I want to change where in the elf file execution starts. For example I have a basic hello world program in a elf file. The actual code is located at an offset of 0x1000 bytes into the file. I want to move that code to, lets say, a 0x900 offset and modify the file so that it starts executing at 0x900. I know this sounds kinda useless but it does serve a purpose.
First you compile/assemble (clang/as/...) your program into a hello.o ELF object file. At this point, you would normally let the compiler driver finish the job and emit an ELF executable.
You can instead use the linker (lld/ld/...) and specify the entry point with --entry 0x900. You can also do this with a linker script. Note that if you do this, you have to handle a bunch of stuff that the compiler driver normally handles for you. The warning from the Oracle linker manual says:
When you invoke the link-editor directly, you have to supply every
object file and library required to create the intended output. The
link-editor makes no assumptions about the object modules or libraries
that you meant to use in creating the output.
I'm writing an EFI application that loads an ELF into memory and jumps to it, but I don't know what header I should analyse first (program or section header). I have a function that reads the program headers to load the ELF into memory (which works) and a function that reads the section headers to load the ELF into memory (which also works).
The program loader should look at the program header only. The section headers are for tools such as debuggers. I don't think this is spelled out explicitly in the original ELF specification or the System V ABI specification, but it is very much implied:
System V Application Binary Interface
Even today, when new features are defined which are used by the dynamic linker, references are added the dynamic to the dynamic section, even though in theory, the information could also be obtained from the section header (but there are probably some exceptions for certain architectures).
I am learning about ELF. The file class can be one of ELFCLASS32, ELFCLASS64 or ELFCLASSNONE.
However, I cannot find any example usage of ELFCLASSNONE.
What is it used for ? And when ? Is it actually used anywhere ?
Is it actually used anywhere ?
No.
(It's only used to detect invalid ELF files.)
Used where?
Anywhere validity of the ELF file is verified. Here is an example from the Linux kernel tools.
even there, ELFCLASSNONE is not used.
You don't know what parts of the ELF header readelf examined before it concluded that .bashrc is not an ELF file. It may have looked at ei_ident[EI_CLASS] and compared the value with ELFCLASSNONE (though likely it didn't).
If you make a copy of e.g. /bin/date, and write a 0 byte into 5th byte of the copy (EI_IDENT == 4) to corrupt it, then run readelf -h on that copy, you'll probably get an "invalid ELF class" or similar message.
Problem scenario:
In simple words, do we have a Trace32 command to read symbols (and its contents) from ELF file that was loaded on to target ? We have this special case where application specific debug symbols of the ELF file are made as part of '.noload' section in ELF, which means the symbols/contents are present part of the ELF file (available when read using readelf -a xxxx.elf_file_name) but are not part of the final binary image generated i.e. the '.noload' section in ELF file is stripped away when generating xxx.bin which is flashed to target memory.
Debug symbols in '.noload' section are statically assigned values and these values do not change during runtime.
When I tried to read the debug symbols part of the '.noload' section (after compiling into binary and loading onto Trace32), I see 'MMU fail' flagged on trace32 popup window which means trace32 is trying to read symbol contents from memory but is not accessible, since symbols part of the '.noload' section was not loaded at all though they have addresses mapped.
Any inputs:
- I need help with a trace32 command that can directly read symbol content from ELF file than from target memory.
- Also not sure if I can use 'readelf ' in practice scripts ? Any help in this direction if we do not have any solution for above query ?
Use command
Data.LOAD.Elf myfile.elf [<optional address offset>] /NoCODE
The option /NoCODE instructs TRACE32 to only load the debgug symbols from your ELF but not to load any code to your target. You can than view the symbols with command sYmbol.Browse.
However if you use TRACE32 to load your application to your target, you don't have to create a binary from you ELF first. With TRACE32 you can also load the PROGBITS sections of your ELF directly to your target.
In this case you would simply use the Data.LOAD.Elf command without the /NoCODE option (after enabling flash programming).
Since you are using an MMU you might want to activate logical memory space IDs with command SYStem.Option.MMUSPACES ON. Then load your symbols with
Data.LOAD.Elf myfile.elf <space-ID>:<offset> /NoCODE
where 'space-ID' matches with the space-ID used by you MMU for the Task and 'offset' is usually zero.
If you are debugging your application on an embedded Linux than you should use the TRACE32 OS awareness for Linux and the Linux symbol auto-loader to load the symbols to the correct addresses for you.
I don't think there is any reason why you should use 'readelf' from within TRACE32. Anyway you can invoke any command line program with commands OS.Area or OS.Command.
I've been reading ELF standard here. From what I understand, each ELF contains ELF header, program headers (why more than one?) and section headers. Can anyone please explain:
How are ELF files generated? is it the compiler responsibility?
What are sections and why do we need them?
What are program headers and why do we need them?
Inside program headers, what's the meaning of the fields p_vaddr and p_paddr?
Does each section have it's own section header?
Alternatively, does any one have a link to a more friendly documenation of ELF?
How are ELF files generated? is it the compiler responsibility?
They can be generated by a compiler, an assembler, or any other tool that can generate them. Even your own program you wrote for generating ELF files ;) They're just streams of bytes after all, so they can be generated by just writing bytes into a file in binary mode. You can do that too.
What are sections and why do we need them?
ELF files are subdivided into sections. Sections are the smallest continuous regions in the file. You can think of them as pages in an organizer, each with its own name and type that describes what does it contain inside. Linkers use this information to combine different parts of the program coming from different modules into one executable file or a library, by merging sections of the same type (gluing pages together, if you will).
In executable files, sections are optional, but they're usually there to describe what's in the file and where does it begin, and how much bytes does it take.
What are program headers and why do we need them?
They're mostly for making executable files. In order to run a program, sections aren't enough, because you have to specify not only what's there in the file, but also where should it be loaded into memory in the running process. Program headers are just for that purpose: they describe segments, which are regions of memory in the running process, with different access privileges & stuff.
Each program header describes one segment. It tells the loader where should it load a certain region in the file into memory and what permissions should it set for that region (e.g. should it be allowed to execute code from it? should it be writable or just for reading?)
Segments can be further subdivided into sections. For example, if you have to specify that your code segment is further subdivided into code and static read-only strings for the messages the program displays. Or that your data segment is subdivided into funky data and hardcore data :J It's for you to decide.
In executable files, sections are optional, but it's nice to have them, because they describe what's in the file and allow for dumping selected parts of it (e.g. with the objdump tool). Sometimes they are needed, though, for storing dynamic linking information, symbol tables, debugging information, stuff like that.
Inside program headers, what's the meaning of the fields p_vaddr and p_paddr?
Those are the addresses at which the data in the file will be loaded. They map the contents of the file into their corresponding memory locations. The first one is a virtual address, the second one is physical address.
Physical addresses are the "raw" memory addresses. On modern operating systems, those are no longer used in the userland. Instead, userland programs use virtual addresses. The operating system deceives the userland program that it is alone in memory, and that the entire address space is available for it. Under the hood, the operating system maps those virtual addresses to physical ones in the actual memory, and it does it transparently to the program.
Of course, not every address in the virtual address space is available at the same time. There are limitations imposed by the actual physical memory available. So the operating system just maps the memory for the segments the program actually uses (here's where the "segments" part from the ELF file's program headers comes into play). If the process tries to access some unmapped memory, the operating system steps in and says, "sorry, chap, this memory doesn't belong to you". (The program can address it, but it cannot access it.)
Does each section have it's own section header?
Yes. If it doesn't have an entry in the Section Headers Table, it's not a section :q Because they only way to tell if some part of the file is a section, is by looking in to the Section Headers Table which tells you what sections are defined in the file and where you can find them.
You can think of the Section Headers Table as a table of contents in a book. Without the table of contents, there aren't any chapters after all, because they're not listed anywhere. The book may have headings, but the content is not subdivided into logical chapters that can be found through the table of contents. Same goes with sections in ELF files: there can be some regions of data, but you can't tell without the "table of contents" which is the SHT.
This link includes a better explaination.
How are ELF files generated? is it the compiler responsibility?*
It is architecture dependent.
What are sections and why do we need them?
Different section have different information such as code, initialized data, uninitialized data etc. These information will be used by the compiler and linker.
What are program headers and why do we need them?
Program headers are used by the operating system when it loads the executable. These headers contains information about the segments (contiguous memory block with some permissions) such as which parts needs to be loaded, interpreter infor etc.
Inside program headers, what's the meaning of the fields p_vaddr and p_paddr?
In general virtual address and the physical address are same. But could be different depends on the system.
Does each section have it's own section header?
yes. Each section have a section header entry at section header table.
This is the best documentation I've found: http://www.skyfree.org/linux/references/ELF_Format.pdf
Each section has only one section header, but there can be section headers without sections
2 - There are many different sections, ex: relocation section recoeds many infomation for relocation symbol. I use the infomation to load a elf object and run/relocate the object.
Antoher example: debug section records debug information, gdb use the data for showing debug message.
Symbol section records symbol information.
3 - programming header used by loader, loader loads a elf execute file by looking up programming header.