baremetal: fix heap and stack position in preparation for CLI arguments

README.adoc

@@ -19482,6 +19482,27 @@ It is not possible to call those C functions from the examples that don't use a
 
 For this reason, we tend to create examples with bootloaders, as it is easier to write them portably.
 
+=== Baremetal linker script
+
+For things to work in baremetal, we often have to layout memory in specific ways.
+
+Notably, since we start with <<arm-paging,paging>> disabled, there are more constraints on where memory can or cannot go.
+
+Especially for C programs, this memory layout is specified by a "linker script", which is present at: link:baremetal/link.ld[]
+
+Note how our linker script also exposes some symbols to C:
+
+....
+lkmc_heap_low = .;
+lkmc_heap_top = .;
+....
+
+Those for example are required to implement `malloc` in Newlib. We can play with those variables more explicitly with link:baremetal/linker_variables.c[]:
+
+....
+./run --arch aarch64 --baremetal baremetal/linker_variables.c
+....
+
 === Semihosting
 
 Semihosting is a publicly documented interface specified by ARM Holdings that allows us to do some magic operations very useful in development.

baremetal/arch/aarch64/multicore.c

@@ -18,7 +18,7 @@ __asm__(
 "    bne .Lsleep_forever\n"
 /* Prepare the stack for CPU1. This is what we need
  * this assembly function for. */
-"    ldr x0, =(stack_top - 0x1000)\n"
+"    ldr x0, =(lkmc_stack_top - 0x1000)\n"
 "    mov sp, x0\n"
 "    bl main_cpu1\n"
 ".Lsleep_forever:\n"

baremetal/arch/arm/multicore.c

@@ -11,7 +11,7 @@ __asm__(
 "lkmc_cpu_not_0:\n"
 "    cmp r0, 1\n"
 "    bne .Lsleep_forever\n"
-"    ldr sp, =(stack_top - 0x1000)\n"
+"    ldr sp, =(lkmc_stack_top - 0x1000)\n"
 "    bl main_cpu1\n"
 ".Lsleep_forever:\n"
 "    wfe\n"

baremetal/lib/aarch64.S

@@ -20,7 +20,7 @@ _start:
     isb
 
     /* Prepare the stack for main, mandatory for C code. */
-    ldr x0, =stack_top
+    ldr x0, =lkmc_stack_top
     mov sp, x0
 
     /* https://cirosantilli.com/linux-kernel-module-cheat#magic-failure-string */

baremetal/lib/arm.S

@@ -8,7 +8,7 @@ _start:
     bne lkmc_cpu_not_0
 
     /* Prepare the stack for main, mandatory for C code. */
-    ldr sp, =stack_top
+    ldr sp, =lkmc_stack_top
 
     /* Enable floating point.
      * Code copied from: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.ddi0409h/CHDEGGFF.html

baremetal/lib/syscalls.c

@@ -12,8 +12,8 @@ enum {
     UART_FR_RXFE = 0x10,
 };
 
-extern char heap_low;
-extern char heap_top;
+extern char lkmc_heap_low;
+extern char lkmc_heap_top;
 
 char *heap_end = 0;
 
@@ -90,10 +90,10 @@ _CLOCK_T_ _times_r (struct _reent *r, struct tms *ptms) {
 caddr_t _sbrk(int incr) {
     char *prev_heap_end;
     if (heap_end == 0) {
-        heap_end = &heap_low;
+        heap_end = &lkmc_heap_low;
     }
     prev_heap_end = heap_end;
-    if (heap_end + incr > &heap_top) {
+    if (heap_end + incr > &lkmc_heap_top) {
         /* Heap and stack collision */
         return (caddr_t)0;
     }

baremetal/link.ld

@@ -1,3 +1,4 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#baremetal-linker-script */
 ENTRY(_start)
 SECTIONS
 {
@@ -10,10 +11,18 @@ SECTIONS
   }
   /* gem5 uses the bss as a measure of the kernel size. */
   .bss : { *(.bss) }
-  heap_low = .;
+  /* Fix the addresses of everything that comes after, no matter
+   * the exact size of the code present in .text. This allows us to
+   * place CLI arguments in memory at a known location! */
+  . = ADDR(.text) + 0x1000000;
+  lkmc_heap_low = .;
   . = . + 0x1000000;
-  heap_top = .;
+  lkmc_heap_top = .;
   . = . + 0x1000000;
-  stack_top = .;
+  lkmc_stack_top = .;
+  . = . + 0x1000000;
+  lkmc_argv = .;
+  . = . + 0x4;
+  lkmc_argc = .;
 }
 

baremetal/linker_variables.c

@@ -0,0 +1,16 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#baremetal-linker-script */
+
+#include <inttypes.h>
+#include <stdint.h>
+#include <stdio.h>
+
+extern int32_t lkmc_argc;
+extern int32_t lkmc_heap_low;
+
+int main(int argc, char **argv) {
+    (void)argc;
+    (void)argv;
+    printf("&lkmc_heap_low %p\n", (void*)&lkmc_heap_low);
+    printf("&lkmc_argc %p\n", (void*)&lkmc_argc);
+    printf("lkmc_argc %" PRId32 "\n", lkmc_argc);
+}

build-baremetal

@@ -102,6 +102,7 @@ Build the baremetal examples with crosstool-NG.
             )
         cc_flags.extend([
             '-Wl,--section-start=.text={:#x}'.format(entry_address), LF,
+            '-Wl,--section-start=.lkmc_memory={:#x}'.format(entry_address + 0x1000000), LF,
             '-T', self.env['baremetal_link_script'], LF,
         ])
         with thread_pool.ThreadPool(