Re: Debian SID kernel doesn't boot on PowerBook 3400c

On Fri, 6 Aug 2021, Christophe Leroy wrote:

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Can you check if they DO NOT happen at preceding commit c16728835~

$ git checkout c16728835~
Previous HEAD position was c16728835eec powerpc/32: Manage KUAP in C
HEAD is now at 0b45359aa2df powerpc/8xx: Create C version of kuap
save/restore/check helpers
$ git am ../message.mbox
warning: Patch sent with format=flowed; space at the end of lines might be
lost.
Applying: powerpc/32: Dismantle EXC_XFER_STD/LITE/TEMPLATE
$ cp ../dot-config-powermac-5.13 .config
$ make ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu- -j4 clean
olddefconfig vmlinux

Linux version 5.12.0-rc3-pmac-00077-gc9f6e8dd045

3) PB 3400c
Hangs at boot (Mac OS screen)

4) Wallstreet
X fails, errors in console log (different than test 2), see
Wallstreet_console-2.txt.

This log shows that the errors "xfce4-session[1775]: bus error (7)" and
"kernel BUG at arch/powerpc/kernel/interrupt.c:49!" happen prior to commit
c16728835eec ("powerpc/32: Manage KUAP in C").

As mentionned by Nic, this is due to r11 being cloberred. For the time being
the only r11 clobber identified is the one I have provided a fix for. I'm
wondering whether it was applied for all further tests or not.

Your fix was applied to this build with "git am ../message.mbox".

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Could you test with CONFIG_PPC_KUAP and CONFIG_PPC_KUAP_DEBUG

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$scripts/config -e CONFIG_PPC_KUAP
$ scripts/config -e CONFIG_PPC_KUAP_DEBUG
$ make ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu- -j4 clean
olddefconfig vmlinux
$ grep CONFIG_PPC_KUAP .config
CONFIG_PPC_KUAP=y
CONFIG_PPC_KUAP_DEBUG=y

Linux version 5.12.0-rc3-pmac-00078-g5cac2bc3752

9) PB 3400c
Hangs at boot (Mac OS screen)

10) Wallstreet
X failed at first login, worked at second login, one error in console
log ("BUG: Unable to handle kernel instruction fetch"), see
Wallstreet_console-5.txt.

One might expect to see "Kernel attempted to write user page (b3399774) -
exploit attempt?" again here (see c16728835eec build above) but instead
this log says "Oops: Kernel access of bad area, sig: 11".

Maybe the test should be done a second time. As r11 is garbage it may or 
may not be a user address. If it is a user address the we get "Kernel 
attempted to write user page". If it is a random kernel address, we 
likely get "Kernel access of bad area" instead.

Your fix was applied here also.