Messages in this thread | | | From | Dmitry Vyukov <> | Date | Tue, 24 May 2022 08:38:55 +0200 | Subject | Re: [PATCH v2] kcov: update pos before writing pc in trace function |
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On Tue, 24 May 2022 at 05:08, Liu, Congyu <liu3101@purdue.edu> wrote: > > It was actually first found in the kernel trace module I wrote for my research > project. For each call instruction I instrumented one trace function before it > and one trace function after it, then expected traces generated from > them would match since I only instrumented calls that return. But it turns > out that it didn't match from time to time in a non-deterministic manner. > Eventually I figured out it was actually caused by the overwritten issue > from interrupt. I then referred to kcov for a solution but it also suffered from > the same issue...so here's this patch :).
Ah, interesting. Thanks for sharing.
> ________________________________________ > From: Dmitry Vyukov <dvyukov@google.com> > Sent: Monday, May 23, 2022 4:38 > To: Liu, Congyu > Cc: andreyknvl@gmail.com; kasan-dev@googlegroups.com; linux-kernel@vger.kernel.org > Subject: Re: [PATCH v2] kcov: update pos before writing pc in trace function > > 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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