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    Subject[PATCH 6.0 862/862] lib/Kconfig.debug: Add check for non-constant .{s,u}leb128 support to DWARF5
    Date
    From: Nathan Chancellor <nathan@kernel.org>

    commit 0a6de78cff600cb991f2a1b7ed376935871796a0 upstream.

    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@kernel.org>
    Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
    Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    lib/Kconfig.debug | 9 +++++++--
    1 file changed, 7 insertions(+), 2 deletions(-)

    --- 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

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