Re: [PATCH 2/3] kcov: wire up compiler instrumentation for CONFIG_KCOV_EXT_RECORDS

[Dmitry Vyukov]

On Wed, 11 Mar 2026 at 22:06, Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> This is the first half of CONFIG_KCOV_EXT_RECORDS.
>
> Set the appropriate compiler flags to call separate hooks for function
> entry/exit, and provide these hooks, but don't make it visible in the KCOV
> UAPI yet.
>
> With -fsanitize-coverage=trace-pc-entry-exit, the compiler behavior changes
> as follows:
>
> - The __sanitizer_cov_trace_pc() call on function entry is replaced with a
>   call to __sanitizer_cov_trace_pc_entry(); so for now,
>   __sanitizer_cov_trace_pc_entry() must be treated the same way as
>   __sanitizer_cov_trace_pc().
>  - On function exit, an extra call to __sanitizer_cov_trace_pc_exit()
>    happens; since function exit produced no coverage in the old UAPI,
>    __sanitizer_cov_trace_pc_exit() should do nothing for now.
>
> Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
>  include/linux/kcov.h  |  2 ++
>  kernel/kcov.c         | 30 +++++++++++++++++++++++-------
>  lib/Kconfig.debug     | 14 ++++++++++++++
>  scripts/Makefile.kcov |  2 ++
>  tools/objtool/check.c |  2 ++
>  5 files changed, 43 insertions(+), 7 deletions(-)
>
> diff --git a/include/linux/kcov.h b/include/linux/kcov.h
> index 0143358874b0..e5502d674029 100644
> --- a/include/linux/kcov.h
> +++ b/include/linux/kcov.h
> @@ -81,6 +81,8 @@ typedef unsigned long long kcov_u64;
>  #endif
>
>  void __sanitizer_cov_trace_pc(void);
> +void __sanitizer_cov_trace_pc_entry(void);
> +void __sanitizer_cov_trace_pc_exit(void);
>  void __sanitizer_cov_trace_cmp1(u8 arg1, u8 arg2);
>  void __sanitizer_cov_trace_cmp2(u16 arg1, u16 arg2);
>  void __sanitizer_cov_trace_cmp4(u32 arg1, u32 arg2);
> diff --git a/kernel/kcov.c b/kernel/kcov.c
> index 0b369e88c7c9..2cc48b65384b 100644
> --- a/kernel/kcov.c
> +++ b/kernel/kcov.c
> @@ -202,15 +202,10 @@ static notrace unsigned long canonicalize_ip(unsigned long ip)
>         return ip;
>  }
>
> -/*
> - * Entry point from instrumented code.
> - * This is called once per basic-block/edge.
> - */
> -void notrace __sanitizer_cov_trace_pc(void)
> +static void notrace kcov_add_pc_record(unsigned long record)
>  {
>         struct task_struct *t;
>         unsigned long *area;
> -       unsigned long ip = canonicalize_ip(_RET_IP_);
>         unsigned long pos;
>
>         t = current;
> @@ -230,11 +225,32 @@ void notrace __sanitizer_cov_trace_pc(void)
>                  */
>                 WRITE_ONCE(area[0], pos);
>                 barrier();
> -               area[pos] = ip;
> +               area[pos] = record;
>         }
>  }
> +
> +/*
> + * Entry point from instrumented code.
> + * This is called once per basic-block/edge.
> + */
> +void notrace __sanitizer_cov_trace_pc(void)
> +{
> +       kcov_add_pc_record(canonicalize_ip(_RET_IP_));
> +}
>  EXPORT_SYMBOL(__sanitizer_cov_trace_pc);
>
> +#ifdef CONFIG_KCOV_EXT_RECORDS
> +void notrace __sanitizer_cov_trace_pc_entry(void)
> +{
> +       unsigned long record = canonicalize_ip(_RET_IP_);
> +
> +       kcov_add_pc_record(record);
> +}
> +void notrace __sanitizer_cov_trace_pc_exit(void)
> +{
> +}
> +#endif
> +
>  #ifdef CONFIG_KCOV_ENABLE_COMPARISONS
>  static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip)
>  {
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 93f356d2b3d9..dddc330ad3ca 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -2215,6 +2215,20 @@ config KCOV
>
>           For more details, see Documentation/dev-tools/kcov.rst.
>
> +config KCOV_EXT_RECORDS
> +       bool "Support extended KCOV records with function entry/exit records"
> +       depends on KCOV
> +       depends on 64BIT
> +       # TODO: check CLANG_VERSION instead once this has landed in an LLVM
> +       # release

I think the compiler option check is actually better, since it will
allow us to test earlier and supports compiler backports.
But it may be good to add a reference to the compiler patch in the
commit description.

Otherwise:

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

> +       depends on $(cc-option,-fsanitize-coverage=trace-pc-entry-exit)
> +       help
> +         Extended KCOV records allow distinguishing between multiple types of
> +         records: Normal edge coverage, function entry, and function exit.
> +
> +         This will likely cause a small additional slowdown compared to normal
> +         KCOV.
> +
>  config KCOV_ENABLE_COMPARISONS
>         bool "Enable comparison operands collection by KCOV"
>         depends on KCOV
> diff --git a/scripts/Makefile.kcov b/scripts/Makefile.kcov
> index 78305a84ba9d..aa0be904268f 100644
> --- a/scripts/Makefile.kcov
> +++ b/scripts/Makefile.kcov
> @@ -1,10 +1,12 @@
>  # SPDX-License-Identifier: GPL-2.0-only
>  kcov-flags-y                                   += -fsanitize-coverage=trace-pc
> +kcov-flags-$(CONFIG_KCOV_EXT_RECORDS)          += -fsanitize-coverage=trace-pc-entry-exit
>  kcov-flags-$(CONFIG_KCOV_ENABLE_COMPARISONS)   += -fsanitize-coverage=trace-cmp
>
>  kcov-rflags-y                                  += -Cpasses=sancov-module
>  kcov-rflags-y                                  += -Cllvm-args=-sanitizer-coverage-level=3
>  kcov-rflags-y                                  += -Cllvm-args=-sanitizer-coverage-trace-pc
> +kcov-rflags-$(CONFIG_KCOV_EXT_RECORDS)         += -Cllvm-args=-sanitizer-coverage-trace-pc-entry-exit
>  kcov-rflags-$(CONFIG_KCOV_ENABLE_COMPARISONS)  += -Cllvm-args=-sanitizer-coverage-trace-compares
>
>  export CFLAGS_KCOV := $(kcov-flags-y)
> diff --git a/tools/objtool/check.c b/tools/objtool/check.c
> index a30379e4ff97..ae3127227621 100644
> --- a/tools/objtool/check.c
> +++ b/tools/objtool/check.c
> @@ -1251,6 +1251,8 @@ static const char *uaccess_safe_builtin[] = {
>         "write_comp_data",
>         "check_kcov_mode",
>         "__sanitizer_cov_trace_pc",
> +       "__sanitizer_cov_trace_pc_entry",
> +       "__sanitizer_cov_trace_pc_exit",
>         "__sanitizer_cov_trace_const_cmp1",
>         "__sanitizer_cov_trace_const_cmp2",
>         "__sanitizer_cov_trace_const_cmp4",
>
> --
> 2.53.0.473.g4a7958ca14-goog
>