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<h1 id="ld1"><a class="header" href="#ld1">ld(1)</a></h1>
<pre><code>ld [opts] files...
-T <script> use <script> as linker script
--trace report each file the linker touches
--start-group archives --end-group
search archives repearepeatedly until no new
undefined references are created
(eg helpfull with list of static libraries)
</code></pre>
<h2 id="linker-script"><a class="header" href="#linker-script">Linker Script</a></h2>
<p><code>output</code> sections are defined as follows (full description at <a href="https://sourceware.org/binutils/docs/ld/Output-Section-Attributes.html">output
section</a> and <a href="https://sourceware.org/binutils/docs/ld/Input-Section.html">input section</a>).</p>
<pre><code>section_name [vaddr] : [AT(paddr)] {
file_pattern (section_pattern)
}
</code></pre>
<p>The following gives an example of an <code>output</code> section with two <code>input</code> section rules.</p>
<pre><code>.foo : {
abc.o (.foo)
*.o (.foo.*)
}
</code></pre>
<h3 id="example-virtual-vs-physical-load-address"><a class="header" href="#example-virtual-vs-physical-load-address">Example: virtual vs physical (load) address</a></h3>
<p>Sometimes code is initially located at a different location as when being run.
For example in embedded cases, where code may initially resides in a <em>rom</em> and
startup code will copy a section with writable <em>data</em> into <em>ram</em>. Code accessing
the writable data accesses the data in the <em>ram</em>.</p>
<p>In this case we need different addresses for the same data.</p>
<ul>
<li>The <code>virtual</code> or <em>runtime</em> address, this is the address used when the linker
resolves accesses to the data. Hence, this is the address the data will have
when the code is running.</li>
<li>The <code>physical</code> or <em>load</em> address, this is the address the data is stored at
initially. Startup code typically copies the initial values from the
<code>physical</code> to the <code>virtual</code> address.</li>
</ul>
<p>The following shows an example linker script which uses <em>virtual</em> and <em>physical</em>
addresses. The full source files can be found <a href="https://github.com/johannst/notes/tree/master/src/development/ld">here</a>.</p>
<pre><code>OUTPUT_FORMAT(elf64-x86-64)
ENTRY(_entry)
SECTIONS {
/* Set the initial location counter (vaddr) */
. = 0x00800000;
/* Create .text output section at current vaddr */
.text : {
*(.text*)
}
ASSERT(. == 0x00800000 + SIZEOF(.text), "inc loc counter automatically")
/* Create .data section at location counter aligned to the next 0x100 (vaddr) */
/* Set the load address to 0x00100000 (paddr) */
.data ALIGN(0x100) : AT(0x00100000) {
HIDDEN(_data_vaddr = .);
HIDDEN(_data_paddr = LOADADDR(.data));
*(.data*)
}
/* Create .rodata with explicit vaddr */
/* Re-adjust the paddr location counter */
.rodata 0x00804000 : AT(ADDR(.rodata)) {
*(.rodata*)
}
ASSERT(. == 0x00804000 + SIZEOF(.rodata), "inc loc counter automatically")
.stack ALIGN (0x1000) : {
. += 0x1000;
HIDDEN(_stack_top = .);
}
/DISCARD/ : {
*(.*)
}
}
/* Some example assertions */
ASSERT(ADDR(.data) != LOADADDR(.data), "DATA vaddr and paddr must be different")
ASSERT(SIZEOF(.stack) == 0x1000, "STACK section must be 0x1000")
</code></pre>
<p>We can use the following assembly snippet to explore the linker script.</p>
<pre><code class="language-x86asm">.section .text, "ax", @progbits
.global _entry
_entry:
mov $_stack_top, %rsp
mov $asm_array, %rax
mov (asm_len), %eax
hlt
jmp _entry
.section .data.asm, "aw", @progbits
asm_array:
.4byte 0xa
.4byte 0xb
.4byte 0xc
.4byte 0xd
.rept 4
.4byte 0xff
.endr
.section .rodata.asm, "a", @progbits
asm_len:
.4byte 8
</code></pre>
<blockquote>
<p><code>gcc -c data.S && ld -o link-nomem -T link-nomem.ld data.o</code></p>
</blockquote>
<p>The elf load segments show the difference in <em>physical</em> and <em>virtual</em> address
for the segment containing the <code>.data</code> section.</p>
<pre><code class="language-sh">> readelf -W -l link-nomem
# There are 4 program headers, starting at offset 64
#
# Program Headers:
# Type Offset VirtAddr PhysAddr FileSiz MemSiz Flg Align
# LOAD 0x001100 0x0000000000800100 0x0000000000100000 0x000020 0x000020 RW 0x1000
# LOAD 0x002000 0x0000000000800000 0x0000000000800000 0x000018 0x000018 R E 0x1000
# LOAD 0x003000 0x0000000000804000 0x0000000000804000 0x000004 0x000004 R 0x1000
# LOAD 0x000000 0x0000000000805000 0x0000000000805000 0x000000 0x001000 RW 0x1000
#
# Section to Segment mapping:
# Segment Sections...
# 00 .data
# 01 .text
# 02 .rodata
# 03 .stack
</code></pre>
<p>Startup code could copy data from <code>_data_paddr</code> to <code>_data_vaddr</code>.</p>
<pre><code class="language-sh">> nm link-nomem
# 0000000000800100 d asm_array
# 0000000000804000 r asm_len
# 0000000000100000 a _data_paddr
# 0000000000800100 d _data_vaddr
# 0000000000800000 T _entry
# 0000000000806000 b _stack_top
</code></pre>
<p>The linker resolves symbols to their <em>virtual</em> address, this can be seen by the
access to the <code>asm_array</code> variable.</p>
<pre><code class="language-sh">> objdump -d link-nomem
# Disassembly of section .text:
#
# 0000000000800000 <_entry>:
# 800000: 48 c7 c4 00 60 80 00 mov $0x806000,%rsp
# 800007: 48 c7 c0 00 01 80 00 mov $0x800100,%rax ;; mov $asm_array, %rax
# 80000e: 8b 04 25 00 40 80 00 mov 0x804000,%eax
# 800015: f4 hlt
# 800016: eb e8 jmp 800000 <_entry>
</code></pre>
<p>The following linker script shows an example with the <code>MEMORY</code> command.</p>
<pre><code>OUTPUT_FORMAT(elf64-x86-64)
ENTRY(_entry)
MEMORY {
ROM : ORIGIN = 0x00100000, LENGTH = 0x4000
RAM : ORIGIN = 0x00800000, LENGTH = 0x4000
}
SECTIONS {
/* Create .text output section at ROM (vaddr) */
.text : {
*(.text*)
} > ROM
ASSERT(. == ORIGIN(ROM) + SIZEOF(.text), "inc loc counter automatically")
/* Create .data output section at RAM (vaddr) */
/* Set load addr to ROM, right after .text (paddr) */
.data : {
HIDDEN(_data_vaddr = .);
HIDDEN(_data_paddr = LOADADDR(.data));
*(.data*)
} > RAM AT > ROM
/* Append .rodata output section at ROM (vaddr) */
.rodata : {
*(.rodata*)
} > ROM
/* Append .stack output section at RAM (vaddr) aligned up to next 0x1000 */
.stack : ALIGN (0x1000) {
. += 0x1000;
HIDDEN(_stack_top = .);
} > RAM
/DISCARD/ : {
*(.*)
}
}
/* Some example assertions */
ASSERT(ADDR(.data) != LOADADDR(.data), "DATA vaddr and paddr must be different")
ASSERT(ADDR(.rodata) == LOADADDR(.rodata), "RODATA vaddr and paddr must be euqal")
ASSERT(ADDR(.stack) == ORIGIN(RAM) + 0x1000, "STACK section must aligned to 0x1000")
ASSERT(SIZEOF(.stack) == 0x1000, "STACK section must be 0x1000")
</code></pre>
<h2 id="references"><a class="header" href="#references">References</a></h2>
<ul>
<li><a href="https://sourceware.org/binutils/docs/ld/">ld manual</a></li>
<li><a href="https://sourceware.org/binutils/docs/ld/Input-Section.html">ld script: input sections</a></li>
<li><a href="https://sourceware.org/binutils/docs/ld/Output-Section-Attributes.html">ld script: output sections</a></li>
<li><a href="https://github.com/johannst/notes/tree/master/src/development/ld">notes/ld example files</a></li>
</ul>
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