stub: Fix above 4G boot

The kernel x86 boot protocol does not expect this value to be set when
using EFI handover protocol. If the stub is loaded above 4G we would
end up discarding the high bits, breaking boot.

Fixes: #24707

diff --git a/src/boot/efi/linux_x86.c b/src/boot/efi/linux_x86.c
index d506070926c10ceb7fcfcfbe46f85fefeaf111cd..756494b9c7c668c236bfdc16e34da71aee431816 100644 (file)
--- a/src/boot/efi/linux_x86.c
+++ b/src/boot/efi/linux_x86.c
@@ -18,6 +18,7 @@
 #include "macro-fundamental.h"
 #include "util.h"
 
+#define KERNEL_SECTOR_SIZE 512u
 #define SETUP_MAGIC             0x53726448      /* "HdrS" */
 
 struct setup_header {
@@ -106,17 +107,22 @@ struct boot_params {
 
 typedef void(*handover_f)(void *image, EFI_SYSTEM_TABLE *table, struct boot_params *params) __regparm0__;
 
-static void linux_efi_handover(EFI_HANDLE image, struct boot_params *params) {
-        handover_f handover;
-        UINTN start = (UINTN)params->hdr.code32_start;
-
+static void linux_efi_handover(EFI_HANDLE image, uintptr_t kernel, struct boot_params *params) {
         assert(params);
 
+        kernel += (params->hdr.setup_sects + 1) * KERNEL_SECTOR_SIZE; /* 32bit entry address. */
+
 #ifdef __x86_64__
-        asm volatile ("cli");
-        start += 512;
+        asm volatile("cli");
+        kernel += KERNEL_SECTOR_SIZE; /* 64bit entry address. */
 #endif
-        handover = (handover_f)(start + params->hdr.handover_offset);
+
+        kernel += params->hdr.handover_offset; /* 32/64bit EFI handover address. */
+
+        /* Note in EFI mixed mode this now points to the correct 32bit handover entry point, allowing a 64bit
+         * kernel to be booted from a 32bit sd-stub. */
+
+        handover_f handover = (handover_f) kernel;
         handover(image, ST, params);
 }
 
@@ -130,7 +136,6 @@ EFI_STATUS linux_exec(
         struct boot_params *boot_params;
         EFI_HANDLE initrd_handle = NULL;
         EFI_PHYSICAL_ADDRESS addr;
-        uint8_t setup_sectors;
         EFI_STATUS err;
 
         assert(image);
@@ -162,8 +167,10 @@ EFI_STATUS linux_exec(
         memset(boot_params, 0, 0x4000);
         boot_params->hdr = image_params->hdr;
         boot_params->hdr.type_of_loader = 0xff;
-        setup_sectors = image_params->hdr.setup_sects > 0 ? image_params->hdr.setup_sects : 4;
-        boot_params->hdr.code32_start = (uint32_t) POINTER_TO_PHYSICAL_ADDRESS(linux_buffer) + (setup_sectors + 1) * 512;
+
+        /* Spec says: For backwards compatibility, if the setup_sects field contains 0, the real value is 4. */
+        if (boot_params->hdr.setup_sects == 0)
+                boot_params->hdr.setup_sects = 4;
 
         if (cmdline) {
                 addr = 0xA0000;
@@ -194,7 +201,7 @@ EFI_STATUS linux_exec(
         err = initrd_register(initrd_buffer, initrd_length, &initrd_handle);
         if (err != EFI_SUCCESS)
                 return err;
-        linux_efi_handover(image, boot_params);
+        linux_efi_handover(image, (uintptr_t) linux_buffer, boot_params);
         (void) initrd_unregister(initrd_handle);
         initrd_handle = NULL;
         return EFI_LOAD_ERROR;