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[PATCH v3 7/7] pc-bios/s390-ccw: Don't use __bss_start with the "larl" i

From: Thomas Huth
Subject: [PATCH v3 7/7] pc-bios/s390-ccw: Don't use __bss_start with the "larl" instruction
Date: Thu, 29 Jun 2023 12:48:21 +0200

start.S currently cannot be compiled with Clang 16 and binutils 2.40:

 ld: start.o(.text+0x8): misaligned symbol `__bss_start' (0xc1e5) for
     relocation R_390_PC32DBL

According to the built-in linker script of ld, the symbol __bss_start
can actually point *before* the .bss section and does not need to have
any alignment, so in certain situations (like when using the internal
assembler of Clang), the __bss_start symbol can indeed be unaligned
and thus it is not suitable for being used with the "larl" instruction
that needs an address that is at least aligned to halfwords.
The problem went unnoticed so far since binutils <= 2.39 did not
check the alignment, but starting with binutils 2.40, such unaligned
addresses are now refused.

Fix it by loading the address indirectly instead.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2216662
Reported-by: Miroslav Rezanina <mrezanin@redhat.com>
Suggested-by:  Andreas Krebbel <andreas.krebbel@de.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
 pc-bios/s390-ccw/start.S | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/pc-bios/s390-ccw/start.S b/pc-bios/s390-ccw/start.S
index 429a2b30a1..061b06591c 100644
--- a/pc-bios/s390-ccw/start.S
+++ b/pc-bios/s390-ccw/start.S
@@ -19,7 +19,8 @@ _start:
     larl    %r15,stack + STACK_SIZE - STACK_FRAME_SIZE   /* Set up stack */
     /* clear bss */
-    larl    %r2,__bss_start
+    larl    %r2,bss_start_literal   /* __bss_start might be unaligned ... */
+    lg      %r2,0(%r2)              /* ... so load it indirectly */
     larl    %r3,_end
     slgr    %r3,%r2    /* get sizeof bss */
     ltgr    %r3,%r3    /* bss empty? */
@@ -45,7 +46,6 @@ done:
     xc      0(1,%r1),0(%r1)
  * void disabled_wait(void)
@@ -113,6 +113,8 @@ io_new_code:
     br      %r14
     .align  8
+    .quad   __bss_start
     .quad   0x0002000180000000,0x0000000000000000

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