Is there a Linux system call to create a “view” of a range of a file?

Question

Does Linux provide a system call which can create a "view" of a limited byte range of a backing file? I'm envisioning something that for example would act on an open file descriptor and either modify it or generate a new file descriptor where file offsets are relative to the beginning of the range and end at the end of the range.

The use-case would be to limit a non-cooperating subprocess to accessing only a particular portion of an input file.

@Gilles I was avoiding a pipe (named or otherwise) because I wanted to avoid the extra copy of data through userspace, and thought I'd need the subprocess to seek arbitrarily in its constrained input. But now that you've mentioned it, a pipe plus the splice() system call seems like it would be nearly right for the stream case. — llasram, Feb 27 '19 at 20:26
While the accepted answer works, it's not really a "system call" as the question asked. For system calls, the mmap() call comes to mind, as long as the subprocess is spawned from the process that does the mapping... — twalberg, Feb 27 '19 at 21:20
See also mandatory file locking where you could lock a region of the file so that process could not read or write it. — Stéphane Chazelas, Feb 27 '19 at 23:27

score 12 · Accepted Answer · edited Feb 27 '19 at 17:16

12

One way of doing this is to use a loop device. This approach does have two requirements which may make it less useful: you need to be root to set it up, and the non-cooperating subprocess must be able to write to a block device. Oh, and it doesn’t deal with conflicting changes.

To set the loop device up, run

losetup -o 1024 --sizelimit 2048 --show -f yourfile

replacing 1024, 2048 and yourfile with appropriate values — -o specifies the start offset, --sizelimit the size (counting from the offset). Note that sizelimit has to be a multiple of 512.

This will output the name of the loop device which has been set up; adjust the permissions as necessary, and give it to your non-cooperating sub-process. When you no longer need the device, delete it with

losetup -d /dev/loopN

replacing N as appropriate.

edited Feb 27 '19 at 17:16

Stéphane Chazelas

544,893

answered Feb 27 '19 at 17:08

Stephen Kitt

434,908

Hmm, what do you mean with conflicting changes? Do you mean writes that come through the loop device not being in sync with direct writes to the backing file? – ilkkachu Feb 27 '19 at 18:43
Not 100% what I wanted, but in the absence of anything closer I'll take it. Thanks! – llasram Feb 27 '19 at 19:32
@ilkkachu yes, I mean that there needs to be some “agreement” between the processes about who is allowed to write. (It can be simple enough — if the “parent” process doesn’t write while the loop device is set up for the non-cooperating subprocess, everything will be fine.) – Stephen Kitt Feb 27 '19 at 19:33
@llasram right, I know this isn’t perfect; the FIFO approach in Gilles’ link is interesting to look at too. – Stephen Kitt Feb 27 '19 at 19:34

Is there a Linux system call to create a “view” of a range of a file?

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