On Fri, Oct 14, 2022 at 1:48 PM Nathan Chancellor <nathan@xxxxxxxxxx> wrote: > > 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 > Signed-off-by: Nathan Chancellor <nathan@xxxxxxxxxx> Thanks for keeping these LLVM_IAS=0 builds alive a little longer. My hope is the GNU binutils can relax their requirement for debug info sections to improve support for DWARF v5. Reviewed-by: Nick Desaulniers <ndesaulniers@xxxxxxxxxx> > --- > > v2: > - Rebase on commit bb1435f3f575 ("Kconfig.debug: add toolchain > checks for DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT") from the kbuild > tree. > - Limit CONFIG_AS_HAS_NON_CONST_LEB128 dependency to GNU as. There > is no point to applying this dependency to the integrated > assembler because it will always pass. > - Apply the CONFIG_AS_HAS_NON_CONST_LEB128 dependency to > CONFIG_DEBUG_INFO_DWARF_TOOLCHAIN_DEFAULT as well, due to the > aforementioned kbuild change. > - Move comment block to above CONFIG_AS_HAS_NON_CONST_LEB128, as the > configuration is now used in two places. > - Drop Conor's tested by, as the patch is different enough to > potentially require new testing. > > v1: https://lore.kernel.org/20220928182523.3105953-1-nathan@xxxxxxxxxx/ > > lib/Kconfig.debug | 9 +++++++-- > 1 file changed, 7 insertions(+), 2 deletions(-) > > diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug > index db8d9271cabf..5c1c63575895 100644 > --- a/lib/Kconfig.debug > +++ b/lib/Kconfig.debug > @@ -231,6 +231,11 @@ 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 > @@ -253,7 +258,7 @@ 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) > + depends on !CC_IS_CLANG || AS_IS_LLVM || CLANG_VERSION < 140000 || (AS_IS_GNU && AS_VERSION >= 23502 && AS_HAS_NON_CONST_LEB128) > help > The implicit default version of DWARF debug info produced by a > toolchain changes over time. > @@ -277,7 +282,7 @@ config DEBUG_INFO_DWARF4 > config DEBUG_INFO_DWARF5 > bool "Generate DWARF Version 5 debuginfo" > select DEBUG_INFO > - depends on !CC_IS_CLANG || AS_IS_LLVM || (AS_IS_GNU && AS_VERSION >= 23502) > + 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 > 5.0+ accepts the -gdwarf-5 flag but only had partial support for some > > base-commit: bb1435f3f575b5213eaf27434efa3971f51c01de > -- > 2.38.0 > -- Thanks, ~Nick Desaulniers
