Useful Tools

Before discussing performance, it is useful to be aware of some available tools and their use in analyzing the contents of an ELF file.

Frequently reference is made to the size of either the sections or the segments that are defined within an ELF file (for a complete description of the ELF format see Chapter 7, Object Files). The size of a file can be displayed using the size(1) command. For example:

$ size -x libfoo.so.1
59c + 10c + 20 = 0x6c8

$ size -xf libfoo.so.1
..... + 1c(.init) + ac(.text) + c(.fini) + 4(.rodata) + \
..... + 18(.data) + 20(.bss) .....

The first example indicates the size of the shared objects text, data, and bss, a categorization that has traditionally been used throughout previous releases of the SunOS operating system. The ELF format provides a finer granularity for expressing data within a file by organizing the data into sections. The second example displays the size of each of the file's loadable sections.

Sections are allocated to units known as segments, some of which describe how portions of a file will be mapped into memory. These loadable segments can be displayed by using the dump(1) command and examining the LOAD entries. For example:

$ dump -ov libfoo.so.1

libfoo.so.1:
 ***** PROGRAM EXECUTION HEADER *****
Type        Offset      Vaddr       Paddr
Filesz      Memsz       Flags       Align

LOAD        0x94        0x94        0x0
0x59c       0x59c       r-x         0x10000

LOAD        0x630       0x10630     0x0
0x10c       0x12c       rwx         0x10000

Here, there are two segments in the shared object libfoo.so.1, commonly referred to as the text and data segments. The text segment is mapped to allow reading and execution of its contents (r-x), whereas the data segment is mapped to allow its contents to be modified (rwx). Notice that the memory size (Memsz) of the data segment differs from the file size (Filesz). This difference accounts for the .bss section, which is actually part of the data segment.

Programmers, however, usually think of a file in terms of the symbols that define the functions and data elements within their code. These symbols can be displayed using nm(1). For example:

$ nm -x libfoo.so.1

[Index]   Value      Size      Type  Bind  Other Shndx   Name
.........
[39]    |0x00000538|0x00000000|FUNC |GLOB |0x0  |7      |_init
[40]    |0x00000588|0x00000034|FUNC |GLOB |0x0  |8      |foo
[41]    |0x00000600|0x00000000|FUNC |GLOB |0x0  |9      |_fini
[42]    |0x00010688|0x00000010|OBJT |GLOB |0x0  |13     |data
[43]    |0x0001073c|0x00000020|OBJT |GLOB |0x0  |16     |bss
.........

The section that contains a symbol can be determined by referencing the section index (Shndx) field from the symbol table and by using dump(1) to display the sections within the file. For example:

$ dump -hv libfoo.so.1

libfoo.so.1:
           **** SECTION HEADER TABLE ****
[No]    Type    Flags   Addr      Offset    Size      Name
.........
[7]     PBIT    -AI     0x538     0x538     0x1c      .init

[8]     PBIT    -AI     0x554     0x554     0xac      .text

[9]     PBIT    -AI     0x600     0x600     0xc       .fini
.........
[13]    PBIT    WA-     0x10688   0x688     0x18      .data

[16]    NOBI    WA-     0x1073c   0x73c     0x20      .bss
.........

Using the output from both the previous nm(1) and dump(1) examples, the association of the functions _init, foo, and _fini to the sections .init, .text and .fini can be seen. These sections, because of their read-only nature, are part of the text segment.

Similarly, it can be seen that the data arrays data, and bss are associated with the sections .data and .bss respectively. These sections, because of their writable nature, are part of the data segment.

The previous dump(1) display has been simplified for this example.

Armed with this tool information, you can analyze the location of code and data within any ELF file you generate. This knowledge will be useful when following the discussions in later sections.