On Fri, May 9, 2014 at 3:14 PM, Lucas De Marchi
[off-list ref] wrote:

Since the beginning depmod just warned about dependency loops and upon
creation of modules.dep{,.bin} it skipped the modules that were part of
a loop. However just skipping the modules may come as a surprise to
kernel module developers: they will need to try to load the module (or
to pay attention to the log messages) to notice thavt the module has not
been put in the index. Also, differently from module-init-tools we were
not skipping modules that depend on modules with dependency loops,
leading to a segfault in depmod.

So this is a summary of the change in behavior with this patch:

Loop 1)
    A -> B -> C -
    ^           |
    '------------

    Before:
        depmod: WARNING: found 3 modules in dependency cycles!
        depmod: WARNING: /tmp/test-kmod/lib/modules/3.14.2-1-ARCH/kernel/moduleB.ko in dependency cycle!
        depmod: WARNING: /tmp/test-kmod/lib/modules/3.14.2-1-ARCH/kernel/moduleC.ko in dependency cycle!
        depmod: WARNING: /tmp/test-kmod/lib/modules/3.14.2-1-ARCH/kernel/moduleA.ko in dependency cycle!

        return code: 0

    After:
        depmod: ERROR: Found 3 modules in dependency cycles!
        depmod: ERROR: /tmp/test-kmod/lib/modules/3.14.2-1-ARCH/kernel/moduleB.ko in dependency cycle!
        depmod: ERROR: /tmp/test-kmod/lib/modules/3.14.2-1-ARCH/kernel/moduleC.ko in dependency cycle!
        depmod: ERROR: /tmp/test-kmod/lib/modules/3.14.2-1-ARCH/kernel/moduleA.ko in dependency cycle!

        return code: 2

Loop 2)
    A -> B -> C -
         ^      |
         '-------

    Before:
        depmod: WARNING: found 2 modules in dependency cycles!
        depmod: WARNING: /tmp/test-kmod//lib/modules/3.14.2-1-ARCH/kernel/moduleB.ko in dependency cycle!
        depmod: WARNING: /tmp/test-kmod//lib/modules/3.14.2-1-ARCH/kernel/moduleC.ko in dependency cycle!
        Segmentation fault (core dumped)

    After:
        depmod: ERROR: Found 2 modules in dependency cycles!
        depmod: ERROR: /tmp/test-kmod/lib/modules/3.14.2-1-ARCH/kernel/moduleB.ko in dependency cycle!
        depmod: ERROR: /tmp/test-kmod/lib/modules/3.14.2-1-ARCH/kernel/moduleC.ko in dependency cycle!

        return code: 2

The segfault above could be fixed, but let's just fail everything
because dependency cycles should be fixed in the modules rather than
just be skipped in the index.
---
 tools/depmod.c | 55 +++++++++----------------------------------------------
 1 file changed, 9 insertions(+), 46 deletions(-)

This is now applied. It's better to fail the upgrade than to provide
the user a broken setup, that maybe can't even boot successfully. The
old behavior can be achieved by just disabling the module in the
kernel config.

--
Lucas De Marchi