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ID Category Severity Type Date Submitted Last Update
0000993 [1003.1(2008)/Issue 7] Base Definitions and Headers Editorial Clarification Requested 2015-10-18 10:22 2016-07-21 15:23
Reporter EdSchouten View Status public  
Assigned To ajosey
Priority normal Resolution Open  
Status Under Review  
Name Ed Schouten
Organization Nuxi
User Reference
Section dlfcn.h
Page Number n/a
Line Number n/a
Interp Status ---
Final Accepted Text
Summary 0000993: Standardizing dladdr()
Description Though POSIX already standardizes the <dlfcn.h> functions to load shared libraries at runtime, it does not standardize the dladdr() function. My observation is that this function is used in a couple of important places, to generate backtraces or to improve error messages. It seems to be implemented universally as far as I know, always using the same prototype:

typedef struct {
  const char *dli_fname; // Pathname of shared object.
  void *dli_fbase; // Base address of shared object.
  const char *dli_sname; // Name of nearest symbol.
  void *dli_saddr; // Address of nearest symbol.
} Dl_info;

int dladdr(const void *, Dl_info *);
Desired Action Due to its universal adoption, would it make sense to standardize it?
Tags No tags attached.
Attached Files

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-  Notes
(0002882)
wahern (reporter)
2015-11-04 20:01

It's missing completely on AIX.

I've only ever used the dli_fname member (from modules which attempt to reopen and pin themselves in memory). FWIW, that at least appears to work the same from FreeBSD, Linux/glibc, Linux/musl, NetBSD, OpenBSD, OS X, and Solaris.
(0002883)
shware_systems (reporter)
2015-11-05 14:00

Also FWIW, while a function like this may make writing a generic debugger easier, as a legitimate purpose, its primary use in an application that I can see would be for reverse engineering a module that's been obfuscated and as such it constitutes a security hole. If it were to be included in the standard as a formalization of common practice I would hope it be as part of an option group, not a candidate for the base set of interfaces, and visible only when NDEBUG is defined similar to the assert macros.
(0002886)
EdSchouten (updater)
2015-11-05 18:17

I'm not sure I agree with this reasoning. The information that is returned by dladdr() is in many cases already public knowledge.

- You can also extract this information outside of the process using the nm(1) utility.
- Even if your system does not provide this function, but does provide the dl_iterate_phdr() function (present on many systems that use ELF), you can just search for the .dynsym section and parse the symbol entries manually.
(0002891)
dalias (reporter)
2015-11-06 16:33

In regard to comment 2883, the only security model in the standard is the user/group system. While additional security models are permitted (see "appropriate privileges" etc.), anything "DRM-like" is not a security measure by any stretch of the word and there is no precedent for attempting to offer such snake oil. However the whole issue is irrelevant to dladdr. An implementation wanting to attempt such futile measures could simply have dladdr fail for such modules. (Then, the user need only patch the module to make it start working again, or implement their own unencumbered but nonportable dladdr that only works on the particular implementation they need it on.)

In regard to comment 2886, dl_iterate_phdr is also not provided in the standard and thus not a substitute for dladdr. Even if it were provided, however, there are implementations in which dl_iterate_phdr does not provide sufficient information to implement dladdr on top of it. In particular, for any implementation using function descriptors that are dynamically allocated (ELF/FDPIC ABIs), the headers lack sufficient information to determine the identity of a function pointer; additional runtime state known only to the dynamic linker is needed.
(0003236)
nico (reporter)
2016-05-27 16:07

dladdr() is extremely useful, and no, nm(1) and other such tools don't help.

Suppose that you have a library for parsing configuration files where you want to open a configuration file by relative path -- relative to the *caller*'s object. First, you need the return address (and since there's no portable way to get, an address given by the caller). Next you call dladdr(), obtain the caller's object (or executable) path, and now you can find the absolute path given that configuration file relative path. There is no portable way to do this without dladdr() or similar because the caller may not know its path due to its being intended to be relocatable!

Making applications relocatable (that is, not tied to an absolute install path) is difficult. dladdr(), along with $ORIGIN, helps a lot.

POSIX should provide more interfaces to make it possible to write relocatable code.

There are other uses of dladdr(), naturally. Reflection/introspection, for example, especially in codebases that pass around function pointer tables obtained or filled with dlsym() on objects loaded with dlopen() -- this is a case where the information is available in other ways, but where arranging to make that available conveniently may not be easy without much refactoring.

I can probably think of other uses of dladdr(). Making relocation easier seems quite compelling to me.
(0003237)
nico (reporter)
2016-05-27 16:13

The hoops that we jump through to make code relocatable! Anyone who has had to use libtool has some idea (e.g., abuse LD_LIBRARY_PATH in order to run built-but-not-yet-installed artifacts). There is also Conda (which binary-edits install paths in build artifacts at install time). There are libraries to help find files by path relative to the application's install path (which can generally be determined via getauxval() or, failing that, main()'s argv[0]), but doing so relative to *objects* (as opposed to the executable) really does require dladdr().
(0003300)
Don Cragun (manager)
2016-07-21 15:23

We will ask The Open Group if they would like to sponsor this additional interface.

In the future, it would be nice if requests like this included text to be included in the standard, rather than hoping a sponsor organization will not only approve the suggestion but also take the additional effort to create all of the needed documentation.

- Issue History
Date Modified Username Field Change
2015-10-18 10:22 EdSchouten New Issue
2015-10-18 10:22 EdSchouten Status New => Under Review
2015-10-18 10:22 EdSchouten Assigned To => ajosey
2015-10-18 10:22 EdSchouten Name => Ed Schouten
2015-10-18 10:22 EdSchouten Organization => Nuxi
2015-10-18 10:22 EdSchouten Section => dlfcn.h
2015-10-18 10:22 EdSchouten Page Number => n/a
2015-10-18 10:22 EdSchouten Line Number => n/a
2015-11-04 20:01 wahern Note Added: 0002882
2015-11-05 14:00 shware_systems Note Added: 0002883
2015-11-05 18:17 EdSchouten Note Added: 0002886
2015-11-06 16:33 dalias Note Added: 0002891
2016-05-27 16:07 nico Note Added: 0003236
2016-05-27 16:13 nico Note Added: 0003237
2016-07-21 15:23 Don Cragun Note Added: 0003300
2016-09-14 14:24 emaste Issue Monitored: emaste


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