In PIE binary, code section changes during execution. Why this happens?

Question

Due to the nature of the PIE binary, all data in binary cannot be accessed by absolute address.
So, there are two ways to access the data relatively

TWO Way

1. During execution, loader loads the data's location at GOT entry.
   And binary access to that address.

2. Calculate the address of data using _GLOBAL_OFFSET_TABLE location, and access to it.

But, I saw another way to access data relatively.
In below binary, binary changed <.text> section's code.
It was very weird.

.global main
main:
 push stderr

I compiled it into pie binary.

jiwon@jiwon$ gcc -fPIE -pie -o test test.s
jiwon@jiwon$ objdump -D test_pie | grep "<main>" -A5
000005c0 <main>:
 5c0:   ff 35 00 00 00 00       pushl  0x0
 5c6:   66 90                   xchg   %ax,%ax
 5c8:   66 90                   xchg   %ax,%ax
 5ca:   66 90                   xchg   %ax,%ax

As you can see above disassembly, push stderr assembled into pushl 0x0.
And.. When I execute the binary,

pwndbg> disass /r main
Dump of assembler code for function main:
=> 0x004005c0 <+0>: ff 35 48 18 40 00   push   DWORD PTR ds:0x401848
   0x004005c6 <+6>: 66 90   xchg   ax,ax
   0x004005c8 <+8>: 66 90   xchg   ax,ax
   0x004005ca <+10>:    66 90   xchg   ax,ax

The <.text> section changed to point the stderr!
I think It's very weird because <.text> section has -WX permission in general application.
But In this case, <.text> is RWX permission.

Question:

1. Why this happens? Why compiler choose to use this weird way, instead of above TWO Way?
2. Is this common situation in pie binary..?

Answer 1

Position independent executable is produced by combined work of compiler and linker, not just by the linker. In your case, you passed GCC an already assembled code, probably not what the compiler would have produced with the PIE flag.

The linker is not the one who produces relative load and store instructions, the compiler does that, and the linker makes sure proper relocation sections are placed correctly and such.