Re: module: add debugging alias parsing support

[Jessica Yu]

+++ Luis R. Rodriguez [29/11/17 18:36 -0800]:
> Debugging modules can often lead to an alias question. We purposely
> don't have alias parsing support upstream as this is all dealt with
> in userpace with the assumption that in-kernel we just process aliases
> and userspace Does The Right Thing (TM) about aliases.
>
> Obviously userspace can be buggy though, and it can lie to us. We
> currently have no easy way to determine this. Parsing aliases is
> an example debugging facility we can use to help with these sorts
> of problems.
>
> You can debug by adding to your dynamic debug:
>
> GRUB_CMDLINE_LINUX_DEFAULT="dyndbg=\"func module_process_aliases +p;\" "
>
> Upon boot for example here a few entries:
>
> module ext4 num_aliases: 5
> alias[0] = fs-ext4
> alias[1] = ext3
> alias[2] = fs-ext3
> alias[3] = ext2
> alias[4] = fs-ext2
>
> module xfs num_aliases: 1
> alias[0] = fs-xfs
>
> module floppy num_aliases: 3
> alias[0] = block-major-2-*
> alias[1] = acpi*:PNP0700:*
> alias[2] = pnp:dPNP0700*
>
> module ata_piix num_aliases: 89
> alias[0] = pci:v00008086d00008C81sv*sd*bc*sc*i*
> alias[1] = pci:v00008086d00008C80sv*sd*bc*sc*i*
> alias[2] = pci:v00008086d00008C89sv*sd*bc*sc*i*
> alias[3] = pci:v00008086d00008C88sv*sd*bc*sc*i*
> ... etc ...
>
> Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Luis R. Rodriguez <mcgrof@xxxxxxxxxx>

Hi Luis,

Just some quick questions - are there any plans to use these in-kernel
module aliases anywhere else? Or are you using them just for debugging?

And if it's only for debugging, what's the benefit of printing the
aliases in-kernel when one could already use modinfo to print the
exact same information (namely, in-kernel module name, which was
recently added by Kees, + aliases)? We're essentially parsing the
.modinfo section of the module binary in two different places, just
that this patchset does it in-kernel and modinfo does it from
userspace, so I'm curious why we want to bring this into the kernel as
well without any additional users..

Thanks!

Jessica