Technovelty - Symbol Versions and dependencies
Jan 2, A common tool to quickly parse ELF files is the readelf utility from GNU fields of Segments and Sections and the relationship between them. Ben, I ran readelf on a Linux x g++ built kull, and ran stdout and . However, we have yet to prove that relationship and certainly don't know the. Aug 3, The readelf program can dump all the information in an ELF file. ELF is What is going on with the relationship between Qatar and the GCC?.
ELF header readelf -h: ELF sections readelf -S: The ELF segments are missing, as they are built during linking. What symbols are there in the symbol table?
Additionally, object files have a relocation table, i. Let's look at hello. Offset Info Type Sym.
We notice that one of the external symbols is puts. Since that is part of the C library, the linker must resolve its location and replace all occurences with the symbol's address. This is where the difference between static and dynamic linking shows. During static linking, the actual symbol address is filled in; while dynamic linking uses dynamic relocation tables the Global Offset Table and the Procedure Linkage Table whose values are resolved by the loader. More details on this in the next lab.
What does -M do? In general we encourage you to check out the manpages to find out. Sometimes however it is possible that the code we are dealing with doesn't have any useful metadata associated with it, e. Let's take for example the hello2 binary generated from hello2. S in the lab archive: In this case however, objdump does not assume any target architecture, so we must pass it explicitly using -m.
S source file, we notice that the disassembled code maps almost directly. The last part of the binary does not contain any meaningful code, because here objdump attempts to also disassemble data. Code is also data! This has interesting security implications, as we will see throughout the course. To obtain raw data we can just dump the binary using hexdump or xxd: Shellcode [2p] The purpose of this task is to get you acquainted with some tools that can be used to manipulate ELF files. Inspect the source code of shellcode.
LOAD 0xe28 0xe28 0xe28 0x 0x RW a trick that might work is making the stack executable execstack -s. Compile run and save the generated shellcode Compile gcc -O0 -o shellcode shellcode. Is it a false positive? File is reading some magic bytes, this is misleading Try to execute. Who is throwing the error? The loader, which resides in the operating system How to actually run the generated shellcode.
The problem so far is that the shellcode SC ends in a segment that does not have the executable bit set. One solution to this is, at runtime, remap the segment page with the exec flag — this solution requires writing some code.
We can focus on another solution: ELF object file from the raw binary objcopy -I binary -O elfx Where are the segments? It should be WA! The segments are linktime info, we didn't link yet Adjust the. It should be WAX! How do we actually use the data from this.
- GNU Binary Utilities
- Lab 03 - Executables. Static Analysis
- Symbol Versions and dependencies
What symbols are exported? What does it do? It uses the variables previously listed to call the code. ELF the contents of the.
Let's run it and give it a brief view: However, it's small enough, so we can try to reverse engineer it by hand. To do that, answer the following questions: What is the file's entry point? What instructions get executed started from that entry point?
What operands does the call instruction receive during execution? Where are ret instructions placed relative to the call operands? What other control-flow altering instructions are executed besides call and ret?
elf - Understanding the relocation table output from readelf - Stack Overflow
Normally we use tools such as IDA or Radare2 to reverse engineer binaries. In this case however, we challenge you to use only your brain, a pen and a piece of paper. It's a bit tedious, but the end result should be fun.
You can dump data from within objdump using the -s flag. Use this to figure out what pointers to contents from. Dumping the code, we can see that stripped calls a bunch of functions starting with 0xc9: We stopped at the first encountered ret, assuming that this is where we exit from the function.
We'll see this is not quite true! Let's note the functions that are called starting from the entry point: Let's look at f1, to see what it does: If we look carefully, we see that it sets eax to 0x4, which is the system call code for write, while ebx the argument for the file descriptor is set to 0x1 stdout.
If one of the files named in member By default, new members are added at the end of the file; but you may use one of the modifiers a, b, or i to request placement relative to some existing member. The modifier v used with this operation elicits a line of output for each file inserted, along with one of the letters a or r to indicate whether the file was appended no old member deleted or replaced. Normally only the member name is shown; if you also want to see the modes permissionstimestamp, owner, group, and size, you can request that by also specifying the v modifier.
If you do not specify a member, all files in the archive are listed. If there is more than one file with the same name say, fie in an archive say b. You can use the v modifier with this operation, to request that ar list each name as it extracts it. If you do not specify a member, all files in the archive are extracted.
A number of modifiers mod may immediately follow the p keyletter, to specify variations on an operation's behavior: If you use the modifier a, the name of an existing archive member must be present as the relpos argument, before the archive specification. If you use the modifier b, the name of an existing archive member must be present as the relpos argument, before the archive specification.
The specified archive is always created if it did not exist, when you request an update. But a warning is issued unless you specify in advance that you expect to create it, by using this modifier. GNU ar will normally permit file names of any length. This will cause it to create archives which are not compatible with the native ar program on some systems.
If this is a concern, the f modifier may be used to truncate file names when putting them in the archive. If you use the modifier i, the name of an existing archive member must be present as the relpos argument, before the archive specification. N Uses the count parameter. This is used if there are multiple entries in the archive with the same name.
Executable and Linkable Format 101 - Part 1 Sections and Segments
Extract or delete instance count of the given name from the archive. If you do not specify this modifier, files extracted from the archive are stamped with the time of extraction. P Use the full path name when matching names in the archive. This option will cause GNU ar to match file names using a complete path name, which can be convenient when extracting a single file from an archive created by another tool.
You may use this modifier flag either with any operation, or alone. Running ar s on an archive is equivalent to running ranlib on it. S Do not generate an archive symbol table. This can speed up building a large library in several steps. The resulting archive can not be used with the linker.
In order to build a symbol table, you must omit the S modifier on the last execution of ar, or you must run ranlib on the archive. If you would like to insert only those of the files you list that are newer than existing members of the same names, use this modifier. The u modifier is allowed only for the operation r replace. In particular, the combination qu is not allowed, since checking the timestamps would lose any speed advantage from the operation q.
Many operations display additional information, such as filenames processed, when the modifier v is appended.