Message-ID: <CAEf4BzYfE0Jo7E4-Tadkgx1P=67H01E90zGJpFzFyuzXGy_PnQ@mail.gmail.com>
Date:   Mon, 28 Feb 2022 17:45:15 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alan Maguire <alan.maguire@...cle.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin Lau <kafai@...com>, Song Liu <songliubraving@...com>,
        Yonghong Song <yhs@...com>,
        john fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
        Yucong Sun <sunyucong@...il.com>,
        Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH v3 bpf-next 2/4] libbpf: add auto-attach for uprobes based
 on section name

On Thu, Feb 24, 2022 at 7:40 AM Alan Maguire <alan.maguire@...cle.com> wrote:
>
> On Thu, 24 Feb 2022, Andrii Nakryiko wrote:
>
> > On Wed, Feb 23, 2022 at 1:33 AM Alan Maguire <alan.maguire@...cle.com> wrote:
> > >
> > > On Fri, 4 Feb 2022, Andrii Nakryiko wrote:
> > >
> > > > On Mon, Jan 31, 2022 at 8:13 AM Alan Maguire <alan.maguire@...cle.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?

What do I think about dlopen()'ing some random library under root by
libbpf into the host process?.. I'd say that's a bad idea.

I'd probably start with just checking /lib, /usr/lib (and maybe those
32-bit and 64-bit specific ones, depending on host architecture; not
sure about all the details there, tbh). Or just say that the path to
the shared library has to be specified.

There is a similar problem with doing something like
SEC("uprobe/bash:readline"). Do we want to "search" for bash? I think
bpftrace is supporting that, but I haven't checked what it is doing.


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

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