crit: add method to retrieve shared memory size of process

[behouba]

This PR aims to add a method for retrieving the size of shared memory of processes. The function iterates over the virtual memory areas (VMAs) associated with the process, summing up the sizes of VMAs with the MAP_SHARED flag.

@rst0git PTAL, is this approach accurate ?

[codecov]

Codecov Report

Patch coverage: 77.50% and project coverage change: +1.01% 🎉

Comparison is base (2210c02) 49.49% compared to head (02d11ab) 50.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #146      +/-   ##
==========================================
+ Coverage   49.49%   50.50%   +1.01%     
==========================================
  Files          24       24              
  Lines        2760     2865     +105     
==========================================
+ Hits         1366     1447      +81     
- Misses       1203     1220      +17     
- Partials      191      198       +7     

Files Changed	Coverage Δ
crit/mempages.go	93.54% <76.92%> (-1.95%)	⬇️
test/crit/main.go	62.28% <77.57%> (+10.07%)	⬆️

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[rst0git]

In Linux we have two types of shared memory: POSIX and System V shared memory.

• With POSIX shared memory, you can use shm_open() to create a shared memory object and mmap() to map the object into the address space of a process.

  • ./zdtm.py run -t zdtm/static/maps01 -f h -k always can be used to create such image files.

• With System V, a shared memory segment can be created with shmget() and attached to the address space of process using shmat().

  • ./zdtm.py run -t zdtm/static/shm -f ns -k always can be used to create image files for testing.

We can identify shared memory in mm-*.img by the use of MAP_SHARED in VMA flags.

sudo crit show mm-46373.img
...
                {
                    "start": "0x7fb90ef70000",
                    "end": "0x7fb90ef73000",
                    "pgoff": 0,
                    "shmid": 3,
                    "prot": "PROT_READ | PROT_WRITE",
                    "flags": "MAP_SHARED",
                    "status": "VMA_AREA_REGULAR | VMA_FILE_SHARED",
                    "fd": -1,
                    "fdflags": "0x2"
                },

sudo crit show mm-56.img
...
                {
                    "start": "0x7f6132b7c000",
                    "end": "0x7f6132b7d000",
                    "pgoff": 81920,
                    "shmid": 3,
                    "prot": "PROT_READ | PROT_WRITE",
                    "flags": "MAP_SHARED",
                    "status": "VMA_AREA_REGULAR | VMA_FILE_SHARED | 0x4000",
                    "fd": -1,
                    "fdflags": "0x2"
                },

[rst0git]

LGTM