layout: Add explict .stack section

This ensures that the stack region is mapped by the ELF loader and
doesn't conflict with any hypervisor memory regions. To do this without
increasing binary size, we place the stack right after the .bss section.

Note that we still need an assert to make sure that our minimal page
table setup in ram32.s covers our program + stack.

Also, being able to see the stack as a section makes debugging easier.

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

layout.ld

@@ -8,9 +8,6 @@ PHDRS
 
 /* Loaders like to put stuff in low memory (< 1M), so we don't use it. */
 ram_min = 1M;
-ram_max = 2M;
-/* Our stack grows down from ram_max. TODO: Add a guard for stack overflows. */
-stack_size = 64K;
 
 SECTIONS
 {
@@ -28,16 +25,18 @@ SECTIONS
 	.data   : { *(.data .data.*)     }
 	data_size = . - data_start;
 
-	/* The BSS section isn't mapped from any file data. It is simply zeroed
-	   in RAM. So our file size should be computed from here.                 */
-	file_size = . - ram_min;
+	/* The BSS section isn't mapped from file data. It is just zeroed in RAM. */
 	.bss : {
 		bss_start = .;
 		*(.bss .bss.*)
 		bss_size = . - bss_start;
 	}
 
-	ASSERT((. <= ram_max - stack_size), "firmware size too big for RAM region")
+	/* Our stack grows down and is page-algined. TODO: Add stack guard pages. */
+	.stack (NOLOAD) : ALIGN(4K) { . += 64K; }
+	stack_start = .;
+	/* ram32.s only maps the first 2 MiB, and that must include the stack.    */
+	ASSERT((. <= 2M), "Stack overflows initial identity-mapped memory region")
 
 	/* Match edk2's GccBase.lds DISCARD section */
 	/DISCARD/ : {

src/asm/ram64.s

@@ -7,8 +7,8 @@ linux64_start:
     xorq %rdi, %rdi
 
 ram64_start:
-    # Setup the stack (at the end of our RAM region)
-    movq $ram_max, %rsp
+    # Initialize the stack pointer (Rust code always uses the stack)
+    movq $stack_start, %rsp
 
     # PVH start_info is in %rdi, the first paramter of the System V ABI.
     # BootParams are in %rsi, the second paramter of the System V ABI.