, Apr 3, 2022 at 6:14 PM Andrii Nakryiko
> <andrii.nakryiko@gmail.com> wrote:
> > 
> > On Wed, Mar 30, 2022 at 8:27 AM Alan Maguire
> > <alan.maguire@oracle.com> wrote:
> > > 
> > > Now that u[ret]probes can use name-based specification, it makes
> > > sense to add support for auto-attach based on SEC() definition.
> > > The format proposed is
> > > 
> > >        
> > > SEC("u[ret]probe/binary:[raw_offset|[function_name[+offset]]")
> > > 
> > > For example, to trace malloc() in libc:
> > > 
> > >         SEC("uprobe/libc.so.6:malloc")
> > > 
> > > ...or to trace function foo2 in /usr/bin/foo:
> > > 
> > >         SEC("uprobe//usr/bin/foo:foo2")
> > > 
> > > Auto-attach is done for all tasks (pid -1).  prog can be an
> > > absolute
> > > path or simply a program/library name; in the latter case, we use
> > > PATH/LD_LIBRARY_PATH to resolve the full path, falling back to
> > > standard locations (/usr/bin:/usr/sbin or /usr/lib64:/usr/lib) if
> > > the file is not found via environment-variable specified
> > > locations.
> > > 
> > > Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> > > ---
> > >  tools/lib/bpf/libbpf.c | 74
> > > ++++++++++++++++++++++++++++++++++++++++++++++++--
> > >  1 file changed, 72 insertions(+), 2 deletions(-)
> > > 
> > 
> > [...]
> > 
> > > +static int attach_uprobe(const struct bpf_program *prog, long
> > > cookie, struct bpf_link **link)
> > > +{
> > > +       DECLARE_LIBBPF_OPTS(bpf_uprobe_opts, opts);
> > > +       char *func, *probe_name, *func_end;
> > > +       char *func_name, binary_path[512];
> > > +       unsigned long long raw_offset;
> > > +       size_t offset = 0;
> > > +       int n;
> > > +
> > > +       *link = NULL;
> > > +
> > > +       opts.retprobe = str_has_pfx(prog->sec_name,
> > > "uretprobe/");
> > > +       if (opts.retprobe)
> > > +               probe_name = prog->sec_name +
> > > sizeof("uretprobe/") - 1;
> > > +       else
> > > +               probe_name = prog->sec_name + sizeof("uprobe/") -
> > > 1;
> > 
> > I think this will mishandle SEC("uretprobe"), let's fix this in a
> > follow up (and see a note about uretprobe selftests)
> 
> So I actually fixed it up a little bit to avoid test failure on s390x
> arch. But now it's a different problem, complaining about not being
> able to resolve libc.so.6. CC'ing Ilya, but I was wondering if it's
> better to use more generic "libc.so" instead of "libc.so.6"? Have you
> tried that?

I believe it's a Debian-specific issue (our s390x CI image is Debian).
libc is still called libc.so.6, but it's located in
/lib/s390x-linux-gnu.
This must also be an issue on Intel and other architectures.
I'll send a patch.