Re: [PATCH v3 bpf-next 2/4] libbpf: add auto-attach for uprobes based on section name

[Alan Maguire <alan.maguire@oracle.com>]

On Thu, 24 Feb 2022, Andrii Nakryiko wrote:

> On Wed, Feb 23, 2022 at 1:33 AM Alan Maguire <alan.maguire@oracle.com> wrote:
> >
> > On Fri, 4 Feb 2022, Andrii Nakryiko wrote:
> >
> > > On Mon, Jan 31, 2022 at 8:13 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//path/to/prog:[raw_offset|[function_name[+offset]]")
> > > >
> > > > For example, to trace malloc() in libc:
> > > >
> > > >         SEC("uprobe//usr/lib64/libc.so.6:malloc")
> > >
> > > I assume that path to library can be relative path as well, right?
> > >
> > > Also, should be look at trying to locate library in the system if it's
> > > specified as "libc"? Or if the binary is "bash", for example. Just
> > > bringing this up, because I think it came up before in the context of
> > > one of libbpf-tools.
> > >
> >
> > This is a great suggestion for usability, but I'm trying to puzzle
> > out how to carry out the location search for cases where the path
> > specified is not a relative or absolute path.
> >
> > A few things we can can do - use search paths from PATH and
> > LD_LIBRARY_PATH, with an appended set of standard locations
> > such as /usr/bin, /usr/sbin for cases where those environment
> > variables are missing or incomplete.
> >
> > However, when it comes to libraries, do we search in /usr/lib64 or
> > /usr/lib? We could use whether the version of libbpf is 64-bit or not I
> > suppose, but it's at least conceivable that the user might want to
> > instrument a 32-bit library from a 64-bit libbpf.  Do you think that
> > approach is sufficient, or are there other things we should do? Thanks!
> 
> How does dynamic linker do this? When I specify "libbpf.so", is there
> some documented algorithm for finding the library? If it's more or
> less codified, we could implement something like that. If not, well,
> too bad, we can do some useful heuristic, but ultimately there will be
> cases that won't be supported. Worst case user will have to specify an
> absolute path.
> 

There's a nice description in [1]:

       If filename is NULL, then the returned handle is for the main
       program.  If filename contains a slash ("/"), then it is
       interpreted as a (relative or absolute) pathname.  Otherwise, the
       dynamic linker searches for the object as follows (see ld.so(8)
       for further details):

       o   (ELF only) If the calling object (i.e., the shared library or
           executable from which dlopen() is called) contains a DT_RPATH
           tag, and does not contain a DT_RUNPATH tag, then the
           directories listed in the DT_RPATH tag are searched.

       o   If, at the time that the program was started, the environment
           variable LD_LIBRARY_PATH was defined to contain a colon-
           separated list of directories, then these are searched.  (As
           a security measure, this variable is ignored for set-user-ID
           and set-group-ID programs.)

       o   (ELF only) If the calling object contains a DT_RUNPATH tag,
           then the directories listed in that tag are searched.

       o   The cache file /etc/ld.so.cache (maintained by ldconfig(8))
           is checked to see whether it contains an entry for filename.

       o   The directories /lib and /usr/lib are searched (in that
           order).

Rather than re-inventing all of that however, we could use it
by dlopen()ing the file when it is a library (contains .so) and
is not a relative/absolute path, and then use dlinfo()'s
RTLD_DI_ORIGIN command to extract the path discovered, and then
dlclose() it. It would require linking libbpf with -ldl however.
What do you think?

Alan
 
[1] https://man7.org/linux/man-pages/man3/dlopen.3.html

> >
> > Alan
>