[RFC PATCH v2 03/14] kcov: add barriers to recursion guard in kcov_df_write

[Yunseong Kim]

The recursion guard (bit-31 of kcov_df_seq) prevents reentry when
copy_from_kernel_nofault() or other called functions are instrumented
with INSTRUMENT_ALL. Without compiler barriers, the guard set/clear
can be reordered relative to the function body, making the protection
ineffective under optimization.

Add barrier() after setting the guard and before clearing it, ensuring
the compiler does not move instrumented operations outside the guarded
region.

Cc: Peter Zijlstra <pet...@infradead.org>
Signed-off-by: Yunseong Kim <yunseo...@est.tech>
---
kernel/kcov_dataflow.c | 2 ++
1 file changed, 2 insertions(+)

diff --git a/kernel/kcov_dataflow.c b/kernel/kcov_dataflow.c
index df7e8bf70bfa..5248293280d5 100644
--- a/kernel/kcov_dataflow.c
+++ b/kernel/kcov_dataflow.c
@@ -86,6 +86,7 @@ kcov_df_write(u64 type_marker, u64 pc, u64 meta, void *ptr,
if (t->kcov_df_seq & (1U << 31))
return;
t->kcov_df_seq |= (1U << 31);
+ barrier();

area = (u64 *)t->kcov_df_area;
if (!area)
@@ -147,6 +148,7 @@ kcov_df_write(u64 type_marker, u64 pc, u64 meta, void *ptr,
}
}
out:
+ barrier();
t->kcov_df_seq &= ~(1U << 31);
}


--
2.43.0

[Alexander Potapenko]

On Thu, Jun 11, 2026 at 6:21 PM Yunseong Kim <yunseo...@est.tech> wrote:
>
> The recursion guard (bit-31 of kcov_df_seq) prevents reentry when
> copy_from_kernel_nofault() or other called functions are instrumented
> with INSTRUMENT_ALL. Without compiler barriers, the guard set/clear
> can be reordered relative to the function body, making the protection
> ineffective under optimization.
>
> Add barrier() after setting the guard and before clearing it, ensuring
> the compiler does not move instrumented operations outside the guarded
> region.
>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Signed-off-by: Yunseong Kim <yunseo...@est.tech>
> ---
> kernel/kcov_dataflow.c | 2 ++

Please merge this patch into the one introducing kcov_dataflow.c

> 1 file changed, 2 insertions(+)
>
> diff --git a/kernel/kcov_dataflow.c b/kernel/kcov_dataflow.c
> index df7e8bf70bfa..5248293280d5 100644
> --- a/kernel/kcov_dataflow.c
> +++ b/kernel/kcov_dataflow.c
> @@ -86,6 +86,7 @@ kcov_df_write(u64 type_marker, u64 pc, u64 meta, void *ptr,
> if (t->kcov_df_seq & (1U << 31))
> return;
> t->kcov_df_seq |= (1U << 31);
> + barrier();

Please make sure barriers have comments explaining which barriers they
pair with (see kernel/kcov.c)

[Yunseong Kim]

Hi Alexander,

> On Thu, Jun 11, 2026 at 6:21 PM Yunseong Kim <yunseo...@est.tech> wrote:
>>
>> The recursion guard (bit-31 of kcov_df_seq) prevents reentry when
>> copy_from_kernel_nofault() or other called functions are instrumented
>> with INSTRUMENT_ALL. Without compiler barriers, the guard set/clear
>> can be reordered relative to the function body, making the protection
>> ineffective under optimization.
>>
>> Add barrier() after setting the guard and before clearing it, ensuring
>> the compiler does not move instrumented operations outside the guarded
>> region.
>>
>> Cc: Peter Zijlstra <pet...@infradead.org>
>> Signed-off-by: Yunseong Kim <yunseo...@est.tech>
>> ---
>> kernel/kcov_dataflow.c | 2 ++
>
> Please merge this patch into the one introducing kcov_dataflow.c
>

Understood. I'll merge them in v3.

>> 1 file changed, 2 insertions(+)
>>
>> diff --git a/kernel/kcov_dataflow.c b/kernel/kcov_dataflow.c
>> index df7e8bf70bfa..5248293280d5 100644
>> --- a/kernel/kcov_dataflow.c
>> +++ b/kernel/kcov_dataflow.c
>> @@ -86,6 +86,7 @@ kcov_df_write(u64 type_marker, u64 pc, u64 meta, void *ptr,
>> if (t->kcov_df_seq & (1U << 31))
>> return;
>> t->kcov_df_seq |= (1U << 31);
>> + barrier();
>
> Please make sure barriers have comments explaining which barriers they
> pair with (see kernel/kcov.c)

Thanks for the pointer. I see the existing implementation now and will align
my changes with it.

Best regards,
Yunseong