Implement dladdr

[kadler]

dladdr is a GNU extension to the common dl family of functions: dlopen, dlsym, etc.

It has been implemented on many other operating systems:

• Solaris
• FreeBSD
• macOS
• HP-UX

It seems like the FreeBSD and macOS implementations are ports of or just compatible with the Solaris version.

One thing we need to think about is what to return for the dli_fname? Do we include the member name (if any) or just the file path. A given path that includes the member in the /path/to/lib(member.so) syntax is a valid file path, so that could be confusing, however it would be unique to AIX/PASE. The HP-UX version at least has defined system-specific fields, so I think we can follow them.

[kadler]

Alternatively, the function is defined to return 0 on error and a "non-zero" value on success. We could use any non-zero value for success, which means we could return whether dli_fname contains a member or not.

The bigger issue is where do we store these fields? Both dli_fname and dli_sname fields are not allocated by dladdr on any system I've tested and do not need to be freed by the caller.

One possibility is to add some internal buffers to the Dl_info struct to store them, then make the pointers point to the buffers as necessary. Of course, this means we need to determine ahead of time what's the maximum length these could be. For dli_fname, PATH_MAX might be a good suggestion, but then there's the member name (so PATH_MAX * 2 + 2?). What's the longest symbol name possible, though?

EDIT: According to the AIX docs the symbols are stored in string table entries, which contain a 4-byte length of the symbol name. Since this includes the length of the length field too, it would be 4 GiB - 4 bytes (or 2 GiB - 4 bytes if signed). That's the theoretical max, but that would likely cause lots of other problems since XCOFF offsets are only 4 bytes long so the max file size is 4 GiB total.

[NattyNarwhal]

Whenever I implemented hacky dladdrs, I special-cased the AIX impl to free too, because dladdr was being wrapped for some other purpose. Storing it in the structure would probably be better, but I'd still be concerned about lifetimes. I also did libfoo.a(bar.o) for the library names.

One thing I considered but couldn't do was frob the shared library loader, usla, since it almost certainly has a copy of the data you'd need and could just point to it. No idea where to start with that, but you have the private knowledge to know if it'd be possible or if it'd be a good idea. And of course, speaking of things you can do I can't, you could put a Qp2dladdr in 7.5 👀

[kadler]

Whenever I implemented hacky dladdrs, I special-cased the AIX impl to free too, because dladdr was being wrapped for some other purpose.

That's what OpenSSL's wrapper does: https://github.com/openssl/openssl/blob/38fc02a7084438e384e152effa84d4bf085783c9/crypto/dso/dso_dlfcn.c#L425-L427

And of course, speaking of things you can do I can't, you could put a Qp2dladdr

Yeah, that might make more sense.