Binary Structure

Adding New Sections to an ELF

There are situations in which we want to add new sections in the executable file (ELF format - Executable and Linking Format) for our application or library. The reason these sections are useful is that the library (or application) becomes much easier to configure, thus serving more purposes. For example, the Unikraft virtual filesystem (i.e. the vfscore library) uses such a section in which it registers the used filesystem (ramfs, 9pfs), and we are going to discuss this in the following sections. Another component that makes use of additional sections is the scheduler. The scheduler interface allows us to register a set of functions at build time that will be called when a thread is created or at the end of its execution.

The way we can add such a section in our application/library is the following:

1. Create a file with the .ld extension (e.g. extra.ld) with the following content:

   SECTIONS
   {
       .my_section : {
           PROVIDE(my_section_start = .);
           KEEP (*(.my_section_entry))
           PROVIDE(my_section_end = .);
       }
   }
   INSERT AFTER .text;
   

2. Add the following line to Makefile.uk:

   LIBYOURAPPNAME_SRCS-$(CONFIG_LIBYOURAPPNAME) += $(LIBYOURAPPNAME_BASE)/extra.ld
   

   This will add the .my_section section after the .text section in the ELF file. The .my_section_entry field will be used to register an entry in this section, and access to it is generally gained via traversing the section’s endpoints (i.e. from my_section_start to my_section_end).

Before running the program let’s analyze the source code. We want to register the my-structure structure in the newly added section. In Unikraft core libraries this is usually done using macros. So we will do the same.

#define MY_REGISTER(s, f) static const struct my_structure      \
        __section(".my_section_entry")                          \
        __my_section_var __used =                               \
                {.name = (s),                                   \
                .func = (f)};

This macro receives the fields of the structure and defines a variable called __my_section_var in the newly added section. This is done via __section(). We also use the __used attribute to tell the compiler not to optimize out the variable. Note that this macro uses different compiler attributes. Most of these are in uk/essentials.h, so please make sure you include it when working with macros.

Next, let’s analyze the method by which we can go through this section to find the entries. We must first import the endpoints of the section. It can be done as follows:

extern const struct my_structure my_section_start;
extern const struct my_structure my_section_end;

Using the endpoints we can write the macro for iterating through the section:

#define for_each_entry(iter)                                    \
        for (iter = &my_section_start;                          \
                iter < &my_section_end;                         \
                iter++)

Let’s configure the program. Use the make menuconfig command to set the KVM platform as in the following image.

Save the configuration, exit the menuconfig tab and run make. Now, let’s run it. You can use the following command:

$ qemu-guest -k build/01-extrald-app_kvm-x86_64

The program’s output should be the following:

To see that the information about the section size and its start address is correct we will examine the binary using the readelf utility. The readelf utility is used to display information about ELF files, like sections or segments. More about it here Use the following command to display information about the ELF sections:

$ readelf -S build/01-extrald-app_kvm-x86_64

The output should look like this:

We can see that my_section is indeed among the sections of the ELF. Looking at its size we see that it is 0x10 bytes (the equivalent of 16 in decimal). We also notice that the start address of the section is 0x1120f0, the same as the one we got from running the program.