[PATCH 2/2] riscv: Restrict DWARF5 when building with LLVM to known working versions

[Nathan Chancellor]

On Thu, Nov 30, 2023 at 08:16:41AM -0800, Fangrui Song wrote:
> On Wed, Nov 29, 2023 at 3:30 PM Nathan Chancellor <nathan at kernel.org> wrote:
> >
> > LLVM prior to 18.0.0 would generate incorrect debug info for DWARF5 due
> > to linker relaxation, which was worked around in clang by defaulting
> > RISC-V to DWARF4 [1]. Unfortunately, this workaround does not work for
> > the kernel because the DWARF version can be independently changed from
> > the default in Kconfig.
> >
> > Do not allow DWARF5 to be selected for RISC-V when using linker
> > relaxation (ld.lld >= 15.0.0) and a version of LLVM that does not have
> > the fixes (the integrated assembler [2] and ld.lld [3] < 18.0.0)
> > necessary to generate the correct debug info.
> >
> > Link: https://github.com/llvm/llvm-project/commit/bbc0f99f3bc96f1db16f649fc21dd18e5b0918f6 [1]
> > Link: https://github.com/llvm/llvm-project/commit/1df5ea29b43690b6622db2cad7b745607ca4de6a [2]
> > Link: https://github.com/llvm/llvm-project/commit/7ffabb61a5569444b5ac9322e22e5471cc5e4a77 [3]
> > Signed-off-by: Nathan Chancellor <nathan at kernel.org>
> > ---
> >  arch/riscv/Kconfig | 9 +++++++++
> >  lib/Kconfig.debug  | 1 +
> >  2 files changed, 10 insertions(+)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 72be1d8122a3..81b473cb47b0 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -186,6 +186,15 @@ config RISCV_USE_LINKER_RELAXATION
> >         # https://github.com/llvm/llvm-project/commit/6611d58f5bbcbec77262d392e2923e1d680f6985
> >         depends on !LD_IS_LLD || LLD_VERSION >= 150000
> >
> > +# https://github.com/llvm/llvm-project/commit/bbc0f99f3bc96f1db16f649fc21dd18e5b0918f6
> > +config ARCH_HAS_BROKEN_DWARF5
> > +       def_bool y
> > +       depends on RISCV_USE_LINKER_RELAXATION
> > +       # https://github.com/llvm/llvm-project/commit/1df5ea29b43690b6622db2cad7b745607ca4de6a
> > +       depends on AS_IS_LLVM && AS_VERSION < 180000
> > +       # https://github.com/llvm/llvm-project/commit/7ffabb61a5569444b5ac9322e22e5471cc5e4a77
> > +       depends on LD_IS_LLD && LLD_VERSION < 180000
> > +
> >  config ARCH_MMAP_RND_BITS_MIN
> >         default 18 if 64BIT
> >         default 8
> > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> > index cc7d53d9dc01..a0ebce05a368 100644
> > --- a/lib/Kconfig.debug
> > +++ b/lib/Kconfig.debug
> > @@ -282,6 +282,7 @@ config DEBUG_INFO_DWARF4
> >  config DEBUG_INFO_DWARF5
> >         bool "Generate DWARF Version 5 debuginfo"
> >         select DEBUG_INFO
> > +       depends on !ARCH_HAS_BROKEN_DWARF5
> >         depends on !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_LEB128)
> >         help
> >           Generate DWARF v5 debug info. Requires binutils 2.35.2, gcc 5.0+ (gcc
> >
> > --
> > 2.43.0
> >
> 
> A comment and "depends on" condition introduced by commit
> 0a6de78cff600cb991f2a1b7ed376935871796a0 "lib/Kconfig.debug: Add check
> for non-constant .{s,u}leb128 support to DWARF5"
> can now be refined.
> 
> > ib/Kconfig.debug
> > (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_LEB128)
> 
> AS_VERSION can now be 24100.

That will impact all architectures that use GNU as, even though the
issue was really only seen on RISC-V. It probably doesn't matter since
most architectures work with the integrated assembler and that is what
we tell people to use but I think the current check will continue to
work.

Alternatively, I could do something like this but it is quite ugly and
verbose :/

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index a0ebce05a368..aea0ef5a5d1f 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -231,11 +231,6 @@ config DEBUG_INFO
 	  in the "Debug information" choice below, indicating that debug
 	  information will be generated for build targets.
 
-# 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
-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
@@ -258,7 +253,7 @@ config DEBUG_INFO_NONE
 config DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT
 	bool "Rely on the toolchain's implicit default DWARF version"
 	select DEBUG_INFO
-	depends on !CC_IS_CLANG || AS_IS_LLVM || CLANG_VERSION < 140000 || (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_LEB128)
+	depends on !CC_IS_CLANG || AS_IS_LLVM || CLANG_VERSION < 140000 || (AS_IS_GNU && ((!RISCV && AS_VERSION >= 23502) || (RISCV && AS_VERSION >= 24100)))
 	help
 	  The implicit default version of DWARF debug info produced by a
 	  toolchain changes over time.
@@ -283,7 +278,7 @@ config DEBUG_INFO_DWARF5
 	bool "Generate DWARF Version 5 debuginfo"
 	select DEBUG_INFO
 	depends on !ARCH_HAS_BROKEN_DWARF5
-	depends on !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_LEB128)
+	depends on !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && ((!RISCV && AS_VERSION >= 23502) || (RISCV && AS_VERSION >= 24100)))
 	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