Description of problem:

The kernel build system allows CONFIG_IPV6=y, which selects CONFIG_XFRM_NALGO=y, which is dependent upon CONFIG_CRYPTO_API.  Ufortunately, CONFIG_CRYPTO_API can be independently configured as a loadable module, and is by Red Hat default.  The result is a kernel link failure due to the missing symbol crypto_has_alg().

Version-Release number of selected component (if applicable):

2.6.18-92.1.10.el5.i686

How reproducible:

Always.

Steps to Reproduce:
1. Unpack the 2.6.18-92.1.10.el5 source RPM, prep as usual
2. Opt (I used "make menuconfig") to build the IPv6 driver into the static kernel image
3. Build the kernel
  
Actual results:

  GEN     .version
  CHK     include/linux/compile.h
  UPD     include/linux/compile.h
  CC      init/version.o
  LD      init/built-in.o
  LD      .tmp_vmlinux1
net/built-in.o: In function `xfrm_find_algo':
/usr/src/linux-2.6.18-92.1.10.el5.sws/net/xfrm/xfrm_nalgo.c:450: undefined reference to `crypto_has_alg'
make: *** [.tmp_vmlinux1] Error 1

Expected results:

The kernel configuration system should force a dependant driver to build statically when another driver that depends on it is statically built.  Or disallow a static build if a dependant module is built for dynamic loading.  Either way, the symbols of all statically-built device drivers must reconcile at link time.

Additional info:

[boilerplate_disclaimer]
Yes, I am all too aware that Red Hat doesn't support user-configured kernels.
[/boilerplate_disclaimer]

That said, I think you will want to know about a hidden dependency between device drivers.

FYI.