Messages in this thread | | | From | Dmitry Vyukov <> | Date | Mon, 23 May 2022 10:38:39 +0200 | Subject | Re: [PATCH v2] kcov: update pos before writing pc in trace function |
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On Mon, 23 May 2022 at 07:35, Congyu Liu <liu3101@purdue.edu> wrote: > > In __sanitizer_cov_trace_pc(), previously we write pc before updating pos. > However, some early interrupt code could bypass check_kcov_mode() > check and invoke __sanitizer_cov_trace_pc(). If such interrupt is raised > between writing pc and updating pos, the pc could be overitten by the > recursive __sanitizer_cov_trace_pc(). > > As suggested by Dmitry, we cold update pos before writing pc to avoid > such interleaving. > > Apply the same change to write_comp_data(). > > Signed-off-by: Congyu Liu <liu3101@purdue.edu>
This version looks good to me. I wonder how you encountered this? Do you mind sharing a bit about what you are doing with kcov?
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
Thanks
> --- > PATCH v2: > * Update pos before writing pc as suggested by Dmitry. > > PATCH v1: > https://lore.kernel.org/lkml/20220517210532.1506591-1-liu3101@purdue.edu/ > --- > kernel/kcov.c | 14 ++++++++++++-- > 1 file changed, 12 insertions(+), 2 deletions(-) > > diff --git a/kernel/kcov.c b/kernel/kcov.c > index b3732b210593..e19c84b02452 100644 > --- a/kernel/kcov.c > +++ b/kernel/kcov.c > @@ -204,8 +204,16 @@ void notrace __sanitizer_cov_trace_pc(void) > /* The first 64-bit word is the number of subsequent PCs. */ > pos = READ_ONCE(area[0]) + 1; > if (likely(pos < t->kcov_size)) { > - area[pos] = ip; > + /* Previously we write pc before updating pos. However, some > + * early interrupt code could bypass check_kcov_mode() check > + * and invoke __sanitizer_cov_trace_pc(). If such interrupt is > + * raised between writing pc and updating pos, the pc could be > + * overitten by the recursive __sanitizer_cov_trace_pc(). > + * Update pos before writing pc to avoid such interleaving. > + */ > WRITE_ONCE(area[0], pos); > + barrier(); > + area[pos] = ip; > } > } > EXPORT_SYMBOL(__sanitizer_cov_trace_pc); > @@ -236,11 +244,13 @@ static void notrace write_comp_data(u64 type, u64 arg1, u64 arg2, u64 ip) > start_index = 1 + count * KCOV_WORDS_PER_CMP; > end_pos = (start_index + KCOV_WORDS_PER_CMP) * sizeof(u64); > if (likely(end_pos <= max_pos)) { > + /* See comment in __sanitizer_cov_trace_pc(). */ > + WRITE_ONCE(area[0], count + 1); > + barrier(); > area[start_index] = type; > area[start_index + 1] = arg1; > area[start_index + 2] = arg2; > area[start_index + 3] = ip; > - WRITE_ONCE(area[0], count + 1); > } > } > > -- > 2.34.1 >
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