xen/arm: Skip initializing the BSS section when it is empty

If the BSS section is empty, then the function should return.
If one does not check whether the BSS section is empty or not, then there is a
risk of writing 0s outside of BSS section (which may contain critical data).

Fixes: dac84b66cc ("xen: arm64: initial build + config changes, start of day code")
Signed-off-by: Ayan Kumar Halder <ayan.kumar.halder@amd.com>
Reviewed-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>

xen/arch/arm/arm32/head.S

@@ -185,12 +185,15 @@ zero_bss:
         PRINT("- Zero BSS -\r\n")
         mov_w r0, __bss_start        /* r0 := vaddr(__bss_start) */
         mov_w r1, __bss_end          /* r1 := vaddr(__bss_end)   */
+        cmp   r1, r0
+        beq   skip_bss
 
         mov   r2, #0
 1:      str   r2, [r0], #4
         cmp   r0, r1
         blo   1b
 
+skip_bss:
         mov   pc, lr
 ENDPROC(zero_bss)
 

xen/arch/arm/arm64/head.S

@@ -346,6 +346,8 @@ FUNC_LOCAL(zero_bss)
         PRINT("- Zero BSS -\r\n")
         ldr   x0, =__bss_start       /* x0 := vaddr(__bss_start) */
         ldr   x1, =__bss_end         /* x1 := vaddr(__bss_end)   */
+        cmp   x1, x0
+        beq   skip_bss
 
 1:      str   xzr, [x0], #8
         cmp   x0, x1