linux-kernel - Re: [2.6.22.6] nfsd: fh_verify() `malloc failure' with lots of free memory leads to NFS hang

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Message-ID: <873ax7ua9j.fsf@hades.wkstn.nix>
Date:	Fri, 21 Sep 2007 19:46:32 +0100
From:	Nix <nix@...eri.org.uk>
To:	"J. Bruce Fields" <bfields@...ldses.org>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [2.6.22.6] nfsd: fh_verify() `malloc failure' with lots of free memory leads to NFS hang

On 18 Sep 2007, J. Bruce Fields told this:
> Also I suppose we should check which version of nfs-utils that fix is in
> and make sure distributions are getting the fixed nfs-utils before they
> get the new libc, or we're going to see this bug a lot....

Further info. This behaviour, although it is allowed by POSIX, is not
*specified*, as such, by it (ah! ambiguity! life would be so dull
without you!), and is not replicated by any other C library on the face
of the planet.

Further, it's not even the behaviour of glibc, as such.

It's the behaviour of glibc 2.6.x as patched with this Debian-specific
patch:

,----[ any/submitted-fileops-and-signals.diff ]
| Index: glibc-2.6/libio/fileops.c
| ===================================================================
| --- glibc-2.6.orig/libio/fileops.c
| +++ glibc-2.6/libio/fileops.c
| @@ -517,10 +517,19 @@
|      fp->_cur_column = INTUSE(_IO_adjust_column) (fp->_cur_column - 1, data,
|  						 count) + 1;
|    _IO_setg (fp, fp->_IO_buf_base, fp->_IO_buf_base, fp->_IO_buf_base);
| -  fp->_IO_write_base = fp->_IO_write_ptr = fp->_IO_buf_base;
| -  fp->_IO_write_end = (fp->_mode <= 0
| +  if (__builtin_expect (count == to_do, 1))
| +    {
| +      fp->_IO_write_base = fp->_IO_write_ptr = fp->_IO_buf_base;
| +      fp->_IO_write_end = (fp->_mode <= 0
|  		       && (fp->_flags & (_IO_LINE_BUF+_IO_UNBUFFERED))
|  		       ? fp->_IO_buf_base : fp->_IO_buf_end);
| +    }
| +  else if (count > 0 && fp->_IO_write_base == data)
| +    {
| +      memmove (data, data+count, 
| +               fp->_IO_write_ptr - fp->_IO_write_base - count);
| +      fp->_IO_write_ptr -= count;
| +    }
|    return count;
|  }
|  
`----

This patch also breaks at least bash and zsh as well as nfs-utils (see
<http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=429021>) and I sort of
suspect it's not long for this world as a result.

So this is much less nasty than we feared (well, and more nasty if you
happen to be a Debian unstable or testing user).

-- 
`Some people don't think performance issues are "real bugs", and I think 
such people shouldn't be allowed to program.' --- Linus Torvalds
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