How to view the section layout info recorded in an ELF file? - ld

Question

Suppose the following linker script is used to layout an executable file.
SECTIONS
{
. = 0x10000;
.text : { *(.text) }
.data : { *(.data) }
.bss : { *(.bss) }
}
OUTPUT_FORMAT(ELF)
My questions are:
I think such layout info must be stored in the output ELF file so the loader can load the executable based on that info. Right?
If 1 is true, how to view such layout info in a ELF? objdump?

Answer 1

I think such layout info must be stored in the output ELF file so the loader can load the executable based on that info. Right?
ELF stands for executable and linking format.
The .text, .data etc. are part of the linking format. While they usually are present in the fully-linked binary, they can be stripped.
The info that the loader needs to load the executable is stored as a table of segments (Elf{36,64}_Phdr[]).
If 1 is true, how to view such layout info in a ELF? objdump?
You can examine segments (and, if the optional section table is present, the mapping of sections to segments) with readelf -Wl a.out.

Answer 2

You can deduce the information from the linker command script by taking a look at the output of the -S option in readelf.
$ readelf -S $(which ls)
There are 28 section headers, starting at offset 0x1f6f8:
Section Headers:
[Nr] Name Type Address Offset
Size EntSize Flags Link Info Align
[ 0] NULL 0000000000000000 00000000
0000000000000000 0000000000000000 0 0 0
[ 1] .interp PROGBITS 0000000000400238 00000238
000000000000001c 0000000000000000 A 0 0 1
[ 2] .note.ABI-tag NOTE 0000000000400254 00000254
0000000000000020 0000000000000000 A 0 0 4
[ 3] .note.gnu.build-i NOTE 0000000000400274 00000274
0000000000000024 0000000000000000 A 0 0 4
[ 4] .gnu.hash GNU_HASH 0000000000400298 00000298
0000000000000104 0000000000000000 A 5 0 8
[ 5] .dynsym DYNSYM 00000000004003a0 000003a0
0000000000000c48 0000000000000018 A 6 1 8
[ 6] .dynstr STRTAB 0000000000400fe8 00000fe8
0000000000000582 0000000000000000 A 0 0 1
[ 7] .gnu.version VERSYM 000000000040156a 0000156a
...
If you want to know where this comes from, take a look at the default linker command scripts for your toolchain; this default is used if you did not explicitly create your own. I think there may be some inheritance between them as well, but I can't say for certain.
$ ls -l /usr/lib/ldscripts/ | grep elf
-rw-r--r-- 1 root root 9027 Mar 22 04:05 elf32_x86_64.x
-rw-r--r-- 1 root root 8880 Mar 22 04:05 elf32_x86_64.xbn
-rw-r--r-- 1 root root 8706 Mar 22 04:05 elf32_x86_64.xc
-rw-r--r-- 1 root root 9024 Mar 22 04:05 elf32_x86_64.xd
-rw-r--r-- 1 root root 8720 Mar 22 04:05 elf32_x86_64.xdc
-rw-r--r-- 1 root root 8680 Mar 22 04:05 elf32_x86_64.xdw
-rw-r--r-- 1 root root 9027 Mar 22 04:05 elf32_x86_64.xn
-rw-r--r-- 1 root root 5443 Mar 22 04:05 elf32_x86_64.xr
-rw-r--r-- 1 root root 8551 Mar 22 04:05 elf32_x86_64.xs
-rw-r--r-- 1 root root 8247 Mar 22 04:05 elf32_x86_64.xsc
-rw-r--r-- 1 root root 8207 Mar 22 04:05 elf32_x86_64.xsw
-rw-r--r-- 1 root root 5489 Mar 22 04:05 elf32_x86_64.xu
-rw-r--r-- 1 root root 8666 Mar 22 04:05 elf32_x86_64.xw
-rw-r--r-- 1 root root 8396 Mar 22 04:05 elf_i386.x
-rw-r--r-- 1 root root 8249 Mar 22 04:05 elf_i386.xbn
-rw-r--r-- 1 root root 8209 Mar 22 04:05 elf_i386.xc
-rw-r--r-- 1 root root 8389 Mar 22 04:05 elf_i386.xd
-rw-r--r-- 1 root root 8219 Mar 22 04:05 elf_i386.xdc
...

Answer 3

I searched a bit and found something.
For 1. According ELF format on wikipedia, there are program headers that record the address layout info.
For 2. readelf is a promising tool.