+ lib-kconfigdebug-add-check-for-non-constant-suleb128-support-to-dwarf5.patch added to mm-nonmm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: lib/Kconfig.debug: add check for non-constant .{s,u}leb128 support to DWARF5
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     lib-kconfigdebug-add-check-for-non-constant-suleb128-support-to-dwarf5.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/lib-kconfigdebug-add-check-for-non-constant-suleb128-support-to-dwarf5.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Nathan Chancellor <nathan@kernel.org>
Subject: lib/Kconfig.debug: add check for non-constant .{s,u}leb128 support to DWARF5
Date: Wed, 28 Sep 2022 11:25:23 -0700

When building with a RISC-V kernel with DWARF5 debug info using clang and
the GNU assembler, several instances of the following error appear:

  /tmp/vgettimeofday-48aa35.s:2963: Error: non-constant .uleb128 is not supported

Dumping the .s file reveals these .uleb128 directives come from
.debug_loc and .debug_ranges:

  .Ldebug_loc0:
          .byte   4                               # DW_LLE_offset_pair
          .uleb128 .Lfunc_begin0-.Lfunc_begin0    #   starting offset
          .uleb128 .Ltmp1-.Lfunc_begin0           #   ending offset
          .byte   1                               # Loc expr size
          .byte   90                              # DW_OP_reg10
          .byte   0                               # DW_LLE_end_of_list

  .Ldebug_ranges0:
          .byte   4                               # DW_RLE_offset_pair
          .uleb128 .Ltmp6-.Lfunc_begin0           #   starting offset
          .uleb128 .Ltmp27-.Lfunc_begin0          #   ending offset
          .byte   4                               # DW_RLE_offset_pair
          .uleb128 .Ltmp28-.Lfunc_begin0          #   starting offset
          .uleb128 .Ltmp30-.Lfunc_begin0          #   ending offset
          .byte   0                               # DW_RLE_end_of_list

There is an outstanding binutils issue to support a non-constant operand
to .sleb128 and .uleb128 in GAS for RISC-V but there does not appear to be
any movement on it, due to concerns over how it would work with linker
relaxation.

To avoid these build errors, prevent DWARF5 from being selected when using
clang and an assembler that does not have support for these symbol deltas,
which can be easily checked in Kconfig with as-instr plus the small test
program from the dwz test suite from the binutils issue.

Link: https://sourceware.org/bugzilla/show_bug.cgi?id=27215
Link: https://github.com/ClangBuiltLinux/linux/issues/1719
Link: https://lkml.kernel.org/r/20220928182523.3105953-1-nathan@kernel.org
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Cc: Masahiro Yamada <masahiroy@kernel.org>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Cc: Tom Rix <trix@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/Kconfig.debug |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/lib/Kconfig.debug~lib-kconfigdebug-add-check-for-non-constant-suleb128-support-to-dwarf5
+++ a/lib/Kconfig.debug
@@ -231,6 +231,9 @@ config DEBUG_INFO
 	  in the "Debug information" choice below, indicating that debug
 	  information will be generated for build targets.
 
+config AS_HAS_NON_CONST_LEB128
+	def_bool $(as-instr,.uleb128 .Lexpr_end4 - .Lexpr_start3\n.Lexpr_start3:\n.Lexpr_end4:)
+
 choice
 	prompt "Debug information"
 	depends on DEBUG_KERNEL
@@ -275,6 +278,10 @@ config DEBUG_INFO_DWARF5
 	bool "Generate DWARF Version 5 debuginfo"
 	select DEBUG_INFO
 	depends on !CC_IS_CLANG || (CC_IS_CLANG && (AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502)))
+	# Clang is known to generate .{s,u}leb128 with symbol deltas with
+	# DWARF5, which some targets may not support.
+	# https://sourceware.org/bugzilla/show_bug.cgi?id=27215
+	depends on !CC_IS_CLANG || AS_HAS_NON_CONST_LEB128
 	help
 	  Generate DWARF v5 debug info. Requires binutils 2.35.2, gcc 5.0+ (gcc
 	  5.0+ accepts the -gdwarf-5 flag but only had partial support for some
_

Patches currently in -mm which might be from nathan@kernel.org are

lib-kconfigdebug-add-check-for-non-constant-suleb128-support-to-dwarf5.patch