Mount filesystem image placed in a static variable - c++

Question

It's possible to loop mount a binary file that contains a filesystem image. I'd like to put that binary file into a C static variable, and then mount that. Is this possible? If so, what C API magic do I need?

Answer 1

There are several steps we'll want to take
Create a file system image
Embed this image into the binary
Mount the embedded image as a read-only file system.
It sounds like you already know how to perform stems 1 and 2, but not how to do step 3.
I prepared ab.sqfs image and a.out which contains that image at offset 0x3010. Here are the commands to mount this filesystem:
# optional, look at the bytes of the filesystem from step 1
xxd -l 16 -g1 ab.sqfs
00000000: 68 73 71 73 07 00 00 00 6c 61 ce 60 00 00 02 00 hsqs....la.`....
# optional: confirm that we have the correct file offset to the start of FS image
xxd -l 16 -g1 -s 0x3010 a.out
00003010: 68 73 71 73 07 00 00 00 6c 61 ce 60 00 00 02 00 hsqs....la.`....
# create a loop device which "points" into the file:
sudo losetup -r -o 0x3010 loop0 a.out
losetup: a.out: Warning: file does not fit into a 512-byte sector; the end of the file will be ignored.
# optional: confirm that (just created) /dev/loop0 contains expected bytes
sudo xxd -l 16 -g1 /dev/loop0
00000000: 68 73 71 73 07 00 00 00 6c 61 ce 60 00 00 02 00 hsqs....la.`....
# create directory on which the FS will be mounted
mkdir /tmp/mnt
# finally mount the FS:
sudo mount -oro /dev/loop0 /tmp/mnt
# optional: verify contents of /tmp/mnt
ls -lR /tmp/mnt
... has exactly the files I've put into it.
what C API magic do I need?
You can run the losetup and mount commands under strace to observe what they do. The key steps for losetup are:
openat(AT_FDCWD, "/tmp/a.out", O_RDONLY|O_CLOEXEC) = 3
openat(AT_FDCWD, "/dev/loop0", O_RDONLY|O_CLOEXEC) = 4
ioctl(4, LOOP_SET_FD, 3) = 0
ioctl(4, LOOP_SET_STATUS64, {lo_offset=0x3010, lo_number=0, lo_flags=LO_FLAGS_READ_ONLY, lo_file_name="/tmp/a.out", ...}) = 0
And for mount:
mount("/dev/loop0", "/tmp/mnt", "squashfs", MS_RDONLY, NULL) = 0
These calls can be performed by the application itself, or by "shelling out" to external losetup and mount commands.