add second elf article

Signed-off-by: innocentzero <md-isfarul-haque@proton.me>

Author: InnocentZero

content/posts/understanding_elfs_2.md

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+title = "Understanding ELFs, part 2"
+date = 2025-01-29
+authors = ["InnocentZero"]
++++
+
+## Segments
+
+Sections gather all the information needed to link a given executable. Segments, on the other
+hand, contain information needed to load the program into memory. Segments can be imagined as
+a tool to make linux loader's life easier, as they group sections by attributes into single
+segments in order to make the loading process more efficient. Otherwise the loader would load
+each individual section into memory independently.
+
+Similar to sections, Segments, also called _Program Headers_, also have a **Program Header Table**
+that lists all the segments. This table is read by the loader and helps map the ELF into memory.
+These can be seen via `readelf -l executable`.
+
+```
+Elf file type is DYN (Position-Independent Executable file)
+Entry point 0x1040
+There are 13 program headers, starting at offset 64
+
+Program Headers:
+  Type           Offset             VirtAddr           PhysAddr
+                 FileSiz            MemSiz              Flags  Align
+  PHDR           0x0000000000000040 0x0000000000000040 0x0000000000000040
+                 0x00000000000002d8 0x00000000000002d8  R      0x8
+  INTERP         0x0000000000000318 0x0000000000000318 0x0000000000000318
+                 0x000000000000001c 0x000000000000001c  R      0x1
+      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
+  LOAD           0x0000000000000000 0x0000000000000000 0x0000000000000000
+                 0x0000000000000630 0x0000000000000630  R      0x1000
+  LOAD           0x0000000000001000 0x0000000000001000 0x0000000000001000
+                 0x0000000000000191 0x0000000000000191  R E    0x1000
+  LOAD           0x0000000000002000 0x0000000000002000 0x0000000000002000
+                 0x00000000000000bc 0x00000000000000bc  R      0x1000
+  LOAD           0x0000000000002dd0 0x0000000000003dd0 0x0000000000003dd0
+                 0x0000000000000248 0x0000000000000250  RW     0x1000
+  DYNAMIC        0x0000000000002de0 0x0000000000003de0 0x0000000000003de0
+                 0x00000000000001e0 0x00000000000001e0  RW     0x8
+  NOTE           0x0000000000000338 0x0000000000000338 0x0000000000000338
+                 0x0000000000000040 0x0000000000000040  R      0x8
+  NOTE           0x0000000000000378 0x0000000000000378 0x0000000000000378
+                 0x0000000000000044 0x0000000000000044  R      0x4
+  GNU_PROPERTY   0x0000000000000338 0x0000000000000338 0x0000000000000338
+                 0x0000000000000040 0x0000000000000040  R      0x8
+  GNU_EH_FRAME   0x0000000000002018 0x0000000000002018 0x0000000000002018
+                 0x0000000000000024 0x0000000000000024  R      0x4
+  GNU_STACK      0x0000000000000000 0x0000000000000000 0x0000000000000000
+                 0x0000000000000000 0x0000000000000000  RW     0x10
+  GNU_RELRO      0x0000000000002dd0 0x0000000000003dd0 0x0000000000003dd0
+                 0x0000000000000230 0x0000000000000230  R      0x1
+
+ Section to Segment mapping:
+  Segment Sections...
+   00     
+   01     .interp 
+   02     .interp .note.gnu.property .note.gnu.build-id .note.ABI-tag .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt 
+   03     .init .plt .text .fini 
+   04     .rodata .eh_frame_hdr .eh_frame 
+   05     .init_array .fini_array .dynamic .got .got.plt .data .bss 
+   06     .dynamic 
+   07     .note.gnu.property 
+   08     .note.gnu.build-id .note.ABI-tag 
+   09     .note.gnu.property 
+   10     .eh_frame_hdr 
+   11     
+   12     .init_array .fini_array .dynamic .got 
+```
+
+## Types of Segments
+
+There are many types of segments. The most common and important ones are:
+
+- `PT_PHDR`: Contains the program header.
+- `PT_LOAD`: Actually loaded in the memory. Every other section is mapped to this.
+- `PT_INTERP`: Holds the `.interp` section responsible for providing the interpreter.
+- `PT_NULL`: First entry of the table, unassigned.
+- `PT_DYNAMIC`: Holds the `.dynamic` section.
+
+
+An interesting thing to note here is the `GNU_STACK` segment in the output. It has a peculiar
+size of 0. This implies that the stack size is decided by the kernel. Its size is always
+ignored and it is just there for permission management.
+
+Another thing to be mentioned is `GNU_EH_FRAME` that specifies the frame unwinding information.
+Usually the same as `.eh_frame_hdr` section.