For the stage2 of zipl's boot loaders only 3 heap pages are available
[0x6000, 0x6000 + 0x3000]. Therefore, stage2 code needs to be very careful
when and for how long it allocates a heap page.
The heap OOM problem in stage2 was hidden until the commit 252be376
("zipl/boot: fix potential heap overflow in stage2"). Before this commit,
stage2 assumed that heap is 4 pages large which is apparently very wrong
but get_zeroed_page() still allowed allocating an extra 4th page by
overriding the memory segment following the heap area of stage2 (stage3
parameter area).
Try to avoid allocating a heap page and keeping it allocated over a long
period of time in the DASD dumpers. Free a heap page as soon as possible
if not required anymore.
And be extra careful with printf() because it always tries to allocate
a free heap page.
Fixes: 252be376 ("zipl/boot: fix potential heap overflow in stage2")
Signed-off-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Reviewed-by: Philipp Rudo <prudo@linux.ibm.com>
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>
800df6bef8