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SubjectRe: Makefile gcc -o /dev/null: the dissapearing of /dev/null
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On Saturday 29 September 2001 14:21, Jan-Benedict Glaw wrote:

> No-go. It's perfectly okay to remove an opened file. Test it yourself.
> You may even replace a running (!) executable...

Which isn't actually too frightening since the filehandle of an open file
counts as a link to the inode, so the inode won't actually be removed until
that file handle is closed. The associated disk space won't be freed until
then either, etc. And if you create a new file with the same name, it
doesn't change which inode your executable is currently bound to. It's like
renaming your existing file to something else and renaming some other file to
have that original name. Who cares? That's not the inode our file handle
points to. Deleting the last directory link just prevents new filehandles
from being opened against it (except maybe by dup(2), or forking a child that
inherits the parent's file handles...)

Slightly oversimplifying, running an executable opens the file and mmaps the
sucker in to your process space and jumps to the code, page faulting stuff in
as needed. If the directory entry that was used to open it is then renamed
or removed, the process doesn't care. It already has the file open, it's
happy.

This is also part of the magic (in linux/fs/exec.c) that makes shared
libraries work. mmaping in the same file twice can share physical pages
without having to do anything special. One cheap and easy (and ugly) way of
doing writable shared memory is to create a writable temp file, mmap it in
twice (once in each process), and then unlink it from the directory while
keeping it open and mapped in in both processes. It's ugly. It's easy.
It's portable. It's really ugly. It creates unnecessary disk access on a
lot of systems, but then so does swapping...

Now opening an executable and WRITING to the file while it was running would
be fairly unpleasant. I believe changing its contents would change stuff in
memory, because of the mmap. (Correct me if I'm wrong.) But I seriously
doubt that's allowed.

The situation is slightly confused since so much stuff does an
open/truncate/write instead of an unlink/create/write. But if it didn't,
stuff like "cp bootnet.img /dev/fd0" wouldn't work as expected, and "echo
blah > /dev/null" would delete /dev/null and replace it with a short text
file. Not good. Since truncate on block devices is ignored, the above two
examples work as expected. (It also automatically maintains ownership and
permissions and such when editing other people's files that are world
writable or being twiddled by root. And there's no race condition between
deleting the old file and creating the new one...)

Rob
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