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issue with split debug info on riscv for clang-14 and older #1914

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nickdesaulniers opened this issue Aug 14, 2023 · 5 comments
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issue with split debug info on riscv for clang-14 and older #1914

nickdesaulniers opened this issue Aug 14, 2023 · 5 comments
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@nathanchance
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[ARCH] risc-v This bug impacts ARCH=riscv [BUG] linux A bug that should be fixed in the mainline kernel. [FIXED][LINUX] 6.6 This bug was fixed in Linux 6.6 [Reported-by] kbuild test robot Reported-by: kbuild test robot <[email protected]> [WORKAROUND] Applied This bug has an applied workaround

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@nickdesaulniers
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via: https://lore.kernel.org/llvm/ZNSWMgFF529bGy14@yujie-X299/

it seems like there's some issue with split debug info for riscv for clang-14 and older. If we bisect llvm, we can update kconfig to avoid it being selectable by randconfig
@nickdesaulniers nickdesaulniers added [BUG] linux A bug that should be fixed in the mainline kernel. [ARCH] risc-v This bug impacts ARCH=riscv [Reported-by] kbuild test robot Reported-by: kbuild test robot <[email protected]> labels Aug 14, 2023
@nathanchance
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If we bisect llvm

Currently working on it.

@nathanchance
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My bisect landed on llvm/llvm-project@80a4e6f, which just prevents CONFIG_DEBUG_INFO_SPLIT from being selected with -mrelax, which is the default as far as I understand.

I am not really sure how to restrict CONFIG_DEBUG_INFO_SPLIT based on this though. That change came out of llvm/llvm-project#56642 and based on some of the comments in that thread, it seems like GCC silently produces broken .dwo files, so maybe we just want something like

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index d6798513a8c2..52a75c51b158 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -355,6 +355,9 @@ endchoice # "Compressed Debug information"
 config DEBUG_INFO_SPLIT
        bool "Produce split debuginfo in .dwo files"
        depends on $(cc-option,-gsplit-dwarf)
+       # RISC-V linker relaxation is not currently compatible with split DWARF:
+       # https://github.com/llvm/llvm-project/issues/56642
+       depends on !RISCV
        help
          Generate debug info into separate .dwo files. This significantly
          reduces the build directory size for builds with DEBUG_INFO,

? Would be nice to have a GCC bug report to go with that...

@MaskRay
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MaskRay commented Aug 16, 2023 •
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My bisect landed on llvm/llvm-project@80a4e6f, which just prevents CONFIG_DEBUG_INFO_SPLIT from being selected with -mrelax, which is the default as far as I understand.

Yes. RISC-V linker relaxation doesn't work well with some debug info features in LLVM/Clang. In addition, DWARF v5 -gdwarf-5/-fdebug-dwarf-version=5 can currently be incorrect. I changed Clang main branch and 17.x to default to v4 (llvm/llvm-project-release-prs@3c0e8be).

? Would be nice to have a GCC bug report to go with that...

I think GCC's -gsplit-dwarf has been correct since 12.0 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99090).

@nathanchance
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Thanks a lot for the extra info, I have sent https://lore.kernel.org/[email protected]/ to avoid this error.

@nathanchance nathanchance added the [PATCH] Submitted A patch has been submitted for review label Aug 16, 2023
akiyks pushed a commit to akiyks/linux that referenced this issue Aug 31, 2023
@nathanchance @palmer-dabbelt
lib/Kconfig.debug: Restrict DEBUG_INFO_SPLIT for RISC-V
01c57c1 
When building for ARCH=riscv using LLVM < 14, there is an error with
CONFIG_DEBUG_INFO_SPLIT=y:

  error: A dwo section may not contain relocations

This was worked around in LLVM 15 by disallowing '-gsplit-dwarf' with
'-mrelax' (the default), so CONFIG_DEBUG_INFO_SPLIT is not selectable
with newer versions of LLVM:

  $ clang --target=riscv64-linux-gnu -gsplit-dwarf -c -o /dev/null -x c /dev/null
  clang: error: -gsplit-dwarf is unsupported with RISC-V linker relaxation (-mrelax)

GCC silently had a similar issue that was resolved with GCC 12.x.
Restrict CONFIG_DEBUG_INFO_SPLIT for RISC-V when using LLVM or GCC <
12.x to avoid these known issues.

Link: ClangBuiltLinux#1914
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99090
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Nathan Chancellor <[email protected]>
Reviewed-by: Fangrui Song <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>
lyctw pushed a commit to lyctw/linux that referenced this issue Sep 1, 2023
@nathanchance @palmer-dabbelt
lib/Kconfig.debug: Restrict DEBUG_INFO_SPLIT for RISC-V
89775a2 
When building for ARCH=riscv using LLVM < 14, there is an error with
CONFIG_DEBUG_INFO_SPLIT=y:

  error: A dwo section may not contain relocations

This was worked around in LLVM 15 by disallowing '-gsplit-dwarf' with
'-mrelax' (the default), so CONFIG_DEBUG_INFO_SPLIT is not selectable
with newer versions of LLVM:

  $ clang --target=riscv64-linux-gnu -gsplit-dwarf -c -o /dev/null -x c /dev/null
  clang: error: -gsplit-dwarf is unsupported with RISC-V linker relaxation (-mrelax)

GCC silently had a similar issue that was resolved with GCC 12.x.
Restrict CONFIG_DEBUG_INFO_SPLIT for RISC-V when using LLVM or GCC <
12.x to avoid these known issues.

Link: ClangBuiltLinux/linux#1914
Link: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99090
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Nathan Chancellor <[email protected]>
Reviewed-by: Fangrui Song <[email protected]>
Reviewed-by: Nick Desaulniers <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Palmer Dabbelt <[email protected]>
@nathanchance
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The workaround is in mainline: https://git.kernel.org/linus/89775a27ff6d0396b44de0d6f44dcbc25221fdda

There is an issue upstream for this problem, so I'll close this one up.

@nathanchance nathanchance added [WORKAROUND] Applied This bug has an applied workaround [FIXED][LINUX] 6.6 This bug was fixed in Linux 6.6 and removed [PATCH] Submitted A patch has been submitted for review labels Sep 14, 2023
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[ARCH] risc-v This bug impacts ARCH=riscv [BUG] linux A bug that should be fixed in the mainline kernel. [FIXED][LINUX] 6.6 This bug was fixed in Linux 6.6 [Reported-by] kbuild test robot Reported-by: kbuild test robot <[email protected]> [WORKAROUND] Applied This bug has an applied workaround
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@MaskRay @nickdesaulniers @nathanchance