POSIXfy readlink() call

[Ruslan Ermilov]

On Mon, May 11, 2009 at 04:23:52PM -0400, John Baldwin wrote:
> On Monday 11 May 2009 2:58:14 pm Kostik Belousov wrote:
> > On Mon, May 11, 2009 at 02:46:14PM -0400, John Baldwin wrote:
> > > On Monday 11 May 2009 2:33:09 pm Kostik Belousov wrote:
> > > > On Mon, May 11, 2009 at 02:05:07PM -0400, John Baldwin wrote:
> > > > > On Friday 28 September 2007 10:39:56 pm Ighighi wrote:
> > > >                         ^^^^^
> > > 
> > > Yes, I had this stuck in the back of my head from when it first appeared.
> > > 
> > > > > > The POXIX prototype for readlink(2) is:
> > > > > > ssize_t readlink(const char *restrict path, char *restrict buf, 
> size_t 
> > > > > > bufsize);
> > > > > 
> > > > > It can't simply be corrected as it would change the ABI and thus 
> requires 
> > > a 
> > > > > new system call, etc.  However, do you really expect a symlink to be 
> > > longer 
> > > > > than 2^31 on a 64-bit machine?
> > > > 
> > > > Yes, I agree that this is ABI change.
> > > > 
> > > > Meantime,
> > > > r176215 | ru | 2008-02-12 22:09:04 +0200 (Tue, 12 Feb 2008) | 5 lines
> > > > 
> > > > Change readlink(2)'s return type and type of the last argument
> > > > to match POSIX.
> > > > 
> > > > Prodded by:     Alexey Lyashkov
> > > > 
> > > > I tried to convince ru@ that ABI breakage is not good, but has not
> > > > succeeded.
> > > 
> > > Ugh, is this only in HEAD?  If so, I will back it out for 8.0.  If this 
> made 
> > > it into a release then this is a far bigger mess.  Oh, good, this is only 
> in 
> > > 8.  I will fix this ASAP.  I can just add the new syscall I guess.
> > 
> > You need to symver the syscalls. It requires some ugly games with our
> > syscall stubs, because gnu ld only honor .symver in the same object where
> > the symbol is defined. I did prototyped this some time ago, by including
> > a file with appropriate .symver from all stubs.
> 
> So, after thinking about this out loud some more, it seems the ABI breakage 
> would only be for 64-bit platforms that passed a -ve value as the buffer 
> size.  However, doing so would already either panic due to triggering an 
> assertion, or result in otherwise undefined behavior and that making the new 
> parameter unsigned actually results in the same undefined behavior in the 
> non-panic case.
> 
For the record.  I also suggest (re-)reading a thread

http://lists.freebsd.org/pipermail/freebsd-current/2008-February/thread.html#83314

that resulted from the original commit where I try to make it clear that a
scary ABI breakage Konstantin mentions is pure artificial.


Cheers,
-- 
Ruslan Ermilov
ru at FreeBSD.org
FreeBSD committer