手拆ELF（三，節區頭表）

節區頭表

節區頭的資料結構為Elf32_Shdr，大小為40位元組，其資料結構如下：

/* Section header.  */

typedef struct
{
  Elf32_Word	sh_name;		/* Section name (string tbl index) */
  Elf32_Word	sh_type;		/* Section type */
  Elf32_Word	sh_flags;		/* Section flags */
  Elf32_Addr	sh_addr;		/* Section virtual addr at execution */
  Elf32_Off	    sh_offset;		/* Section file offset */
  Elf32_Word	sh_size;		/* Section size in bytes */
  Elf32_Word	sh_link;		/* Link to another section */
  Elf32_Word	sh_info;		/* Additional section information */
  Elf32_Word	sh_addralign;	/* Section alignment */
  Elf32_Word	sh_entsize;		/* Entry size if section holds table */
} Elf32_Shdr;

注意：32bit和64bit的節區頭沒有實質性區別

sh_name

該成員的值為一個指向節區頭字元表節的索引，通過該索引可以在節區頭字元表節中找到節區的名稱。

sh_type

該成員歸類節區的內容和語義，標明節區是什麼型別，有什麼作用。

/* Legal values for sh_type (section type).  */

#define SHT_NULL	  0		/* Section header table entry unused */
#define SHT_PROGBITS  1		/* Program data */
#define SHT_SYMTAB	  2		/* Symbol table */
#define SHT_STRTAB	  3		/* String table */
#define SHT_RELA	  4		/* Relocation entries with addends */
#define SHT_HASH	  5		/* Symbol hash table */
#define SHT_DYNAMIC	  6		/* Dynamic linking information */
#define SHT_NOTE	  7		/* Notes */
#define SHT_NOBITS	  8		/* Program space with no data (bss) */
#define SHT_REL		  9		/* Relocation entries, no addends */
#define SHT_SHLIB	  10		/* Reserved */
#define SHT_DYNSYM	  11		/* Dynamic linker symbol table */
#define SHT_INIT_ARRAY	  14		/* Array of constructors */
#define SHT_FINI_ARRAY	  15		/* Array of destructors */
#define SHT_PREINIT_ARRAY 16		/* Array of pre-constructors */
#define SHT_GROUP	  	  17		/* Section group */
#define SHT_SYMTAB_SHNDX  18		/* Extended section indeces */
#define	SHT_NUM		  	  19		/* Number of defined types.  */
#define SHT_LOOS	  0x60000000	/* Start OS-specific.  */
#define SHT_GNU_ATTRIBUTES 0x6ffffff5	/* Object attributes.  */
#define SHT_GNU_HASH	  0x6ffffff6	/* GNU-style hash table.  */
#define SHT_GNU_LIBLIST	  0x6ffffff7	/* Prelink library list */
#define SHT_CHECKSUM	  0x6ffffff8	/* Checksum for DSO content.  */
#define SHT_LOSUNW	  0x6ffffffa	/* Sun-specific low bound.  */
#define SHT_SUNW_move	  0x6ffffffa
#define SHT_SUNW_COMDAT   0x6ffffffb
#define SHT_SUNW_syminfo  0x6ffffffc
#define SHT_GNU_verdef	  0x6ffffffd	/* Version definition section.  */
#define SHT_GNU_verneed	  0x6ffffffe	/* Version needs section.  */
#define SHT_GNU_versym	  0x6fffffff	/* Version symbol table.  */
#define SHT_HISUNW	  0x6fffffff	/* Sun-specific high bound.  */
#define SHT_HIOS	  0x6fffffff	/* End OS-specific type */
#define SHT_LOPROC	  0x70000000	/* Start of processor-specific */
#define SHT_HIPROC	  0x7fffffff	/* End of processor-specific */
#define SHT_LOUSER	  0x80000000	/* Start of application-specific */
#define SHT_HIUSER	  0x8fffffff	/* End of application-specific */

macro	description
SHT_NULL	This value marks the section header as inactive. It does not have an associated section. Other members of the section header have undefined values.
SHT_PROGBITS	This section holds information defined by the program, whose format and meaning are determined solely by the program.
SHT_SYMTAB	This section holds a symbol table. Typically, SHT_SYMTAB provides symbols for link editing, though it may also be used for dynamic linking. As a complete symbol table, it may contain many symbols unnecessary for dynamic linking. An object file can also contain a SHT_DYNSYM section.
SHT_STRTAB	This section holds a string table. An object file may have multiple string table sections.
SHT_RELA	This section holds relocation entries with explicit addends, such as type Elf32_Rela for the 32-bit class of object files. An object may have multiple relocation sections.
SHT_HASH	This section holds a symbol hash table. An object participating in dynamic linking must contain a symbol hash table. An object file may have only one hash table.
SHT_DYNAMIC	This section holds information for dynamic linking. An object file may have only one dynamic section.
SHT_NOTE	This section holds information that marks the file in some way.
SHT_NOBITS	A section of this type occupies no space in the file but otherwise resembles SHT_PROGBITS. Although this section contains no bytes, the sh_offset member contains the conceptual file offset.
SHT_REL	This section holds relocation offsets without explicit addends, such as type Elf32_Rel for the 32-bit class of object files. An object file may have multiple relocation sections.
SHT_SHLIB	This section is reserved but has unspecified semantics.
SHT_DYNSYM	This section holds a minimal set of dynamic linking symbols. An object file can also contain a SHT_SYMTAB section.
SHT_LOPROC	This value up to and including SHT_HIPROC is reserved for processor-specific semantics.
SHT_HIPROC	This value down to and including SHT_LOPROC is reserved for processor-specific semantics.
SHT_LOUSER	This value specifies the lower bound of the range of indices reserved for application programs.
SHT_HIUSER	This value specifies the upper bound of the range of indices reserved for application programs. Section types between SHT_LOUSER and SHT_HIUSER may be used by the application, without conflicting with current or future system-defined section types.

sh_flags

Sections support one-bit flags that describe miscellaneous attributes. If a flag bit is set in sh_flags, the attribute is “on” for the section. Otherwise, the attribute is “off” or does not apply. Undefined attributes are set to zero.

/* Legal values for sh_flags (section flags).  */

#define SHF_WRITE	     	 (1 << 0)	/* Writable */
#define SHF_ALLOC	     	 (1 << 1)	/* Occupies memory during execution */
#define SHF_EXECINSTR	     (1 << 2)	/* Executable */
#define SHF_MERGE	     	 (1 << 4)	/* Might be merged */
#define SHF_STRINGS	     	 (1 << 5)	/* Contains nul-terminated strings */
#define SHF_INFO_LINK	     (1 << 6)	/* `sh_info' contains SHT index */
#define SHF_LINK_ORDER	     (1 << 7)	/* Preserve order after combining */
#define SHF_OS_NONCONFORMING (1 << 8)	/* Non-standard OS specific handling
					   required */
#define SHF_GROUP	     	 (1 << 9)	/* Section is member of a group.  */
#define SHF_TLS		     	 (1 << 10)	/* Section hold thread-local data.  */
#define SHF_COMPRESSED	     (1 << 11)	/* Section with compressed data. */
#define SHF_MASKOS	     	 0x0ff00000	/* OS-specific.  */
#define SHF_MASKPROC	     0xf0000000	/* Processor-specific */
#define SHF_ORDERED	     	 (1 << 30)	/* Special ordering requirement
					   (Solaris).  */
#define SHF_EXCLUDE	     	 (1U << 31)	/* Section is excluded unless
					   referenced or allocated (Solaris).*/

macro	description
SHF_WRITE	This section contains data that should be writable during process execution.
SHF_ALLOC	This section occupies memory during process execution. Some control sections do not reside in the memory image of an object file. This attribute is off for those sections.
SHF_EXECINSTR	This section contains executable machine instructions.
SHF_MASKPROC	All bits included in this mask are reserved for processor-specific semantics.

sh_addr

If this section appears in the memory image of a process, this member holds the address at which the section’s first byte should reside. Otherwise, the member contains zero.

sh_offset

This member’s value holds the byte offset from the beginning of the file to the first byte in the section. One section type, SHT_NOBITS, occupies no space in the file, and its sh_offset member locates the conceptual placement in the file.

sh_size

This member holds the section’s size in bytes. Unless the section type is SHT_NOBITS, the section occupies sh_size bytes in the file. A section of type SHT_NOBITS may have a nonzero size, but it occupies no space in the file.

sh_link

This member holds a section header table index link, whose interpretation depends on the section type.

sh_info

This member holds extra information, whose interpretation depends on the section type.

sh_addralign

Some sections have address alignment constraints. If a section holds a doubleword, the system must ensure doubleword alignment for the entire section. That is, the value of sh_addr must be congruent to zero, modulo the value of sh_addralign. Only zero and positive integral powers of two are allowed. Values of zero or one mean the section has no alignment constraints.

sh_entsize

Some sections hold a table of fixed-sized entries, such as a symbol table. For such a section, this member gives the size in bytes for each entry. This member contains zero if the section does not hold a table of fixed-size entries.

例項

手拆ELF（一） 知道節區表的偏移地址為0x000019a8，節區頭大小為40位元組，有29個節區頭，節區頭表大小為29*40=1160=0x488位元組。

young@ubuntu:~/c/elf$ readelf -S elf32
There are 29 section headers, starting at offset 0x19a8:

Section Headers:
  [Nr] Name              Type            Addr     Off    Size   ES Flg Lk Inf Al
  [ 0]                   NULL            00000000 000000 000000 00      0   0  0
  [ 1] .interp           PROGBITS        00000154 000154 000013 00   A  0   0  1
  [ 2] .note.ABI-tag     NOTE            00000168 000168 000020 00   A  0   0  4
  [ 3] .note.gnu.build-i NOTE            00000188 000188 000024 00   A  0   0  4
  [ 4] .gnu.hash         GNU_HASH        000001ac 0001ac 000020 04   A  5   0  4
  [ 5] .dynsym           DYNSYM          000001cc 0001cc 000130 10   A  6   1  4
  [ 6] .dynstr           STRTAB          000002fc 0002fc 000136 00   A  0   0  1
  [ 7] .gnu.version      VERSYM          00000432 000432 000026 02   A  5   0  2
  [ 8] .gnu.version_r    VERNEED         00000458 000458 0000a0 00   A  6   3  4
  [ 9] .rel.dyn          REL             000004f8 0004f8 000040 08   A  5   0  4
  [10] .rel.plt          REL             00000538 000538 000068 08  AI  5  22  4
  [11] .init             PROGBITS        000005a0 0005a0 000023 00  AX  0   0  4
  [12] .plt              PROGBITS        000005d0 0005d0 0000e0 04  AX  0   0 16
  [13] .plt.got          PROGBITS        000006b0 0006b0 000010 08  AX  0   0  8
  [14] .text             PROGBITS        000006c0 0006c0 000434 00  AX  0   0 16
  [15] .fini             PROGBITS        00000af4 000af4 000014 00  AX  0   0  4
  [16] .rodata           PROGBITS        00000b08 000b08 000089 00   A  0   0  4
  [17] .eh_frame_hdr     PROGBITS        00000b94 000b94 00005c 00   A  0   0  4
  [18] .eh_frame         PROGBITS        00000bf0 000bf0 000180 00   A  0   0  4
  [19] .init_array       INIT_ARRAY      00001e9c 000e9c 000004 04  WA  0   0  4
  [20] .fini_array       FINI_ARRAY      00001ea0 000ea0 000004 04  WA  0   0  4
  [21] .dynamic          DYNAMIC         00001ea4 000ea4 000108 08  WA  6   0  4
  [22] .got              PROGBITS        00001fac 000fac 000054 04  WA  0   0  4
  [23] .data             PROGBITS        00002000 001000 000008 00  WA  0   0  4
  [24] .bss              NOBITS          00002008 001008 000004 00  WA  0   0  1
  [25] .comment          PROGBITS        00000000 001008 000029 01  MS  0   0  1
  [26] .symtab           SYMTAB          00000000 001034 000520 10     27  43  4
  [27] .strtab           STRTAB          00000000 001554 000357 00      0   0  1
  [28] .shstrtab         STRTAB          00000000 0018ab 0000fc 00      0   0  1
Key to Flags:
  W (write), A (alloc), X (execute), M (merge), S (strings), I (info),
  L (link order), O (extra OS processing required), G (group), T (TLS),
  C (compressed), x (unknown), o (OS specific), E (exclude),
  p (processor specific)