Messages in this thread Patch in this message | | | From | "Liu, Congyu" <> | Subject | Re: [PATCH v2] kcov: update pos before writing pc in trace function | Date | Tue, 24 May 2022 03:10:37 +0000 |
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+Andrew Morton
________________________________________ From: Liu, Congyu <liu3101@purdue.edu> Sent: Monday, May 23, 2022 23:08 To: Dmitry Vyukov 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
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 :).
________________________________________ 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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