linker and loader upload
TRANSCRIPT
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Linker and Loader
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Agenda
• What is Linker and Loader• ELF Format• Static Linker vs Dynamic Linker• Run An Executable File• Backup
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What is Linker and Loader
A programming tool which combines one or more partial Object Files and libraries into a (more) complete executable object file.
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Compile Process
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ELF – Three Types
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ELF – Format
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ELF – Two Views
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ELF – Two Views
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ELF - Sections
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• Allocator Space• Resolve symbols• Relocation symbols• Create sections
Static Linker
Using ld in binutils
Printf.oStrlen.oRand.o
foo.ofoo2.ofoo3.o
Static Linker
Main.o
Main.oPrintf.oFoo.o
Test executable file
Run
process
Main.oPrintf.oFoo.o
Test executable file
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Static Linker - Allocator Space
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Static Linker – Resolve symbols
Scans input relocatable files from left to right as on command line• Maintains Set E of object files req to form
executable. Set U of unresolved symbols Set D of symbols defined in prev files.• Update E,U and D while scanning input
relocatable files• U must be empty at the end – contents of E
used to form executable
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Static Linker - RelocationA process of assigning load addresses to various parts of a program and adjusting the code and data in the program to reflect the assigned addresses
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Static Linker – Relocation
.rel .text .symtab .strtab
Find and iterate .rel sections. Every entry includes of
Typedef struct {Elf32_Addr r_offset;Elf32_Word r_info;} Elf32_Rel
Find the address in .text section by r_offset
Find the symbol index in .symbol section by
r_info
Find the symbol type by r_info
Got information about the responding symbol
in .symbol sectioin
Compute the latest address by symbol type
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Static Linker – Linker scriptBe passed to GNU ld to exercise greater control over the linking process
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ld –static crt1.o crti.o crtbeginT.o XXX.o –start-group –lgcc –lgcc_eh –lc-end-group crtend.o crtn.o
Static Linker –Init/Finit Sections
Object Definition Owner RuntimeCrt1.o _start Glibc C、 C++Crti.o .init and .finit section Glibc CCrtn.o .init and .finit section Glibc CcrtbeginT.o .init_array and .finit_array
sectionGcc C++
Crtend.o .init_array and .finit_array section
Gcc C++
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Static Linker –Init/Finit Sections
Crti.o.init
.finit
a.o.init
.finit
b.o.init
.finit
Crtn.o.init
.finit
A.out
.init
.finit
Linker
Contain the code of beginning for
init function
Contain the code of beginning for finit function
Contain the code of end for init
functionContain code of to init global variable
Contain code of to de-init global
variableContain the code of end for finit
functionInit function
FInit function
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Static Linker –Init_array/Finit_array Sections
CRTBeginT.o.init_array
.finit_array
a.o.init_array
.finit_array
b.o(b.cpp)
.init_array
.finit_array
CrtEnd.o
.init_array
.finit_arrayA.out
.init_array
.finit_array
Linker
Contain the code of beginning for
init_array functionContain the code of beginning for
finit_array function
Contain the code of end for
init_array functionContain pointer to point global constructor
Contain the code of end for finit_array function
Pointer table.text
Global constructor
Global de-constructor
Contain pointer to point global de-
constructor
pointer1
pointer2pointer3
NumberCollect all pointers who point global constructor into
an array
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Dynamic Linker
Printf.oStrlen.oRand.o
Libc.so
foo.ofoo2.ofoo3.o
Libtest.so Dynamic Linker
Main.o
Main.oRequire
so
Test executable file
Main.oRequire
so
Test executable file
Printf.oStrlen.oRand.o
Libc.so
foo.ofoo2.ofoo3.o
Libtest.so
Run
process
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Dynamic Linker - PIC
Position-independent code (PIC) or position-independent executable (PIE) is a body of machine code that, being placed somewhere in the primary memory,
Compile option: -fpic
Code section can be un-modified when load into memory so that it can be shared in different process
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Dynamic Linker - PLTStands for Procedure Linkage Table which is, put simply, used to call external procedures/functions whose address isn't known in the time of linking, and is left to be resolved by the dynamic linker at run time.
First Cal l Second Call
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Dynamic Linker - PIE
Compile option: -fpie
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• Implicit Referred to as static load or load-time dynamic linking.
• Explicit Referred to as dynamic load or run-time dynamic linking.
Dlopen Dlsym Dlclose
Dynamic Linker – Two Mode
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Dynamic Linker vs Static Linker
Dynamic Static
Memory usage Small Bigger
Storage usage Small Bigger
Performance Low (But with PLT, 5% down)
Update easy Hard
Compatibility Hard No
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• Implicit
Prelink
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Run Executable File – Static Linker
• Fork• Exec• Kernel load executable file• Run from the start entry of
executable
Run a executable fileLoad and Map executable file（load_elf_bina
ry)
Do_execve system call
Kernel
Read header of executable file
Read other headers of
executable file
NO
User space
__libc_init
Do_execve system call finished
Return address is set to entry of executable file
Fork process
New Process
If .interp is existing, read the
ld.so(system/bin/linker)
Run the executable file
from entry point(_start)
main
exit
Running and quit
Init main thread
Init globals
system_properties_init
Call preinit_arra
y and init_array
Set __cxa_atexi
t
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Run Executable File – Dynamic Linker
• Fork• Exec• Kernel load executable file• Kernel load ld.so• Ld.so load the dependent so• Relocation• Run from the start entry of
executable
Run a executable file who relies on other so
Load and Map executable file（load_elf_bina
ry)
Do_execve system call
Kernel
Read header of exe file
Got the path for linker and map it to get entry
point
Yes
User space
main
Return address is set to entry of linker
Fork process
New Process
If .interp is existing, read the ld.so(system/bin/linker)
Run the executable file
from entry point(_start)
exit
Running and quit
Do_execve system call finished
_start -> __linker_init t
Init main thread
Init globals
call_constructors(DT_I
NIT DT_INIT_AR
RAY)
__libc_preinit
__libc_init
prelink_image and
link_image for linker
__linker_init_post_relocation
debuggerd_init
Init globals
init_default_namespac
e
__system_properties_
init
Load needed .So one by one
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Process Memory Map
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Backup
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• Gcc drivers the compile process.• Glibc provide runtime/libc/libm/libthread …• Binutils provides as/ar/ld
Relation Between Gcc, Glibc and Binutils
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• GPL License• 400K• BinUtils• Gcc• Glibc• Multi-thread• LD(BinUtils)• Loader• System Call
Glibc vs Android Bionic
• APACHE License• 200 K• Ld• Linker• pThread• System Call
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• Linker Ld under Linux
• Loader /system/bin/linker
Linker and Loader in Android
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Android Java loader
System.loadlibrary
Runtime.loadlibrary
Runtime.doLoad
JVM_NativeLoad
LoadNativeLibrary
dlopen
nativeLoad