[PATCH] x86/srso: fix build breakage for LD=ld.lld

[Nick Desaulniers]

The assertion added to verify the difference in bits set of the
addresses of srso_untrain_ret_alias and srso_safe_ret_alias would fail
to link in LLVM's ld.lld linker with the following error:

  ld.lld: error: ./arch/x86/kernel/vmlinux.lds:210: at least one side of
  the expression must be absolute
  ld.lld: error: ./arch/x86/kernel/vmlinux.lds:211: at least one side of
  the expression must be absolute

Use ABSOLUTE to evaluate the expression referring to at least one of the
symbols so that LLD can evaluate the linker script.

Also, add linker version info to the comment about xor being unsupported
in either ld.bfd or ld.lld until somewhat recently.

Fixes: fb3bd914b3ec ("x86/srso: Add a Speculative RAS Overflow mitigation")
Link: https://github.com/ClangBuiltLinux/linux/issues/1907
Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Reported-by: Daniel Kolesa <daniel@xxxxxxxxxxxxx>
Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Closes: https://lore.kernel.org/llvm/CA+G9fYsdUeNu-gwbs0+T6XHi4hYYk=Y9725-wFhZ7gJMspLDRA@xxxxxxxxxxxxxx/
Suggested-by: Sven Volkinsfeld <thyrc@xxxxxxx>
Signed-off-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx>
---
Note that CONFIG_LTO_CLANG is still broken due to GDS mitigations. Will
work on a separate fix for that.  Sending this for now to unmuck the
builds.
---
 arch/x86/kernel/vmlinux.lds.S | 12 +++++++++---
 1 file changed, 9 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index e76813230192..ef06211bae4c 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -529,11 +529,17 @@ INIT_PER_CPU(irq_stack_backing_store);
 
 #ifdef CONFIG_CPU_SRSO
 /*
- * GNU ld cannot do XOR so do: (A | B) - (A & B) in order to compute the XOR
+ * GNU ld cannot do XOR until 2.41.
+ * https://sourceware.org/git/?p=binutils-gdb.git;a=commit;h=f6f78318fca803c4907fb8d7f6ded8295f1947b1
+ *
+ * LLVM lld cannot do XOR until lld-17.
+ * https://github.com/llvm/llvm-project/commit/fae96104d4378166cbe5c875ef8ed808a356f3fb
+ *
+ * Instead do: (A | B) - (A & B) in order to compute the XOR
  * of the two function addresses:
  */
-. = ASSERT(((srso_untrain_ret_alias | srso_safe_ret_alias) -
-		(srso_untrain_ret_alias & srso_safe_ret_alias)) == ((1 << 2) | (1 << 8) | (1 << 14) | (1 << 20)),
+. = ASSERT(((ABSOLUTE(srso_untrain_ret_alias) | srso_safe_ret_alias) -
+		(ABSOLUTE(srso_untrain_ret_alias) & srso_safe_ret_alias)) == ((1 << 2) | (1 << 8) | (1 << 14) | (1 << 20)),
 		"SRSO function pair won't alias");
 #endif
 

---
base-commit: 13b9372068660fe4f7023f43081067376582ef3c
change-id: 20230809-gds-8b0456a18548

Best regards,
-- 
Nick Desaulniers <ndesaulniers@xxxxxxxxxx>