Re: [PATCH v1 2/3] perf auxtrace: Add compat_auxtrace_mmap__{read_head|write_tail}

Arnaldo Carvalho de Melo
2021-08-09T20:12:35Z

Em Mon, Aug 09, 2021 at 07:27:26PM +0800, Leo Yan escreveu:
> When perf runs in compat mode (kernel in 64-bit mode and the perf is in
> 32-bit mode), the 64-bit value atomicity in the user space cannot be
> assured, E.g. on some architectures, the 64-bit value accessing is split
> into two instructions, one is for the low 32-bit word accessing and
> another is for the high 32-bit word.
>
> This patch introduces weak functions compat_auxtrace_mmap__read_head()
> and compat_auxtrace_mmap__write_tail(), as their naming indicates, when
> perf tool works in compat mode, it uses these two functions to access
> the AUX head and tail. These two functions can allow the perf tool to
> work properly in certain conditions, e.g. when perf tool works in
> snapshot mode with only using AUX head pointer, or perf tool uses the
> AUX buffer and the incremented tail is not bigger than 4GB.
>
> When perf tool cannot handle the case when the AUX tail is bigger than
> 4GB, the function compat_auxtrace_mmap__write_tail() returns -1 and
> tells the caller to bail out for the error.
>
> These two functions are declared as weak attribute, this allows to
> implement arch specific functions if any arch can support the 64-bit
> value atomicity in compat mode.

Adrian, ARM guys, can you please review this?

Thanks,

- Arnaldo

> Suggested-by: Adrian Hunter <adrian.hunter@intel.com>
> Signed-off-by: Leo Yan <leo.yan@linaro.org>
> ---
> tools/perf/util/auxtrace.c | 88 ++++++++++++++++++++++++++++++++++++--
> tools/perf/util/auxtrace.h | 22 ++++++++--
> 2 files changed, 103 insertions(+), 7 deletions(-)
>
> diff --git a/tools/perf/util/auxtrace.c b/tools/perf/util/auxtrace.c
> index f33f09b8b535..60975df05d54 100644
> --- a/tools/perf/util/auxtrace.c
> +++ b/tools/perf/util/auxtrace.c
> @@ -1669,6 +1669,82 @@ int perf_event__process_auxtrace_error(struct perf_session *session,
> return 0;
> }
>
> +/*
> + * In the compat mode kernel runs in 64-bit and perf tool runs in 32-bit mode,
> + * 32-bit perf tool cannot access 64-bit value atomically, which might lead to
> + * the issues caused by the below sequence on multiple CPUs: when perf tool
> + * accesses either the load operation or the store operation for 64-bit value,
> + * on some architectures the operation is divided into two instructions, one
> + * is for accessing the low 32-bit value and another is for the high 32-bit;
> + * thus these two user operations can give the kernel chances to access the
> + * 64-bit value, and thus leads to the unexpected load values.
> + *
> + * kernel (64-bit) user (32-bit)
> + *
> + * if (LOAD ->aux_tail) { --, LOAD ->aux_head_lo
> + * STORE $aux_data | ,--->
> + * FLUSH $aux_data | | LOAD ->aux_head_hi
> + * STORE ->aux_head --|-------` smp_rmb()
> + * } | LOAD $data
> + * | smp_mb()
> + * | STORE ->aux_tail_lo
> + * `----------->
> + * STORE ->aux_tail_hi
> + *
> + * For this reason, it's impossible for the perf tool to work correctly when
> + * the AUX head or tail is bigger than 4GB (more than 32 bits length); and we
> + * can not simply limit the AUX ring buffer to less than 4GB, the reason is
> + * the pointers can be increased monotonically, whatever the buffer size it is,
> + * at the end the head and tail can be bigger than 4GB and carry out to the
> + * high 32-bit.
> + *
> + * To mitigate the issues and improve the user experience, we can allow the
> + * perf tool working in certain conditions and bail out with error if detect
> + * any overflow cannot be handled.
> + *
> + * For reading the AUX head, it reads out the values for three times, and
> + * compares the high 4 bytes of the values between the first time and the last
> + * time, if there has no change for high 4 bytes injected by the kernel during
> + * the user reading sequence, it's safe for use the second value.
> + *
> + * When update the AUX tail and detects any carrying in the high 32 bits, it
> + * means there have two store operations in user space and it cannot promise
> + * the atomicity for 64-bit write, so return '-1' in this case to tell the
> + * caller an overflow error has happened.
> + */
> +u64 __weak compat_auxtrace_mmap__read_head(struct auxtrace_mmap *mm)
> +{
> + struct perf_event_mmap_page *pc = mm->userpg;
> + u64 first, second, last;
> + u64 mask = (u64)(UINT32_MAX) << 32;
> +
> + do {
> + first = READ_ONCE(pc->aux_head);
> + /* Ensure all reads are done after we read the head */
> + smp_rmb();
> + second = READ_ONCE(pc->aux_head);
> + /* Ensure all reads are done after we read the head */
> + smp_rmb();
> + last = READ_ONCE(pc->aux_head);
> + } while ((first & mask) != (last & mask));
> +
> + return second;
> +}
> +
> +int __weak compat_auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail)
> +{
> + struct perf_event_mmap_page *pc = mm->userpg;
> + u64 mask = (u64)(UINT32_MAX) << 32;
> +
> + if (tail & mask)
> + return -1;
> +
> + /* Ensure all reads are done before we write the tail out */
> + smp_mb();
> + WRITE_ONCE(pc->aux_tail, tail);
> + return 0;
> +}
> +
> static int __auxtrace_mmap__read(struct mmap *map,
> struct auxtrace_record *itr,
> struct perf_tool *tool, process_auxtrace_t fn,
> @@ -1680,8 +1756,9 @@ static int __auxtrace_mmap__read(struct mmap *map,
> size_t size, head_off, old_off, len1, len2, padding;
> union perf_event ev;
> void *data1, *data2;
> + int kernel_is_64_bit = perf_env__kernel_is_64_bit(evsel__env(NULL));
>
> - head = auxtrace_mmap__read_head(mm);
> + head = auxtrace_mmap__read_head(mm, kernel_is_64_bit);
>
> if (snapshot &&
> auxtrace_record__find_snapshot(itr, mm->idx, mm, data, &head, &old))
> @@ -1764,10 +1841,13 @@ static int __auxtrace_mmap__read(struct mmap *map,
> mm->prev = head;
>
> if (!snapshot) {
> - auxtrace_mmap__write_tail(mm, head);
> - if (itr->read_finish) {
> - int err;
> + int err;
> +
> + err = auxtrace_mmap__write_tail(mm, head, kernel_is_64_bit);
> + if (err < 0)
> + return err;
>
> + if (itr->read_finish) {
> err = itr->read_finish(itr, mm->idx);
> if (err < 0)
> return err;
> diff --git a/tools/perf/util/auxtrace.h b/tools/perf/util/auxtrace.h
> index d68a5e80b217..5f383908ca6e 100644
> --- a/tools/perf/util/auxtrace.h
> +++ b/tools/perf/util/auxtrace.h
> @@ -440,23 +440,39 @@ struct auxtrace_cache;
>
> #ifdef HAVE_AUXTRACE_SUPPORT
>
> -static inline u64 auxtrace_mmap__read_head(struct auxtrace_mmap *mm)
> +u64 compat_auxtrace_mmap__read_head(struct auxtrace_mmap *mm);
> +int compat_auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail);
> +
> +static inline u64 auxtrace_mmap__read_head(struct auxtrace_mmap *mm,
> + int kernel_is_64_bit __maybe_unused)
> {
> struct perf_event_mmap_page *pc = mm->userpg;
> - u64 head = READ_ONCE(pc->aux_head);
> + u64 head;
> +
> +#if BITS_PER_LONG == 32
> + if (kernel_is_64_bit)
> + return compat_auxtrace_mmap__read_head(mm);
> +#endif
> + head = READ_ONCE(pc->aux_head);
>
> /* Ensure all reads are done after we read the head */
> smp_rmb();
> return head;
> }
>
> -static inline void auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail)
> +static inline int auxtrace_mmap__write_tail(struct auxtrace_mmap *mm, u64 tail,
> + int kernel_is_64_bit __maybe_unused)
> {
> struct perf_event_mmap_page *pc = mm->userpg;
>
> +#if BITS_PER_LONG == 32
> + if (kernel_is_64_bit)
> + return compat_auxtrace_mmap__write_tail(mm, tail);
> +#endif
> /* Ensure all reads are done before we write the tail out */
> smp_mb();
> WRITE_ONCE(pc->aux_tail, tail);
> + return 0;
> }
>
> int auxtrace_mmap__mmap(struct auxtrace_mmap *mm,
> --
> 2.25.1
>

--

- Arnaldo