[PATCH 2/3] riscv: Workaround mcount name prior to clang-13

[Nick Desaulniers]

On Thu, Mar 25, 2021 at 3:38 PM Nathan Chancellor <nathan at kernel.org> wrote:
>
> Prior to clang 13.0.0, the RISC-V name for the mcount symbol was
> "mcount", which differs from the GCC version of "_mcount", which results
> in the following errors:
>
> riscv64-linux-gnu-ld: init/main.o: in function `__traceiter_initcall_level':
> main.c:(.text+0xe): undefined reference to `mcount'
> riscv64-linux-gnu-ld: init/main.o: in function `__traceiter_initcall_start':
> main.c:(.text+0x4e): undefined reference to `mcount'
> riscv64-linux-gnu-ld: init/main.o: in function `__traceiter_initcall_finish':
> main.c:(.text+0x92): undefined reference to `mcount'
> riscv64-linux-gnu-ld: init/main.o: in function `.LBB32_28':
> main.c:(.text+0x30c): undefined reference to `mcount'
> riscv64-linux-gnu-ld: init/main.o: in function `free_initmem':
> main.c:(.text+0x54c): undefined reference to `mcount'
>
> This has been corrected in https://reviews.llvm.org/D98881 but the
> minimum supported clang version is 10.0.1. To avoid build errors and to
> gain a working function tracer, adjust the name of the mcount symbol for
> older versions of clang in mount.S and recordmcount.pl.
>
> Cc: stable at vger.kernel.org
> Link: https://github.com/ClangBuiltLinux/linux/issues/1331
> Signed-off-by: Nathan Chancellor <nathan at kernel.org>

Thanks for keeping this alive on clang-10, and resolving it for future releases!
Reviewed-by: Nick Desaulniers <ndesaulniers at google.com>

> ---
>  arch/riscv/include/asm/ftrace.h | 14 ++++++++++++--
>  arch/riscv/kernel/mcount.S      | 10 +++++-----
>  scripts/recordmcount.pl         |  2 +-
>  3 files changed, 18 insertions(+), 8 deletions(-)
>
> diff --git a/arch/riscv/include/asm/ftrace.h b/arch/riscv/include/asm/ftrace.h
> index 845002cc2e57..04dad3380041 100644
> --- a/arch/riscv/include/asm/ftrace.h
> +++ b/arch/riscv/include/asm/ftrace.h
> @@ -13,9 +13,19 @@
>  #endif
>  #define HAVE_FUNCTION_GRAPH_RET_ADDR_PTR
>
> +/*
> + * Clang prior to 13 had "mcount" instead of "_mcount":
> + * https://reviews.llvm.org/D98881
> + */
> +#if defined(CONFIG_CC_IS_GCC) || CONFIG_CLANG_VERSION >= 130000
> +#define MCOUNT_NAME _mcount
> +#else
> +#define MCOUNT_NAME mcount
> +#endif
> +
>  #define ARCH_SUPPORTS_FTRACE_OPS 1
>  #ifndef __ASSEMBLY__
> -void _mcount(void);
> +void MCOUNT_NAME(void);
>  static inline unsigned long ftrace_call_adjust(unsigned long addr)
>  {
>         return addr;
> @@ -36,7 +46,7 @@ struct dyn_arch_ftrace {
>   * both auipc and jalr at the same time.
>   */
>
> -#define MCOUNT_ADDR            ((unsigned long)_mcount)
> +#define MCOUNT_ADDR            ((unsigned long)MCOUNT_NAME)
>  #define JALR_SIGN_MASK         (0x00000800)
>  #define JALR_OFFSET_MASK       (0x00000fff)
>  #define AUIPC_OFFSET_MASK      (0xfffff000)
> diff --git a/arch/riscv/kernel/mcount.S b/arch/riscv/kernel/mcount.S
> index 8a5593ff9ff3..6d462681c9c0 100644
> --- a/arch/riscv/kernel/mcount.S
> +++ b/arch/riscv/kernel/mcount.S
> @@ -47,8 +47,8 @@
>
>  ENTRY(ftrace_stub)
>  #ifdef CONFIG_DYNAMIC_FTRACE
> -       .global _mcount
> -       .set    _mcount, ftrace_stub
> +       .global MCOUNT_NAME
> +       .set    MCOUNT_NAME, ftrace_stub
>  #endif
>         ret
>  ENDPROC(ftrace_stub)
> @@ -78,7 +78,7 @@ ENDPROC(return_to_handler)
>  #endif
>
>  #ifndef CONFIG_DYNAMIC_FTRACE
> -ENTRY(_mcount)
> +ENTRY(MCOUNT_NAME)
>         la      t4, ftrace_stub
>  #ifdef CONFIG_FUNCTION_GRAPH_TRACER
>         la      t0, ftrace_graph_return
> @@ -124,6 +124,6 @@ do_trace:
>         jalr    t5
>         RESTORE_ABI_STATE
>         ret
> -ENDPROC(_mcount)
> +ENDPROC(MCOUNT_NAME)
>  #endif
> -EXPORT_SYMBOL(_mcount)
> +EXPORT_SYMBOL(MCOUNT_NAME)
> diff --git a/scripts/recordmcount.pl b/scripts/recordmcount.pl
> index a36df04cfa09..7b83a1aaec98 100755
> --- a/scripts/recordmcount.pl
> +++ b/scripts/recordmcount.pl
> @@ -392,7 +392,7 @@ if ($arch eq "x86_64") {
>      $mcount_regex = "^\\s*([0-9a-fA-F]+):.*\\s_mcount\$";
>  } elsif ($arch eq "riscv") {
>      $function_regex = "^([0-9a-fA-F]+)\\s+<([^.0-9][0-9a-zA-Z_\\.]+)>:";
> -    $mcount_regex = "^\\s*([0-9a-fA-F]+):\\sR_RISCV_CALL(_PLT)?\\s_mcount\$";
> +    $mcount_regex = "^\\s*([0-9a-fA-F]+):\\sR_RISCV_CALL(_PLT)?\\s_?mcount\$";
>      $type = ".quad";
>      $alignment = 2;
>  } elsif ($arch eq "nds32") {
> --
> 2.31.0
>
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-- 
Thanks,
~Nick Desaulniers