Re: 2.4 bug: fifo-write causes diskwrites to read-only fs !

[Rob van Nieuwkerk (robn@verdi.et.tudelft.nl)]

Hi Richard,

> > > > > It turns out that Linux is updating inode timestamps of fifos (named
> > > > > pipes) that are written to while residing on a read-only filesystem.
> > > > > It is not only updating in-ram info, but it will issue *physical*
> > > > > writes to the read-only fs on the disk !

> > > FYI, I created a FIFO with mkfifo, remounted the file-system
> > > R/O, executed `cat` with it's input coming from the FIFO, and
> > > then waited for a few minutes. I then wrote to the FIFO.
> > > The atime did not change with 2.4.20.
> >
> > Just did the same here (on my workstation).  And the times *did* change ..
> > More precisely: the "modification" & "change" were updated, the "access"
> > time remained unchanged.
> >
> 
> Okay. I can now verify the problem. There are two problems as this

Yeah !, I'm no longer alone .. :-)
	.
	.
> As you can clearly see, access time (atime) is not changed.
> However, both ctime and mtime are both changed with every
> FIFO access. Since this FIFO is provably on a R/O file system,
> nothing should change.

Note that the fact that you see the times changing in the fs while it
is mounted doesn't imply a problem in itself: serial and tty device
nodes get their time-stamps updated too on a read-only fs when they
are written.  But these changes are in ram only: when you reboot you
get the old values back.

But with FIFOs the changes *do* get written out to the read-only fs !

Hmm, wonder what happens if you try it on a real read-only medium like
a CDR.  Maybe kernel errors/panic ..

> Now, somebody will probably claim that this is the correct
> POSIX defined behavior <sigh> so you might have to make some
> work-around like use a pipe or socket instead of the FIFO??

Seems very stupid to me if POSIX specifies this.
I don't have the POSIX spec, but maybe it specifies what "read-only"
is supposed to mean somewhere too ..

But let's wait & see .. :-)

	greetings,
	Rob van Nieuwkerk
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