[klibc] klibc and what's the next step?

H. Peter Anvin hpa at zytor.com
Sat Jul 1 17:17:17 PDT 2006

Theodore Tso wrote:
> On Sat, Jul 01, 2006 at 01:08:17PM -0700, Linus Torvalds wrote:
>> The argument that user space is more debuggable has been shown to be 
>> largely a red herring. User space is only more debuggable if it does 
>> something independent, and we've seen that user space is _harder_ to debug 
>> than kernel space if we have events going back and forth.
> Agreed, 100%.
> In addition, userspace is debuggable only only if the initramfs has
> enough debugging code in their (like a real live working shell,
> strace, basic shell commands etc.)  Otherwise, it becomes even more
> hellish to debug.  I wasted a huge amount of time trying to figure out
> why the RHEL4 initramfs was incompatible with modern kernels using the
> MPT Fusion SCSI driver, because I couldn't make it stop and break out
> to a working shell; it had some busybox-like nash thing that wasn't
> designed for user interaction, so all I could do was try to make tiny
> changes to the initramfs, wait for the !@#@# very long boot cycle, and
> watch the initial ramdisk fail to mount the root and crash, and
> repeat, for hours on end.  RHEL4's userspace root mount system was
> ***not*** more debuggable, not in the last.  Adding printk's into a
> kernel and recompiling would have been easier, and far less
> frustrating.
> Hopefully kinit will be better, but it's definitely not the case that
> userpsace is easier to debug.  

It isn't automatically easier, but it *can* be.

In your case above, with kinit, I would have added dash and strace (the 
latter would probably have to be statically linked against glibc; I 
haven't actually tried building strace under klibc myself) -- or even 
gdb -- to initramfs, and have /init drop into a shell.  From there one 
can run strace -f on kinit.

One of the criticisms I've gotten for klibc has been why I have included 
dash and a whole bunch of shell utilities when they're not used by the 
default kernel build.  It certainly hasn't been just to prove a point; 
as a matter of fact, getting dash to build correctly under Kbuild proved 
to be surprisingly difficult.  However, by making sure that one can 
*easily* pull together an interactive environment -- even if one didn't 
have one from the start -- one has more readily access to a debuggable 

Similarly, I try very hard to have small, individually testable modules. 
  I haven't taken that even as far as I'd like to have, in the end, but 
that's on my list of things to do.


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