Instead of relocate the value at that position compute it
entirely and write it.
During EFI boots sym_offs(SYMBOL) are potentially relocated
causing the values to be corrupted.
For PVH and BIOS the change won't be necessary but keep the
code consistent.

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
---
 xen/arch/x86/boot/head.S | 18 ++++++++++--------
 1 file changed, 10 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 5b82221038..abfa3d82f7 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -132,8 +132,7 @@ multiboot2_header:
 gdt_boot_descr:
         .word   .Ltrampoline_gdt_end - trampoline_gdt - 1
 gdt_boot_base:
-        .long   sym_offs(trampoline_gdt)
-        .long   0 /* Needed for 64-bit lgdt */
+        .quad   0 /* Needed for 64-bit lgdt */
 
 vga_text_buffer:
         .long   0xb8000
@@ -392,15 +391,16 @@ __efi64_mb2_start:
 x86_32_switch:
         mov     %r15,%rdi
 
-        /* Store Xen image load base address in place accessible for 32-bit code. */
-        lea     __image_base__(%rip),%esi
-
         cli
 
         /* Initialize GDTR. */
-        add     %esi,gdt_boot_base(%rip)
+        lea     trampoline_gdt(%rip),%esi
+        movl    %esi,gdt_boot_base(%rip)
         lgdt    gdt_boot_descr(%rip)
 
+        /* Store Xen image load base address in place accessible for 32-bit code. */
+        lea     __image_base__(%rip),%esi
+
         /* Reload code selector. */
         pushq   $BOOT_CS32
         lea     cs32_switch(%rip),%edx
@@ -458,7 +458,8 @@ __pvh_start:
         movb    $-1, sym_esi(opt_console_xen)
 
         /* Prepare gdt and segments */
-        add     %esi, sym_esi(gdt_boot_base)
+        lea     sym_esi(trampoline_gdt), %ecx
+        movl    %ecx, sym_esi(gdt_boot_base)
         lgdt    sym_esi(gdt_boot_descr)
 
         mov     $BOOT_DS, %ecx
@@ -562,7 +563,8 @@ trampoline_bios_setup:
          *
          * Initialize GDTR and basic data segments.
          */
-        add     %esi,sym_esi(gdt_boot_base)
+        lea     sym_esi(trampoline_gdt), %ecx
+        movl    %ecx, sym_esi(gdt_boot_base)
         lgdt    sym_esi(gdt_boot_descr)
 
         mov     $BOOT_DS,%ecx
-- 
2.45.2

On 07.08.2024 15:48, Alejandro Vallejo wrote:
> Instead of relocate the value at that position compute it
> entirely and write it.
> During EFI boots sym_offs(SYMBOL) are potentially relocated
> causing the values to be corrupted.
> For PVH and BIOS the change won't be necessary but keep the
> code consistent.

As already hinted at in a reply to an earlier patch - there are ample
further uses of sym_offs(). Imo we need to sort them all in one go,
not by hacking use sites individually. Which hopefully can be done by
simply calculating %esi correctly.

Jan