Why is the program header executable? - linux

Question

I used readelf on several binaries on my linux box and saw something that surprised me in the program headers. This eample is from the 'ld' utility, but it also occurs with anything I compile with gcc.
PHDR 0x000034 0x08048034 0x08048034 0x00120 0x00120 R E 0x4
This segment spans the entirety of the program headers. Why is is marked as executable? It doesn't contain machine code. But also, why is even this present in the headers? I don't really want it in my program image.

Answer 1

The PHDR pointing to the PHDRs tells the loader that the PHDRs themselves should be mapped to the process address space, in order to make them accessible to the program itself.
This is useful mainly for dynamic linking.
The reason the memory is marked as executable is because the PHDRs are smaller than one page, and live right next to the start of the executable code. If the permissions for the PHDRs were different from those of the program text, the linker would have to insert padding between them.

Answer 2

The main File ELF headers are there to easily find the offset in the file where other sections are stored. Then each subheader describes the data in it's section.
Main ELF header looks like this:
/* ELF File Header */
typedef struct
{
unsigned char e_ident[EI_NIDENT]; /* Magic number and other info */
Elf32_Half e_type; /* Object file type */
Elf32_Half e_machine; /* Architecture */
Elf32_Word e_version; /* Object file version */
Elf32_Addr e_entry; /* Entry point virtual address */
Elf32_Off e_phoff; /* Program header table file offset */
Elf32_Off e_shoff; /* Section header table file offset */
Elf32_Word e_flags; /* Processor-specific flags */
Elf32_Half e_ehsize; /* ELF header size in bytes */
Elf32_Half e_phentsize; /* Program header table entry size */
Elf32_Half e_phnum; /* Program header table entry count */
Elf32_Half e_shentsize; /* Section header table entry size */
Elf32_Half e_shnum; /* Section header table entry count */
Elf32_Half e_shstrndx; /* Section header string table index */
} Elf32_Ehdr;
The program header(s) are there because they describe the executable parts of the ELF executable.
The next portion of the program are
the ELF program headers. These
describe the sections of the program
that contain executable program code
to get mapped into the program address
space as it loads.
/* Program segment header. */
typedef struct
{
Elf32_Word p_type; /* Segment type */
Elf32_Off p_offset; /* Segment file offset */
Elf32_Addr p_vaddr; /* Segment virtual address */
Elf32_Addr p_paddr; /* Segment physical address */
Elf32_Word p_filesz; /* Segment size in file */
Elf32_Word p_memsz; /* Segment size in memory */
Elf32_Word p_flags; /* Segment flags */
Elf32_Word p_align; /* Segment alignment */
} Elf32_Phdr;
This is taken from here