[Top][All Lists]

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [PATCH v2 4/4] pc-bios/s390-ccw: Don't use __bss_start with the "lar

From: Thomas Huth
Subject: Re: [PATCH v2 4/4] pc-bios/s390-ccw: Don't use __bss_start with the "larl" instruction
Date: Tue, 27 Jun 2023 12:54:04 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.11.0

On 27/06/2023 11.29, Claudio Imbrenda wrote:
On Tue, 27 Jun 2023 09:47:03 +0200
Thomas Huth <thuth@redhat.com> wrote:

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 using the real start address of the .bss section instead.

Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=2216662
Reported-by: Miroslav Rezanina <mrezanin@redhat.com>
Suggested-by: Nick Clifton <nickc@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
  pc-bios/s390-ccw/start.S | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pc-bios/s390-ccw/start.S b/pc-bios/s390-ccw/start.S
index 111dea261b..a63c4e3ff2 100644
--- a/pc-bios/s390-ccw/start.S
+++ b/pc-bios/s390-ccw/start.S
@@ -18,7 +18,7 @@ _start:
      larl    %r15,stack + STACK_SIZE - 160   /* Set up stack */
/* clear bss */
-    larl    %r2,__bss_start
+    larl    %r2,.bss
      larl    %r3,_end

since we are here, do you have guarantees that _end is always correctly

Yes, you can see the internal linker script by running
"ld --verbose", and there I get:

  .data1          : { *(.data1) }
  _edata = .; PROVIDE (edata = .);
  . = .;
  __bss_start = .;
  .bss            :
   *(.bss .bss.* .gnu.linkonce.b.*)
   /* Align here to ensure that the .bss section occupies space up to
      _end.  Align after .bss to ensure correct alignment even if the
      .bss section disappears because there are no input sections.
      FIXME: Why do we need it? When there is no .bss section, we do not
      pad the .data section.  */
   . = ALIGN(. != 0 ? 64 / 8 : 1);
  . = ALIGN(64 / 8);
  . = SEGMENT_START("ldata-segment", .);
  . = ALIGN(64 / 8);
  _end = .; PROVIDE (end = .);

As you can see, there is no alignment in front of
__bss_start, but there is alignment in front of

if so:

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>



reply via email to

[Prev in Thread] Current Thread [Next in Thread]