Latest working kernel version: 2.6.25.x
Earliest failing kernel version: 2.6.26.3 (probably also 2.6.26.1+2)
Distribution: vanilla kernel
Hardware Environment: n/a
Software Environment: n/a
Problem Description:

When building a kernel with which later out-of-tree modules can be built against, one normally builds the kernel and then runs a "make clean" on the results.  What is left is stripped down but enough to build modules against.  But in 2.6.26.3 this fails because linux/bounds.h is missing (the header is encountered is for example the external module tries to include linux/slab.h).  This bounds.h header is generated by the build process but make clean is erasing it.  It is worth noting that the top-level Makefile has this comment:

# clean - Delete most, but leave enough to build external modules
#

What's happening now violates that principle, i.e. "make clean" is blowing away linux/bounds.h, breaking the ability to later on build external modules.  I've also verified that manually restoring this one missing file also restores the ability to build external modules.

Steps to reproduce:

1. Build the kernel in your usual way.
2. Note the existence of linux/bounds.h - save it somewhere out-of-tree for the moment.
3. Run "make clean"
4. Note that linux/bounds.h is missing.
5. Try to build an external module.  Odds are it will fail due to this missing header.
6. Manually restore linux/bounds.h from your saved copy.
7. Try to build the external module again.  This time it will work.