linux - How the share library be shared by different processes?

Question

I read some documents that share library comiled with -fPIC argument, the .text seqment of the .so will be shared at process fork's dynamic linking stage (eq. the process will map the .so to the same physical address)

i am interested in who (the kernel or ld.so ) and how to accomplish this? maybe i should trace the code, but i dont know where to start it.

Nevertheless, i try to verify the statement.
I decide to check the function address like printf which is in the libc.so that all c program will link. I get the printf virtual address of the process and need to get the physical address. Tried to write a kernel module and pass the address value to kernel, then call virt_to_phys. But it did not work cause the virt_to_phys only works for kmalloc address.

So, process page table look-at might be the solution to find the virtual address map to physical address. Were there any ways to do page table look-at? Or othere ways can fit the verify experiment?

thanks in advance!

Answer 1

The dynamic loader uses mmap(2) with MAP_PRIVATE and appropriate permissions. You can see what it does exactly by running a command from strace -e file,mmap. For instance:

strace -e file,mmap ls 

All the magic comes from mmap(2). mmap(2) creates mappings in the calling process, they are usually backed either by a file or by swap (anonymous mappings). In a file-backed mapping, MAP_PRIVATE means that writes to the memory don't update the file, and cause that page to be backed by swap from that point on (copy-on-write).

The dynamic loader gets the info it needs from ELF's program headers, which you can view with:

readelf -l libfoo.so

From these, the dynamic loader determines what to map as code, read-only data, data and bss (zero-filled segment with zero size in file, non-zero size in memory, and a name only matched in crypticness by Lisp's car and cdr).

So, in fact, code and also data is shared, until a write causes copy-on-write. That is why marking constant data as constant is a potentially important space optimization (see DSO howto).

You can get more info on the mmap(2) manpage, and in Documentation/nommu-mmap.txt (the MMU case, no-MMU is for embedded devices, like ADSL routers and the Nintendo DS).