linux-kernel - Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

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Date:   Fri, 4 Jun 2021 16:50:46 -0700
From:   Fangrui Song <maskray@...gle.com>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, lma@...ihalf.com,
        Guenter Roeck <groeck@...gle.com>,
        Juergen Gross <jgross@...e.com>, lb@...ihalf.com,
        LKML <linux-kernel@...r.kernel.org>, mbenes@...e.com,
        rad@...ihalf.com, upstream@...ihalf.com,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        Nathan Chancellor <nathan@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>
Subject: Re: [PATCH v3 16/16] objtool,x86: Rewrite retpoline thunk calls

On 2021-06-04, 'Nick Desaulniers' via Clang Built Linux wrote:
>On Fri, Jun 4, 2021 at 1:50 PM Nick Desaulniers <ndesaulniers@...gle.com> wrote:
>>
>> (Manually replying to https://lore.kernel.org/lkml/CAFJ_xbq06nfaEWtVNLtg7XCJrQeQ9wCs4Zsoi5Y_HP3Dx0iTRA@mail.gmail.com/)
>>
>> Hi Peter,
>> We're also tracking 2 recent regressions that look like they've come from this
>> patch.
>>
>> https://github.com/ClangBuiltLinux/linux/issues/1384
>> https://github.com/ClangBuiltLinux/linux/issues/1388
>>
>> (Both in linux-next at the moment).
>>
>> The first, it looks like a boot failure. The second is a warning from the
>> linker on a kernel module; even readelf seems unhappy with the results of the
>> output from objtool.
>>
>> I can more easily reproduce the latter, so I'm working on getting a smaller
>> reproducer. I'll let you know when I have it, but wanted to report it ASAP.
>
>Sent a pretty big attachment, privately.  I was able to capture the
>before/after with:
>
>$ $ echo 'CONFIG_GCOV_KERNEL=n
>CONFIG_KASAN=n
>CONFIG_LTO_CLANG_THIN=y' >allmod.config
>$ OBJTOOL_ARGS="--backup" make -kj"$(nproc)" KCONFIG_ALLCONFIG=1
>LLVM=1 LLVM_IAS=1 all
>
>It looks like
>
>$ ./tools/objtool/objtool orc generate  --module  --no-fp
>--no-unreachable  --retpoline  --uaccess  --mcount
>drivers/gpu/drm/amd/amdgpu/amdgpu.lto.o; ld.lld -r -m elf_x86_64
>-plugin-opt=-code-model=kernel -plugin-opt=-stack-alignment=8
>--thinlto-cache-dir=.thinlto-cache -mllvm -import-instr-limit=5
>-plugin-opt=-warn-stack-size=2048 --build-id=sha1  -T
>scripts/module.lds -o drivers/gpu/drm/amd/amdgpu/amdgpu.ko
>drivers/gpu/drm/amd/amdgpu/amdgpu.lto.o
>drivers/gpu/drm/amd/amdgpu/amdgpu.mod.o
>
>is producing the linker error:
>
>ld.lld: error: drivers/gpu/drm/amd/amdgpu/amdgpu.lto.o:
>SHT_SYMTAB_SHNDX has 79581 entries, but the symbol table associated
>has 79582
>
>Readelf having issues with the output:
>$ readelf -s amdgpu.lto.o.orig
><works fine>
>$ readelf -s amdgpu.lto.o
>readelf: Error: Reading 73014451695 bytes extends past end of file for
>string table
>$ llvm-readelf -s amdgpu.lto.o
>llvm-readelf: error: 'amdgpu.lto.o': unable to continue dumping, the
>file is corrupt: section table goes past the end of file
>
>`file` having issues:
>$ file drivers/gpu/drm/amd/amdgpu/amdgpu.lto.o
>drivers/gpu/drm/amd/amdgpu/amdgpu.lto.o: ELF 64-bit LSB relocatable,
>x86-64, version 1 (SYSV), no section header
>
>for comparison:
>$ file ./drivers/spi/spi-ath79.lto.o
>./drivers/spi/spi-ath79.lto.o: ELF 64-bit LSB relocatable, x86-64,
>version 1 (SYSV), not stripped

tools/objtool/elf.c:elf_add_symbol may not update .symtab_shndx .
Speaking of llvm-objcopy, it finalizes the content of .symtab_shndx when .symtab
is finalized. objtool may want to adopt a similar approach.

read_symbols searches for the section ".symtab_shndx". It'd be better to
use the section type SHT_SYMTAB_SHNDX.