CODEWITHSUNDEEP
elf
Vasant K
Vasant K

Reputation: 125

ELF unmapped region in a segment

Below is the output for my readelf -l test

Program Headers:                   
Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg Align
LOAD           0x000000 0x00008000 0x00008000 0x00148 0x00148 R E 0x8000
LOAD           0x000148 0x00010148 0x00010148 0x00000 0x00004 RW  0x8000
NOTE           0x0000b4 0x000080b4 0x000080b4 0x00024 0x00024 R   0x4
GNU_STACK      0x000000 0x00000000 0x00000000 0x00000 0x00000 RW  0x4
Section to Segment mapping:
Segment Sections...
00     .note.gnu.build-id .text 
01     .bss 
02     .note.gnu.build-id 
03

My question is about the first LOAD segment. It encompasses [8000 - 8148], and is mapped to sections .note and .text. My readelf -S output shows that .note section starts from 80b4, and .text starts from 80d8. That means Loadable segment contains a region [8000-80b3] which is unmapped to any section, but still will be loaded to memory by loader.

My question is, if there is any harm if I create a new segment which ranges from[80b4-8148] deleting this segment?

Upvotes: 0

Views: 282

Answers (1)

Employed Russian
Employed Russian

Reputation: 213386

That means Loadable segment contains a region [8000-80b3] which is unmapped to any section, but still will be loaded to memory by loader.

Correct. You will find the Elf32_Ehdr, and likely a set of Elf32_Phdrs in that segment.

Note: for the main binary, it's actually the kernel that does the loading, and not the dynamic linker. You are not wrong in calling it "loader", but usually people use "loader" for the dynamic linker, and not the "part of the kernel that maps in the main binary".

My question is, if there is any harm if I create a new segment which ranges from[80b4-8148] deleting this segment?

The segment has to be page-aligned. A segment with .p_vaddr that is not page-aligned (as I believe you are proposing) will be rejected by the kernel.

Upvotes: 1

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