Re: [PATCH] Add getmntent_r

[Corinna Vinschen <corinna-cygwin at cygwin dot com>]

On Jul 18 05:21, Yaakov (Cygwin/X) wrote:
> Somehow this one fell through the cracks.  Picking up where we left off:
> 
> On 2012-06-06 02:33, Corinna Vinschen wrote:
> >On Jun  5 21:29, Yaakov (Cygwin/X) wrote:
> >>The string returned into buf is in the following format:
> >>
> >>mnt_fsname\0mnt_dir\0mnt_type\0mnt_opts\0mnt_freq" "mnt_passno\0
> >
> >Yes, but this is not something inherent to the functionality of
> >getmntent_r, it's just residue from the way getmntent_r works.  It reads
> >a line from /etc/fstab or /etc/mtab into the buffer via fgets:
> >
> >   mnt_fsname\tmnt_dir\tmnt_type\tmnt_opts\tmnt_freq\n
> >
> >and then creates the content of mntbuf from there, replacing the \t with
> >\0 as it goes along.  So it starts with mnt_opts and mnt_freq strings in
> >buf, but only to sscanf them into their respective mntbuf->mnt_opts and
> >mntbuf->mnt_freq members.
> 
> Since glibc is getting this information from the kernel, that makes sense.
> 
> >In case of Cygwin this is not needed since we don't read from the file
> >but from the internal datastructure.  There's no reason to create
> >garbage in buf just because this is by chance the layout the buffer gets
> >when operating under Linux.
> >
> >The *important* thing is that buf contains the strings the members of
> >mntbuf points to.
> 
> OK, revised patch attached.

Thanks.  Applied with a tweak:  It's really not necessary at all to
create strings for mnt_freq and mnt_passno in buf.  Just copy them
over from mnt to mntbuf and be done with it.

> >>glibc makes no attempt to verify buf or mntbuf; if either of them
> >>are not initialized or too small, you're in "undefined behaviour"
> >>territory (aka SEGV :-).
> >
> >You're basically right.  But it won't SEGV if buf is too small.
> 
> By "too small", I meant sizeof(buf) < buflen, or sizeof(mntbuf) <
> sizeof(struct mntent); then glibc definitely does SEGV.

That's right.  I meant the fact that buflen is just too small to keep
all strings.


Thanks again,
Corinna

-- 
Corinna Vinschen                  Please, send mails regarding Cygwin to
Cygwin Project Co-Leader          cygwin AT cygwin DOT com
Red Hat