Android 警告"unused DT entry“ 解析

xiaoxiao2021-02-28  2

What are "unused DT entry" errors?

If you have reached this page, it's probably because you have compiled or attempted to run some binaries on your ARM based Android system, with the result that your binary/app crashes or generates a lot of warnings in your logcat. Typically something like this:

WARNING: linker: /blahblah/ unused DT entry: type 0x6ffffffe arg 0x1188

Q: What is a "DT entry"?

In a few words, they are descriptive array entries in the file structure of an ELF file. Specifically they are known as Dynamic Array Tags and are requirements for executable and shared objects. However, not all entries are required or available, depending on the processor and kernel architecture.

In our case we are faced with a "Warning" that one of these are "unused". What that means is, that your executable or library (*.so) files has been compiled with the DT entry indicated, but your kernel is not supporting that entry, for various reasons. The best examples are found on ARM based Android systems, where the system library paths are fixed and the cross compilers used for your firmware (OS/kernel) are set not to use these entries. Usually the binaries still run just fine, but the kernel is flagging this warning every time you're using it.

Q: When does this happen?

This can happen when:

Your ARM kernel is cross-compiled using the wrong flags (usually meant for other processor architectures).Your ARM binaries and libraries are cross-compiled using AOS deprecated compilation flags.and probably other ways yet to be discovered..

Starting from 5.1 (API 22) the Android linker warns about the VERNEED and VERNEEDNUM ELF dynamic sections.

The most common flags that cause this error on Android devices are:

DT_RPATH 0x0f (15) The DT_STRTAB string table offset of a null-terminated library search path string. This element's use has been superseded by DT_RUNPATH. DT_RUNPATH 0x1d (29) The DT_STRTAB string table offset of a null-terminated library search path string. DT_VERNEED 0x6ffffffe The address of the version dependency table. Elements within this table contain indexes into the string table DT_STRTAB. This element requires that the DT_VERNEEDNUM element also be present. DT_VERNEEDNUM 0x6fffffff The number of entries in the DT_VERNEEDNUM table.

Tracking down the error above, we find that this message comes from the bionic librarylinker.cpp:

case DT_VERNEED: verneed_ptr_ = load_bias + d->d_un.d_ptr; break; case DT_VERNEEDNUM: verneed_cnt_ = d->d_un.d_val; break; case DT_RUNPATH: // this is parsed after we have strtab initialized (see below). break; default: if (!relocating_linker) { DL_WARN("\"%s\" unused DT entry: type %p arg %p", get_realpath(), reinterpret_cast<void*>(d->d_tag), reinterpret_cast<void*>(d->d_un.d_val)); } break; }

The code (above) supporting this symbol versioning was committed on April 9, 2015. Thus if your NDK build is either set to support API's earlier than this, or using build tools linking to this earlier library, you will get these warnings.

Q: How do I find what DT entries my system or binaries are using?

There are many ways to do this:

You look into your kernel sources for <linux/elf.h>.You look in your Android NDK installation folders and check: # To find all elf.h files: find /<path_to>/ndk/platforms/android-*/arch-arm*/usr/include/linux/ -iname "elf.h" Do an readelf of your binary: $ readelf --dynamic Dynamic section at offset 0x23b960 contains 28 entries: Tag Type Name/Value 0x00000003 (PLTGOT) 0x23ce18 0x00000002 (PLTRELSZ) 952 (bytes) 0x00000017 (JMPREL) 0x15e70 0x00000014 (PLTREL) REL 0x00000011 (REL) 0x11c8 0x00000012 (RELSZ) 85160 (bytes) 0x00000013 (RELENT) 8 (bytes) 0x6ffffffa (RELCOUNT) 10632 0x00000015 (DEBUG) 0x0 0x00000006 (SYMTAB) 0x148 0x0000000b (SYMENT) 16 (bytes) 0x00000005 (STRTAB) 0x918 0x0000000a (STRSZ) 1011 (bytes) 0x00000004 (HASH) 0xd0c 0x00000001 (NEEDED) Shared library: [] 0x00000001 (NEEDED) Shared library: [] 0x0000001a (FINI_ARRAY) 0x238458 0x0000001c (FINI_ARRAYSZ) 8 (bytes) 0x00000019 (INIT_ARRAY) 0x238460 0x0000001b (INIT_ARRAYSZ) 16 (bytes) 0x00000020 (PREINIT_ARRAY) 0x238470 0x00000021 (PREINIT_ARRAYSZ) 0x8 0x0000001e (FLAGS) BIND_NOW 0x6ffffffb (FLAGS_1) Flags: NOW 0x6ffffff0 (VERSYM) 0x108c 0x6ffffffe (VERNEED) 0x1188 0x6fffffff (VERNEEDNUM) 2 0x00000000 (NULL) 0x0

As you can see from the error above, the type corresponds to DT_VERNEED.

From THIS document:


This element holds the string table offset of a null-terminated search library search path string, discussed in "Shared Object Dependencies." The offset is an index into the table recorded in the DT_STRTAB entry. DT_RPATH may give a string that holds a list of directories, separated by colons (:). All LD_LIBRARY_PATH directories are searched after those from DT_RPATH.

Q: So how do you solve or deal with these issues?

There are essentially 3 ways to deal with this:

the quickthe badthe ugly

The Quick (you don't have any sources or just can't be bothered)

Use an "ELF cleaner" to remove the offending DT entries from a all your binaries. This is an easy and quick remedy, especially when you don't have the sources to recompile them properly for your system. There are at least two cleaners out there that you can use.

The Bad (you have the sources)

Is the right way to do it, because you'll become a bad-ass ARM cross compiler guru in the process of getting it to work. You basically need to find and tune the compiler settings in the Makefiles used.

From here:

The Android linker (/system/bin/linker) does not support RPATH or RUNPATH, so we set LD_LIBRARY_PATH=$USR/lib and try to avoid building useless rpath entries with --disable-rpath configure flags. Another option to avoid depending on LD_LIBRARY_PATH would be supplying a custom linker - this is not done due to the overhead of maintaining a custom linker.

The Ugly (You just want your app to work with any dirty binary.)

You tell your Java app not to freak out when checking for null in error handlers and instead get fed these warnings, possibly causing fatal exceptions. Use something like:

class OpensslErrorThread extends Thread { @Override public void run() { try { while(true){ String line = opensslStderr.readLine(); if(line == null){ // OK return; } if(line.contains("unused DT entry")){ Log.i(TAG, "Ignoring \"unused DT entry\" error from openssl: " + line); } else { // throw exception! break; } } } catch(Exception e) { Log.e(TAG, "Exception!") } } }

This is very bad and ugly as it doesn't solve anything, while bloating your code. In addition, the warnings are there for a reason, and that is that in future AOS versions, this will become a full fledged error!

Q. What else?

Many changes in the API's between 18-25 (J to N) has been made in way the Android kernel and libraries are compiled. I cannot provide a remotely close explanation of all that, but perhaps this will help guide you in the right direction. The best sources is of course looking in the Android sources and documentation itself.

For example, HERE or HERE.

And finally the full list:

Name Value d_un Executable Shared Object --------------------------------------------------------------------------------------------- DT_NULL 0 Ignored Mandatory Mandatory DT_NEEDED 1 d_val Optional Optional DT_PLTRELSZ 2 d_val Optional Optional DT_PLTGOT 3 d_ptr Optional Optional DT_HASH 4 d_ptr Mandatory Mandatory DT_STRTAB 5 d_ptr Mandatory Mandatory DT_SYMTAB 6 d_ptr Mandatory Mandatory DT_RELA 7 d_ptr Mandatory Optional DT_RELASZ 8 d_val Mandatory Optional DT_RELAENT 9 d_val Mandatory Optional DT_STRSZ 0x0a (10) d_val Mandatory Mandatory DT_SYMENT 0x0b (11) d_val Mandatory Mandatory DT_INIT 0x0c (12) d_ptr Optional Optional DT_FINI 0x0d (13) d_ptr Optional Optional DT_SONAME 0x0e (14) d_val Ignored Optional DT_RPATH 0x0f (15) d_val Optional Optional DT_SYMBOLIC 0x10 (16) Ignored Ignored Optional DT_REL 0x11 (17) d_ptr Mandatory Optional DT_RELSZ 0x12 (18) d_val Mandatory Optional DT_RELENT 0x13 (19) d_val Mandatory Optional DT_PLTREL 0x14 (20) d_val Optional Optional DT_DEBUG 0x15 (21) d_ptr Optional Ignored DT_TEXTREL 0x16 (22) Ignored Optional Optional DT_JMPREL 0x17 (23) d_ptr Optional Optional DT_BIND_NOW 0x18 (24) Ignored Optional Optional DT_INIT_ARRAY 0x19 (25) d_ptr Optional Optional DT_FINI_ARRAY 0x1a (26) d_ptr Optional Optional DT_INIT_ARRAYSZ 0x1b (27) d_val Optional Optional DT_FINI_ARRAYSZ 0x1c (28) d_val Optional Optional DT_RUNPATH 0x1d (29) d_val Optional Optional DT_FLAGS 0x1e (30) d_val Optional Optional DT_ENCODING 0x1f (32) Unspecified Unspecified Unspecified DT_PREINIT_ARRAY 0x20 (32) d_ptr Optional Ignored DT_PREINIT_ARRAYSZ 0x21 (33) d_val Optional Ignored DT_MAXPOSTAGS 0x22 (34) Unspecified Unspecified Unspecified DT_LOOS 0x6000000d Unspecified Unspecified Unspecified DT_SUNW_AUXILIARY 0x6000000d d_ptr Unspecified Optional DT_SUNW_RTLDINF 0x6000000e d_ptr Optional Optional DT_SUNW_FILTER 0x6000000e d_ptr Unspecified Optional DT_SUNW_CAP 0x60000010 d_ptr Optional Optional DT_SUNW_SYMTAB 0x60000011 d_ptr Optional Optional DT_SUNW_SYMSZ 0x60000012 d_val Optional Optional DT_SUNW_ENCODING 0x60000013 Unspecified Unspecified Unspecified DT_SUNW_SORTENT 0x60000013 d_val Optional Optional DT_SUNW_SYMSORT 0x60000014 d_ptr Optional Optional DT_SUNW_SYMSORTSZ 0x60000015 d_val Optional Optional DT_SUNW_TLSSORT 0x60000016 d_ptr Optional Optional DT_SUNW_TLSSORTSZ 0x60000017 d_val Optional Optional DT_SUNW_CAPINFO 0x60000018 d_ptr Optional Optional DT_SUNW_STRPAD 0x60000019 d_val Optional Optional DT_SUNW_CAPCHAIN 0x6000001a d_ptr Optional Optional DT_SUNW_LDMACH 0x6000001b d_val Optional Optional DT_SUNW_CAPCHAINENT 0x6000001d d_val Optional Optional DT_SUNW_CAPCHAINSZ 0x6000001f d_val Optional Optional DT_HIOS 0x6ffff000 Unspecified Unspecified Unspecified DT_VALRNGLO 0x6ffffd00 Unspecified Unspecified Unspecified DT_CHECKSUM 0x6ffffdf8 d_val Optional Optional DT_PLTPADSZ 0x6ffffdf9 d_val Optional Optional DT_MOVEENT 0x6ffffdfa d_val Optional Optional DT_MOVESZ 0x6ffffdfb d_val Optional Optional DT_POSFLAG_1 0x6ffffdfd d_val Optional Optional DT_SYMINSZ 0x6ffffdfe d_val Optional Optional DT_SYMINENT 0x6ffffdff d_val Optional Optional DT_VALRNGHI 0x6ffffdff Unspecified Unspecified Unspecified DT_ADDRRNGLO 0x6ffffe00 Unspecified Unspecified Unspecified DT_CONFIG 0x6ffffefa d_ptr Optional Optional DT_DEPAUDIT 0x6ffffefb d_ptr Optional Optional DT_AUDIT 0x6ffffefc d_ptr Optional Optional DT_PLTPAD 0x6ffffefd d_ptr Optional Optional DT_MOVETAB 0x6ffffefe d_ptr Optional Optional DT_SYMINFO 0x6ffffeff d_ptr Optional Optional DT_ADDRRNGHI 0x6ffffeff Unspecified Unspecified Unspecified DT_RELACOUNT 0x6ffffff9 d_val Optional Optional DT_RELCOUNT 0x6ffffffa d_val Optional Optional DT_FLAGS_1 0x6ffffffb d_val Optional Optional DT_VERDEF 0x6ffffffc d_ptr Optional Optional DT_VERDEFNUM 0x6ffffffd d_val Optional Optional DT_VERNEED 0x6ffffffe d_ptr Optional Optional DT_VERNEEDNUM 0x6fffffff d_val Optional Optional DT_LOPROC 0x70000000 Unspecified Unspecified Unspecified DT_SPARC_REGISTER 0x70000001 d_val Optional Optional DT_AUXILIARY 0x7ffffffd d_val Unspecified Optional DT_USED 0x7ffffffe d_val Optional Optional DT_FILTER 0x7fffffff d_val Unspecified Optional DT_HIPROC 0x7fffffff Unspecified Unspecified Unspecified share edit edited Feb 4 '17 at 5:59