[build] Allow sufficient stack space for memsize_e820()

The functions memsize_e820() (and its inlined sanitize_e820_map()) use
around 9.5kb of stack space, which overflows the 8kB per-CPU stack and
so overwrites random variables in .bss, leading to undefined
behaviour.

Fix by reordering the per-CPU stacks so that the boot processor uses
the highest stack.  This provides additional space during the
start-of-day call to memsize_e820(), using stack space which will
subsequently be reused for other CPUs' stacks.

Note that we cannot simply increase the per-CPU stack size to 16kB,
since that would cause the .bss section to exceed the limits of base
memory.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 2 insertions, 4 deletions

diff --git a/main.c b/main.c
index 0bc7ca0..75fb5e5 100644
--- a/main.c
+++ b/main.c
@@ -231,11 +231,9 @@ switch_to_main_stack(unsigned cpu_num)
 	extern uintptr_t boot_stack_top; 
 	uintptr_t *src, *dst;
 	int offs;
-	uint8_t * stackAddr, *stackTop;
+	uint8_t *stackTop;
    
-	stackAddr = (uint8_t *) &stacks[cpu_num][0];
-
-	stackTop  = stackAddr + STACKSIZE;
+	stackTop = (uint8_t *) &stacks[MAX_CPUS - cpu_num][0];
    
 	src = (uintptr_t*)&boot_stack_top;
 	dst = (uintptr_t*)stackTop;