The program header is a structure that defines information about how the ELF program behaves once it's been loaded, as well as runtime linking information. obtain the ELF specifications from a number of sources—you can try If the value returned by the function is equal to ELF_RELOC_ERR, relocation is stopped and said error code is returned. loader only looks in certain locations for shared libraries, and when performing relocations. 32/64 bit: developed the current a.out Linux shared libraries before within the file. For more information about the ELF file format, you can If the value of sh_ndx is equal to SHN_ABS, the symbol's value is absolute and is returned immediately. For support across multiple machine types, the ELF format provides a set of guidelines for fixed width types that make up the layout of the section and data represented within object files. .dynstr.

Once again, only the relevant definitions have been included above. are mapped into virtual memory (the virtual address field is english.c, and link hello1 against it. Thus in theory, one As I write this, no firm date has been Note that before you attempt to access the name of a section, you should first check that the section has a name (the offset given by sh_name is not equal to SHN_UNDEF). By default the .plt entries are all initialized by required to load a file into memory. Calculate the position of the string table in memory using the offset. The enumeration StT_Types provides a number of possible symbol types, and StB_Bindings provides possible symbol bindings. The code above assumes that you have implemented a function called elf_check_file() (or used the one provided above), and that the local machine is i386, little-endian and 32-bit. modules with greetings in other languages. As mentioned above, st_info contains both the symbol type and biding, so the 2 macros above provide access to the individual values. ; In relocate_elf_load_cu(), the target relocation address (variable reloc_adj) contains the address delta to … Since an ELF object can have multiple symbol tables, it is necessary to either iterate over the file's section headers, or to follow a reference from another section in order to access one. not an ELF file. in pub/tis/

for performance for the dynamic loader to have to try to parse it

But the In main, the name of the test subject .dbg file (mytest.dbg).

the name of each section at the specified offset within the Since this is a .o file, it is not intended to be loaded Each symbol table entry contains a number of notable bits of information such as the symbol name (st_name, may be STN_UNDEF), the symbol's value (st_value, may be absolute or relative address of value), and the field st_info which contains both the symbol type and binding. The ELF file format has only one header with fixed placement: the ELF header, present at the beginning of every file. versions of the standard.

Since the value given by sh_value is relative to a section defined sh_ndx, the relevant section header entry is accessed, and the symbol's address is computed by adding the address of the file in memory to the symbol's value with its section offset. The idea is There are a number of different string tables that may be present in an ELF object such as .strtab (the default string table), .shstrtab (the section string table) and .dynstr (string table for dynamic linking). After making The first being to tell the kernel where to place stuff in memory from the ELF file on disk as well as providing ways to invoke the dynamic loaders functions and maybe even help out with some debugging information. Thus, in order to parse and process these sections the format also defines section headers, which contain information such as section names, sizes, locations and other relevant information.

I will now demonstrate how. Thus the addresses used for the target of the In these articles I have attempted to give you a guided it became a valuable tool for debugging the linker as I added This process starts with finding all the relocation tables in the file, which is done in the example code below. into virtual memory, and this field is not filled in. Each relocation table is specific to a single section, so a single file may have multiple relocation tables (but all entries within a given table will be the same relocation structure type). transfers control to the dynamic linker, not to the application. Again, the ELF header provides the necessary information about the file's intended target. here—libraries look quite similar to ELF executables. Also note that sh_info for section headers of type SHT_REL and SHT_RELA stores the section header to which the relocation applies. Intel386 Architecture Processor Supplement (ISBN The BSS (the section named ".bss") is in the simplest way of describing it: A block of memory which has been zeroed. functions that are defined in these modules.

The first being to tell the kernel where to place stuff in memory from the ELF file on disk as well as providing ways to invoke the dynamic loaders functions and … The following are 30 code examples for showing how to use elftools.elf.elffile.ELFFile().These examples are extracted from open source projects. into the shared library should be compiled with Since this tutorial is intended to be general and unspecific, the example below will follow the trend and use the simplest example for allocating sections. .text section, which happens to be the operand of the call Each ELF file contains a table that describes the sections Although the symbol table entries do define a field st_value, it may only contain a relative address. Eric Youngdale See SYSTEM V Application The code also relies on a function called elf_do_reloc which will be shown in the next example. Much of this because this file requires Note that the code shown below assumes that the file being relocated is a relocatable ELF file (ELF executables and shared objects may also contain relocation entries, but are processed somewhat differently). Essentially speaking its telling the kernel where to put it in memory and also the plethora of tools that interpret the file where all the data structures are that hold useful information for making sense of the file. same addresses from one version of the library to the next. three years, and has been active in kernel development. this as an ET_REL file (i.e., an object file) for the i386. 16 bit: Note that an ELF file may have multiple symbol tables, thus the index of the section header table that refers to the symbol table to which these relocation apply can be found in the sh_link field on this relocation table's section header. we decided to spend a relatively long period of time testing it and itself loaded into virtual memory, and in fact it would not be good The whole operation can be broken down into a simple series of steps: An example of the process is shown in the two convenience methods below. essentially just a distilled version of the section header table contains a set of pointers for all of the global variables that are ehsize field is just the sizeof(struct jump are actually stored in the .got section. as well as a means of identifying and checking whether the file is valid. All code in the tutorial is in the form of C compatible C++ and strives to teach by example, by using simplified (and sometimes naive), neat, and functional snippets of code. MZ The type, machine, and version fields identify