AIX Version 4.3 Differences Guide

destructive overlay can be caught when it occurs if the kernel code is protected
from overwrites. The AIX kernel has therefore been enhanced to provide some
protection against these types of errors. The first page of memory is now
protected from writes by setting page protection bits in virtual page tables. A
similar scheme has been implemented for other pages in the kernel that contain
nothing but code (since code should never be altered). Any attempt to overlay
protected pages now results in dumps that point directly to the program that tried
to do the overwriting. This cuts out the most expensive and time-consuming part
of memory overlay debugging for a large number of overlay cases.
For kernel text, enough symbol information has been added to the kernel space
so that the kernel text is protected during system initialization. Note that pages
containing a mixture of data and text, or data only, cannot be protected, so some
kernel text remains writable.
Kernel extension text areas are optionally protected. A run-time check enables
the system loader to protect kernel extension text areas. If xmdbg is set by the
bosdebug or bosboot commands, text pages are protected at kernel extension load
time. Pages that share text and data are not protected.
Note: This change has impacted kernel and kernel extension code that attempts
to modify text. Self-modifying kernel extensions will cause the system to crash
unless those extensions also modify the protection of the text pages.
This design protects as many pages in the kernel space as is practical without
disturbing delicate assembler routines or increasing the working set needed to
run the kernel.
2.6.1 Storage Protection Macro
The STORE_PROTECT macro has been added to store-protect whole pages that
reside between two symbols (x and y). This macro is defined as follows:
#define STORE_PROTECT(x,y) if (STARTOFPAGE(y) > NEXTPAGE(x)) \
vm_protect(NEXTPAGE(x),STARTOFPAGE(y)-NEXTPAGE(x),RDONLY)
The STORE_PROTECT macro has the effect of protecting all pages starting with
the next page boundary beyond x until the last page boundary before y. This
macro is used during system initialization for the various regions in the kernel and
conditionally by the loader during kernel extension load time.
During system initialization, k_protect() is called to protect the regions marked by
the bind steps. k_protect() is called from main() in the following sequence:
debugger init(); /* start the kernel debugger */
kmem init(); /* initialize kernel memory heaps */
k_protect(); /* store protect kernel text areas */
strtdisp(); /* start up the dispatcher */
When called, k_protect() does the following:
• Store protects low.o areas, at least the first three pages
• Store protects pinned text sub binds
• Store protects paged text sub binds
18 AIX Version 4.3 Differences Guide