RE: [PATCH v4] gdb, gdbserver: support dlmopen()

["Metzger, Markus T" <markus.t.metzger@intel.com>]

Thanks Kevin and Ben, for your positive feedback on the usefulness of this.

The patch has grown into a small series when I added gdbserver support
and changed several occurrences of direct objfiles traversal with calls to
gdbarch_iterate_over_objfiles_in_search_order() to take namespaces into
account.

The latest version can be found in users/mmetzger/dlmopen.  I'll also send
it as v5 with a few opens left.

>While I'm convinced that other work will be needed to improve GDB's UI
>to both display linker namespaces (e.g.  in the "info shared" command)
>and accept namespace qualifiers when specifying a symbol (e.g. with a
>breakpoint command), I think that this current patch is useful as
>is.  I.e., I'd like to see it (or a modest update) go in as soon as
>possible.

Indeed, there are a few known issues and many direct objfiles traversals
in GDB that all assume that there can be at most one global symbol
of the same name.  Fixing all of them will be a fair amount of work.

Ben Woodard:
>How do I specify the filename.c TU which is part of the libstdc++.so which is
>linked into one of the audit libraries rather than the one that is linked into the
>app. Path to the filename isn't sufficient because they could have both been built
>in the same directory structure from different sources. That is why I believe that
>we need some way to know which shared objects are in which namespace and
>some way to specify which linkage namespace to search.
>
>Since the linkage namespaces are on a linked list in glibc, I thought referring to
>the default namespace as 0 and counting the depth and then reusing the C++
>namespace qualifier might be a good idea but it is not something that I am hung
>up on. If someone has a better idea, great.

I currently keep namespaces hidden and local to SVr4.  The rest of GDB doesn't
know about them.

The official namespace id that dlinfo(RTLD_DI_LMID) provides is available for
dynamic linker notifications but not during the (initial) load map scan - or I
haven't found it.  I use the address of the r_debug(_ext) object as identifier.

We could simply number them, as Ben suggests, using the ordinal in the
r_debug chain.  This may be confusing to users who are aware of the actual
namespace id, though.

To make GDB aware of namespaces, we'd probably want to partition
solibs and objfiles by namespace.  This will be a big patch when we change
program_space::objfiles() to take a namespace id argument and update all
users.

Ideally, I think we'd want objfiles to be shared so we only read the debug
info once per unique instance and relocate it lazily but GDB is currently not
doing that and it doesn't sound entirely trivial.

regards,
markus.
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