[uml-devel] uml-general-protection-fault even more kludgy but mergeable

[Blaisorblade <blaisorblade@yahoo.it>]

About 2.6.11-rc4/uml-general-protection-fault:

Ok, today I was looking a bit more to this patch and I even looked in the 
Intel manuals.

I effectively verified that, in case of a page fault (do_page_fault) the 
kernel only sends a SIGSEGV to the process if (error_code & 4 != 0), as 
expected.

So, if (!(error_code & 4)), we cannot be handling a page fault at all.

However, the comment in the patch is a bit misleading (or misleaded) because 
error_code has a different format for everything else than protection faults. 
(See section 5.13, Error code, of manual "Intel System Programming Guide - 
253668 -15.pdf", order number 253668-15).

In that case, in fact, it will be unused (and hence, probably, set to 0) when 
the error code references the IDT descriptor, it will be clear when the GDT 
is referenced, it will be set when the LDT is referenced (always by the error 
code).

So, in practice, it's correct to call bad_segv() as done in the patch, but we 
miss some cases. Only this time I've verified this well enough that I feel we 
could merge your patch. However, I've not found any possible *realistic* 
source of general protection faults. Especially I didn't find what expected 
(i.e. I/O errors).
-- 
Paolo Giarrusso, aka Blaisorblade
Skype user "PaoloGiarrusso"
Linux registered user n. 292729
http://www.user-mode-linux.org/~blaisorblade





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