Linux shared library download address

Question

I have serious doubts about the shared library. I learned that the virtual address of the library that will be shared by different processes will be the same for all these processes. But I tried to learn the same thing with the proc file system with the following set of commands:

$ cat /proc/*/maps | grep /lib/libc-2.12.1.so 

The output was:

 0025a000-003b1000 r-xp 00000000 08:07 1046574 /lib/libc-2.12.1.so 003b1000-003b2000 ---p 00157000 08:07 1046574 /lib/libc-2.12.1.so 003b2000-003b4000 r--p 00157000 08:07 1046574 /lib/libc-2.12.1.so 003b4000-003b5000 rw-p 00159000 08:07 1046574 /lib/libc-2.12.1.so 0086d000-009c4000 r-xp 00000000 08:07 1046574 /lib/libc-2.12.1.so 009c4000-009c5000 ---p 00157000 08:07 1046574 /lib/libc-2.12.1.so 009c5000-009c7000 r--p 00157000 08:07 1046574 /lib/libc-2.12.1.so 009c7000-009c8000 rw-p 00159000 08:07 1046574 /lib/libc-2.12.1.so 00110000-00267000 r-xp 00000000 08:07 1046574 /lib/libc-2.12.1.so 00267000-00268000 ---p 00157000 08:07 1046574 /lib/libc-2.12.1.so 00268000-0026a000 r--p 00157000 08:07 1046574 /lib/libc-2.12.1.so 0026a000-0026b000 rw-p 00159000 08:07 1046574 /lib/libc-2.12.1.so 00485000-005dc000 r-xp 00000000 08:07 1046574 /lib/libc-2.12.1.so 005dc000-005dd000 ---p 00157000 08:07 1046574 /lib/libc-2.12.1.so 005dd000-005df000 r--p 00157000 08:07 1046574 /lib/libc-2.12.1.so 005df000-005e0000 rw-p 00159000 08:07 1046574 /lib/libc-2.12.1.so 00110000-00267000 r-xp 00000000 08:07 1046574 /lib/libc-2.12.1.so 00267000-00268000 ---p 00157000 08:07 1046574 /lib/libc-2.12.1.so 00268000-0026a000 r--p 00157000 08:07 1046574 /lib/libc-2.12.1.so 0026a000-0026b000 rw-p 00159000 08:07 1046574 /lib/libc-2.12.1.so 00181000-002d8000 r-xp 00000000 08:07 1046574 /lib/libc-2.12.1.so 002d8000-002d9000 ---p 00157000 08:07 1046574 /lib/libc-2.12.1.so 002d9000-002db000 r--p 00157000 08:07 1046574 /lib/libc-2.12.1.so 002db000-002dc000 rw-p 00159000 08:07 1046574 /lib/libc-2.12.1.so 00110000-00267000 r-xp 00000000 08:07 1046574 /lib/libc-2.12.1.so 00267000-00268000 ---p 00157000 08:07 1046574 /lib/libc-2.12.1.so 00268000-0026a000 r--p 00157000 08:07 1046574 /lib/libc-2.12.1.so 0026a000-0026b000 rw-p 00159000 08:07 1046574 /lib/libc-2.12.1.so 0013c000-00293000 r-xp 00000000 08:07 1046574 /lib/libc-2.12.1.so 00293000-00294000 ---p 00157000 08:07 1046574 /lib/libc-2.12.1.so 00294000-00296000 r--p 00157000 08:07 1046574 /lib/libc-2.12.1.so 00296000-00297000 rw-p 00159000 08:07 1046574 /lib/libc-2.12.1.so 00bf7000-00d4e000 r-xp 00000000 08:07 1046574 /lib/libc-2.12.1.so 00d4e000-00d4f000 ---p 00157000 08:07 1046574 /lib/libc-2.12.1.so 00d4f000-00d51000 r--p 00157000 08:07 1046574 /lib/libc-2.12.1.so 00d51000-00d52000 rw-p 00159000 08:07 1046574 /lib/libc-2.12.1.so 00227000-0037e000 r-xp 00000000 08:07 1046574 /lib/libc-2.12.1.so 0037e000-0037f000 ---p 00157000 08:07 1046574 /lib/libc-2.12.1.so 0037f000-00381000 r--p 00157000 08:07 1046574 /lib/libc-2.12.1.so 

Virtual addresses are different for the same shared library for different processes.

Can someone explain to me why this is so?

Answer 1

the ld.so shared library ld.so can change the virtual addresses in which the library is loaded, depending on the needs of the binary file, since the size of code, data, and other sections can vary from one binary to another. The process of reordering an address space is called relocation .

Moving is also the reason you have to compile shared libraries as position-independent code with gcc -fPIC .

Answer 2

The same virtual address for shared libraries is a.out days. ELF does not have this limitation.

In addition, the kernel randomizes the display layout (see ASLR).