Re: B*gg*r mallocs, mmap JOY!

Snow Cat (snowcat@netgate.net)
Mon, 17 Feb 1997 09:55:23 -0800 (PST)

John Carter once wrote:
>
> ftruncate does the job and shuts-up. I like ftruncate. So it is a
> security hole. So what.
>
> So if I'm really curious about what other users are doing on my disk I
> can ftruncate all the free space and grep around looking for old dirt. So
> what. Its my disk and as Pournelle's law says, "One user, one (at
> least) CPU." People are too paranoid. If someone hacks into my machine
> and does the same to me, well, he will discover spying is the most
> boring job in the world and go home and take up street sweeping. I
> _luv_ Linux.
>

This works only on MS-DOG (that needs an lseek and a one-byte write to
extend files anyway). Linux will not allocate blocks immediatelly on
ftruncate. It will pretend that these unallocated holes are all-'\0' and
sets the remainder of a block to 0 when you do try to write it.

Did anyone check NT?

> So its not portable.
>
> True. From various other messages I have recieved and from notes in
> /usr/src/linux/mm/mmap.c there are a lot of nonportable things going
> on.
>

It's semi-portable. POSIX 4 specifies that ftruncate can expand shared
memory objects and it would be real bogus for a modern OS to implement it
differently on files. But anyway, it's pretty easy to write a function that
tries to expand a file with ftruncate one time and if it doesn't work switches
to lseek+1 byte write.

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