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15 November 2020

by Mike Krinkin

Continuing exploring UEFI after some break. Last time I looked at file access, now I’m going to read an ELF file from the file system, load it in memory and transfer control to the ELF image.

As usual all the sources are available on GitHub.

# Mithical ELF creature

What is ELF? ELF is a rather widely used executable (and not only) file format. You might now already that EFI binaries use PE32+ file format, ELF is just another file format that tries to solve similar problem.

Unlike PE32+ ELF is, at this point, basically the default file format for executables and shared libraries. It’s also often supported by other Unix-like operating systems.

To understand how we can load correctly an ELF image in memory we’d have to refer to the ELF Specification.

Instead of the ELF Specification you might refer to the base System V ABI. It covers the subject of loading ELF files marginally better.

ELF is a truly mythical creature - various accounts of it are confusing and/or incomplete.

# Structure of ELF file

1. it helps to quickly identify a particular file as ELF file
3. pointers to other relevant parts of the file.

What kind of basic information should a header contain? Well, ELF files are used for multiple purposes: executables, shared libraries, compiler object files.

Besides that ELF format supports multiple different hardware/software platforms, like 32-bit and 64-bit x86, 32-bit and 64-bit ARM and multiple others. So ELF file also contains information about what kind of platform this file is for.

NOTE: in this article I will only work with 64-bit version of ELF files. Covering both 32-bit and 64-bit will essentially double the amount of the code for very little benefit to the understanding.

Let’s take a look at the header format:

struct elf64_ehdr {
unsigned char e_ident[16];
uint16_t e_type;
uint16_t e_machine;
uint32_t e_version;
uint64_t e_entry;
uint64_t e_phoff;
uint64_t e_shoff;
uint32_t e_flags;
uint16_t e_ehsize;
uint16_t e_phentsize;
uint16_t e_phnum;
uint16_t e_shentsize;
uint16_t e_shnum;
uint16_t e_shstrndx;
};


The first four entries of the e_ident field must contain magic values: 0x7f, 'E', 'L' and 'F'. So checking the first four bytes allows to quickly identify an ELF file. Other elements in e_ident allow to check if this ELF file is 64-bit or 32-bit, if it’s big-endian or little-endian, but those don’t seem to be that important.

The e_type field contains, as the name suggests, the type of the ELF file: executable, shared library, object file, etc.

The e_machine field encodes the hardware architecture this file targets: x86, x86-64, arm, aarch64, etc.

I don’t really know how the fields e_version or e_flags can be useful and the code in the examples never refers to them, so I will not cover them in this post.

One field that is actually quite important is e_entry. When you write program code in a language like C you need to create function main that serves as an entry point into your program - execution of your program starts from calling function main.

NOTE: especially nitpicky and knowledgable readers may say that constructors of global objects in C++ are executed before main, some might say that even in C you can create a code that will be executed before main. For those readers, keep your pants on and skip the part where I explain what entry point is.

Execution of your program has to starts somewhere. In programming languages we have various conventions that are used to denote the entry point of the program. Once you build your program however there are no function names anymore, so you need some other way to tell where the execution of the code should start.

The e_entry field contains the address of the first instruction of the program. Once the ELF image loaded in memory, you should transfer execution by this address to start executing the program.

Hopefully, you now understand the meaning of the e_entry field. The other fields more or less point or describe other part of the ELF file. Most of them are of no interest for our example, but fields e_phoff and e_phnum are of immediate importance for our example.

Those two fields describe where in the ELF file the array of program headers is stored and how big it is. ELF program header, according to the specification, describes a segment of memory or some other piece of information the system loading the ELF file need to prepare for the program to execute.

It’s a rather vague description, so I’m going to make it a little bit more specific by simplifing the problem a little bit. Program to run needs the code and the data this code works with. When you build an executable the code and the data is stored into the executable file. To load the program you need to read them from the file into the right places in memory.

Essentially ELF program headers describe where in the ELF file code and data are stored, and where we should put them in memory.

NOTE: I mentioned that I simplify the problem a little bit here. Code and data aren’t the only things that program headers can describe. Moreover not everything that program headers describe has to be loaded in memory. So there are quite a few different kinds of program headers and in the simplified version of the problem we don’t need to care about all of them.

The structure of 64-bit ELF program header is as follows:

struct elf64_phdr {
uint32_t p_type;
uint32_t p_flags;
uint64_t p_offset;
uint64_t p_filesz;
uint64_t p_memsz;
uint64_t p_align;
};


There are multiple types of program headers, but in this simplified example we only will care about p_type == PT_LOAD - just one type of program header. The purpose of PT_LOAD program header is to exactly describe code and data the program needs to run.

The p_flags field describes permissions of a particular segment in memory. For example, some data might be read-only, code in memory have to be executable, etc. In this example I will not care about setting up permissions, so p_flags is of little relevance at this point.

p_offset and p_filesz describe where the code and data are stored inside the ELF file. Similarly p_vaddr and p_memsz describe where the code and data should be in memory.

It’s important to note here that p_filesz does not have to be equal to p_memsz. When p_filesz is smaller than p_memsz the rest of data in memory should be filled with zeros. The idea here is that zero-initialized data is quite common and there is no need to store zero inside the ELF file for no reason if we can just fill the memory with zeros when we load the ELF file.

In this example I will assume that the code we will be loading is position independent. Essentially, it would allow us to ignore the exact values of p_vaddr field in the program header. It’s however important for us to maintain relative offsets between data and code in memory.

For example, if in the ELF file we have two PT_LOAD program headers that describe two memory segments that need to be loaded at addresses X and Y correspondingly. We can instead load them at addresses X - D and Y - D, so addresses will not match the content of the program headers, but the distance between the two segments in memory will stay the same. On the other hand loading the two segments at addresses X and Y - D will break that requirement.

Important to note that don’t load the segments in memory at addresses written in program headers, when we transfer control to the program we will have to adjust the value in e_entry accordingly.

2. Calculate how much memory we need for the program and allocate it
3. Copy data from the ELF file in memory
4. Call the entry point of the program.

In the previous post we covered basics of the file operations in EFI environment, so I assume that the reader already familiar with it. What I didn’t cover there is how to actually read from a file in EFI. The signature of the read function of the File Protocol is as follows:

struct efi_file_protocol {
...
efi_status_t (*read)(struct efi_file_protocol *, efi_uint_t *, void *);
...
};


We would additionally need to be able to read at a specific offset in the file. The signature of the function to set position in the file is as follows:

struct efi_file_protocol {
...
efi_status_t (*set_position)(struct efi_file_protocol *, uint64_t);
...
};


Here is a helper function that reads the specified amount of data at the specified offset in the file:

static efi_status_t efi_read_fixed(
struct efi_file_protocol *file,
uint64_t offset,
size_t size,
void *dst)
{
efi_status_t status;
unsigned char *buf = dst;

status = file->set_position(file, offset);
if (status != EFI_SUCCESS)
return status;

efi_uint_t remains = size - read;

if (status != EFI_SUCCESS)
return status;

}

return EFI_SUCCESS;
}


NOTE: the function above can handle short reads, but I don’t really know if it’s an issue in EFI environment.

With that out of the picture we can read the ELF header and verify it’s content. I will not provide the verification function here, mostly because I do not hope that this verification function is complete. You can always find it in the repository if you want though (it’s called verify_elf64_header there).

In the code in the repository I read all the program headers in memory, it’s just easier to work with them when they are available in memory. To calculate the required amount of memory we need to find the minimum value of p_vaddr and the maximum value of p_vaddr + p_memsz among all PT_LOAD program headers. The difference between those is how much memory we need.

NOTE: there might be gaps in memory between segments, so it might make sense to avoid allocating a contigous segment of memory for the program, but it’s much simpler to allocate one contigous segment of memory for the program, so that’s what I’m going to do.

I’m also going to use page allocator to allocate memory for the program, so when calculating minimum and maximum address I’m also aligning them to the page size boundary, which is 4KiB.

Once the required amount of memory was calculated the actual allocation of memory and loading is rather simple:

static efi_status_t load_elf_image(struct elf_app *elf)
{
uint64_t size = elf->page_size + (elf->image_end - elf->image_begin);
uint16_t finish_msg[] = u"Loaded ELF image\r\n";
efi_status_t status;

status = elf->system->out->output_string(elf->system->out, start_msg);
if (status != EFI_SUCCESS)
return status;

// Allocate the required number of pages
status = elf->system->boot->allocate_pages(
if (status != EFI_SUCCESS)
return status;

// Save some bookkeeping information for cleanup in case of errors
elf->image_pages = size / elf->page_size;

// Entry point has to be adjusted, given that the ELF image might not

// Fill in everything with zeros, whatever data is not read from the
// ELF file itself has to be zero-initialized.

// Go over all program headers and load their content in memory
for (size_t i = 0; i < elf->header.e_phnum; ++i) {
const struct elf64_phdr *phdr = &elf->program_headers[i];

continue;

if (status != EFI_SUCCESS)
return status;
}

return elf->system->out->output_string(elf->system->out, finish_msg);
}


In the code above struct elf_app is a bookkeeping structure that I created. It mostly exists to group together various ELF file information and simplify cleanup procedures:

struct elf_app {
struct efi_system_table *system;
struct efi_file_protocol *kernel;
uint64_t image_begin;
uint64_t image_end;
uint64_t page_size;

// Only populated when image is loaded into mamory
uint64_t image_pages;
uint64_t image_entry;
};


So far so good. There is quite a bit of code that needs to be created to read, parse and load an ELF file in memory, but most of that code was rather straightforward (assuming some knowledge of ELF). Now comes the tricky part.

The program is now in memory, it’s time to transfer the control to the program. I will assume that entry point of the program will point to a function that takes no arguments and returns an int.

Additionally, I will assume that the calling convention used by the function is System V ABI (this is commonly used calling convention in the Unix world).

Finally, for the sake of testing, I will assume that the program will return back the number 42. That’s basically just to verify that our code works: we will pass control to the loaded image and then check the returned value. That’s why the entry point in this example returns int.

With those assumptions out the way, here is how the code look:

static efi_status_t start_elf_image(struct elf_app *elf)
{
uint16_t msg[] = u"Starting ELF image...\r\n";
uint16_t fail[] = u"Starting ELF image failed\r\n";
uint16_t success[] = u"ELF image successfully returned\r\n";
int (__attribute__((sysv_abi)) *entry)(void);
efi_status_t status;
int ret = 0;

status = elf->system->out->output_string(elf->system->out, msg);
if (status != EFI_SUCCESS)
return status;

entry = (int (__attribute__((sysv_abi)) *)(void))elf->image_entry;
ret = (*entry)();
if (ret != 42) {
elf->system->out->output_string(elf->system->out, fail);
}

elf->system->out->output_string(elf->system->out, success);
return EFI_SUCCESS;
}


Ignoring various output printing and attribute manipulations, the code is rather simple essentially. It just converts the entry point address to a function pointer and then calls the function using this pointer. The only magical part is the attribute that tells the compiler that we use System V ABI calling convention.

# Testing

Now we need an ELF binary to test. All we need from this binary is to return 42 and that’s it. As a result the code is rather simple:

int main(void)
{
return 42;
}


Compiling this code is somewhat tricky however. While this code looks like a regular C program we cannot compile it as such. We often this of main as an entry point of our program and in many ways it’s the case. However, compilers tend to put some additional code in the binary that executes before main and just calls main when it’s done. In other words, e_entry in the ELF header doesn’t necessarily contain the address of the main. To make sure that it’s the case we have to compile and link the program in a somewhat special way.

Here are the commands I used to build the code above:

clang \
-ffreestanding -MMD -mno-red-zone -std=c11 -Wall -Werror -pedantic \
-c kernel.c \
-o kernel.o
lld \
kernel.o \
-o kernel


NOTE: In the repository those commands are in the Makefile, so you can just use make to build everything.

Before trying our EFI code we can explore the binary a little bit using available commandline tools. For example, we can check the ELF header of the file:

$hexdump -n 4 -C kernel 00000000 7f 45 4c 46 |.ELF| 00000004  As you can see the first four bytes are 0x7f, 'E', 'L' and 'F'. Which is the expected magic at the beginning of the ELF file. There are also dedicated tools that parse ELF files. For example, here is how you can take a look at the ELF header in a more human friendly way: $ readelf -h kernel
Magic:   7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00
Class:                             ELF64
Data:                              2's complement, little endian
Version:                           1 (current)
OS/ABI:                            UNIX - System V
ABI Version:                       0
Type:                              EXEC (Executable file)
Version:                           0x1
Start of program headers:          64 (bytes into file)
Start of section headers:          488 (bytes into file)
Flags:                             0x0
Size of this header:               64 (bytes)
Size of program headers:           56 (bytes)
Size of section headers:           64 (bytes)
Section header string table index: 4


From the header you can find that there are example four program header in the resulting binary and they are located starting from the byte 64 from the beginning of the file.

We can also explore the program headers themselves:

\$ readelf -l kernel

Elf file type is EXEC (Executable file)
Entry point 0x201120
There are 4 program headers, starting at offset 64

FileSiz            MemSiz              Flags  Align
PHDR           0x0000000000000040 0x0000000000200040 0x0000000000200040
0x00000000000000e0 0x00000000000000e0  R      0x8
0x0000000000000120 0x0000000000000120  R      0x1000
0x000000000000000b 0x000000000000000b  R E    0x1000
GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
0x0000000000000000 0x0000000000000000  RW     0x0

Section to Segment mapping:
Segment Sections...
00
01
02     .text
03


You can see that while there are four program headers in the file only two of them are PT_LOAD. Also pay attention to flags for the two PT_LOAD program headers: only one of them is executable. So you can probably figure out which one of them is for data and which one of them is for code.

To actually test our code with this binary you need to put the kernel binary together with the bootx64.efi file in the root file system. You can refer for details to the EFI getting started post.

If everything goes as expected you should be able to see the ELF image successfully returned message on the screen. That’s what the start_elf_image function prints when the loaded image returns control back.