Re: [RFC][PATCH 3/3] objtool: Fix noinstr vs KCOV

[Dmitry Vyukov]

On Fri, Jun 12, 2020 at 4:37 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Since many compilers cannot disable KCOV with a function attribute,
> help it to NOP out any __sanitizer_cov_*() calls injected in noinstr
> code.
>
> This turns:
>
> 12:   e8 00 00 00 00          callq  17 <lockdep_hardirqs_on+0x17>
>                 13: R_X86_64_PLT32      __sanitizer_cov_trace_pc-0x4
>
> into:
>
> 12:   0f 1f 44 00 00          nopl   0x0(%rax,%rax,1)
>                 13: R_X86_64_NONE      __sanitizer_cov_trace_pc-0x4
>
> Just like recordmcount does.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

Acked-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

Thanks!

> ---
>  tools/objtool/arch.h                      |    2 ++
>  tools/objtool/arch/x86/decode.c           |   18 ++++++++++++++++++
>  tools/objtool/arch/x86/include/arch_elf.h |    6 ++++++
>  tools/objtool/check.c                     |   19 +++++++++++++++++++
>  4 files changed, 45 insertions(+)
>
> --- a/tools/objtool/arch.h
> +++ b/tools/objtool/arch.h
> @@ -84,4 +84,6 @@ unsigned long arch_jump_destination(stru
>
>  unsigned long arch_dest_reloc_offset(int addend);
>
> +const char *arch_nop_insn(int len);
> +
>  #endif /* _ARCH_H */
> --- a/tools/objtool/arch/x86/decode.c
> +++ b/tools/objtool/arch/x86/decode.c
> @@ -565,3 +565,21 @@ void arch_initial_func_cfi_state(struct
>         state->regs[16].base = CFI_CFA;
>         state->regs[16].offset = -8;
>  }
> +
> +const char *arch_nop_insn(int len)
> +{
> +       static const char nops[5][5] = {
> +               /* 1 */ { 0x90 },
> +               /* 2 */ { 0x66, 0x90 },
> +               /* 3 */ { 0x0f, 0x1f, 0x00 },
> +               /* 4 */ { 0x0f, 0x1f, 0x40, 0x00 },
> +               /* 5 */ { 0x0f, 0x1f, 0x44, 0x00, 0x00 },
> +       };
> +
> +       if (len < 1 || len > 5) {
> +               WARN("invalid NOP size: %d\n", len);
> +               return NULL;
> +       }
> +
> +       return nops[len-1];
> +}
> --- /dev/null
> +++ b/tools/objtool/arch/x86/include/arch_elf.h
> @@ -0,0 +1,6 @@
> +#ifndef _OBJTOOL_ARCH_ELF
> +#define _OBJTOOL_ARCH_ELF
> +
> +#define R_NONE R_X86_64_NONE
> +
> +#endif /* _OBJTOOL_ARCH_ELF */
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -12,6 +12,7 @@
>  #include "check.h"
>  #include "special.h"
>  #include "warn.h"
> +#include "arch_elf.h"
>
>  #include <linux/hashtable.h>
>  #include <linux/kernel.h>
> @@ -744,6 +745,24 @@ static int add_call_destinations(struct
>                         insn->call_dest = reloc->sym;
>
>                 /*
> +                * Many compilers cannot disable KCOV with a function attribute
> +                * so they need a little help, NOP out any KCOV calls from noinstr
> +                * text.
> +                */
> +               if (insn->sec->noinstr &&
> +                   !strncmp(insn->call_dest->name, "__sanitizer_cov_", 16)) {
> +                       if (reloc) {
> +                               reloc->type = R_NONE;
> +                               elf_write_reloc(file->elf, reloc);
> +                       }
> +
> +                       elf_write_insn(file->elf, insn->sec,
> +                                      insn->offset, insn->len,
> +                                      arch_nop_insn(insn->len));
> +                       insn->type = INSN_NOP;
> +               }
> +
> +               /*
>                  * Whatever stack impact regular CALLs have, should be undone
>                  * by the RETURN of the called function.
>                  *
>
>