Re: [PATCH v2 4/6] perf cpumap: Fix alignment for masks in event encoding

From: Ian Rogers
Date: Thu Aug 18 2022 - 18:49:30 EST

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On Thu, Aug 18, 2022 at 2:50 PM Arnaldo Carvalho de Melo
<acme@xxxxxxxxxx> wrote:
>
> Em Tue, Jun 14, 2022 at 07:33:51AM -0700, Ian Rogers escreveu:
> > A mask encoding of a cpu map is laid out as:
> > u16 nr
> > u16 long_size
> > unsigned long mask[];
> > However, the mask may be 8-byte aligned meaning there is a 4-byte pad
> > after long_size. This means 32-bit and 64-bit builds see the mask as
> > being at different offsets. On top of this the structure is in the byte
> > data[] encoded as:
> > u16 type
> > char data[]
> > This means the mask's struct isn't the required 4 or 8 byte aligned, but
> > is offset by 2. Consequently the long reads and writes are causing
> > undefined behavior as the alignment is broken.
> >
> > Fix the mask struct by creating explicit 32 and 64-bit variants, use a
> > union to avoid data[] and casts; the struct must be packed so the
> > layout matches the existing perf.data layout. Taking an address of a
> > member of a packed struct breaks alignment so pass the packed
> > perf_record_cpu_map_data to functions, so they can access variables with
> > the right alignment.
> >
> > As the 64-bit version has 4 bytes of padding, optimizing writing to only
> > write the 32-bit version.
> >
> > Signed-off-by: Ian Rogers <irogers@xxxxxxxxxx>
> > ---
> > tools/lib/perf/include/perf/event.h | 36 +++++++++++--
> > tools/perf/tests/cpumap.c | 19 ++++---
> > tools/perf/util/cpumap.c | 80 +++++++++++++++++++++++------
> > tools/perf/util/cpumap.h | 4 +-
> > tools/perf/util/session.c | 30 +++++------
> > tools/perf/util/synthetic-events.c | 34 +++++++-----
> > 6 files changed, 143 insertions(+), 60 deletions(-)
> >
> > diff --git a/tools/lib/perf/include/perf/event.h b/tools/lib/perf/include/perf/event.h
> > index e7758707cadd..d2d32589758a 100644
> > --- a/tools/lib/perf/include/perf/event.h
> > +++ b/tools/lib/perf/include/perf/event.h
> > @@ -6,6 +6,7 @@
> > #include <linux/types.h>
> > #include <linux/limits.h>
> > #include <linux/bpf.h>
> > +#include <linux/compiler.h>
> > #include <sys/types.h> /* pid_t */
> >
> > #define event_contains(obj, mem) ((obj).header.size > offsetof(typeof(obj), mem))
> > @@ -153,20 +154,47 @@ enum {
> > PERF_CPU_MAP__MASK = 1,
> > };
> >
> > +/*
> > + * Array encoding of a perf_cpu_map where nr is the number of entries in cpu[]
> > + * and each entry is a value for a CPU in the map.
> > + */
> > struct cpu_map_entries {
> > __u16 nr;
> > __u16 cpu[];
> > };
> >
> > -struct perf_record_record_cpu_map {
> > +/* Bitmap encoding of a perf_cpu_map where bitmap entries are 32-bit. */
> > +struct perf_record_mask_cpu_map32 {
> > + /* Number of mask values. */
> > __u16 nr;
> > + /* Constant 4. */
> > __u16 long_size;
> > - unsigned long mask[];
> > + /* Bitmap data. */
> > + __u32 mask[];
> > };
> >
> > -struct perf_record_cpu_map_data {
> > +/* Bitmap encoding of a perf_cpu_map where bitmap entries are 64-bit. */
> > +struct perf_record_mask_cpu_map64 {
> > + /* Number of mask values. */
> > + __u16 nr;
> > + /* Constant 8. */
> > + __u16 long_size;
> > + /* Legacy padding. */
> > + char __pad[4];
> > + /* Bitmap data. */
> > + __u64 mask[];
> > +};
> > +
> > +struct __packed perf_record_cpu_map_data {
>
> In various places I'm getting this:
>
> [perfbuilder@five x-riscv]$ export BUILD_TARBALL=http://192.168.86.14/perf/perf-6.0.0-rc1.tar.xz
> [perfbuilder@five x-riscv]$ time dm .
> 1 5.47 ubuntu:22.04-x-riscv64 : FAIL gcc version 11.2.0 (Ubuntu 11.2.0-16ubuntu1)
> In file included from mmap.c:10:
> /git/perf-6.0.0-rc1/tools/lib/perf/include/perf/event.h:190:34: error: packed attribute causes inefficient alignment for 'type' [-Werror=attributes]
> 190 | __u16 type;
> | ^~~~
> cc1: all warnings being treated as errors
> In file included from util/event.h:12,
> from builtin-diff.c:12:
> /git/perf-6.0.0-rc1/tools/lib/perf/include/perf/event.h:190:34: error: packed attribute causes inefficient alignment for 'type' [-Werror=attributes]
> 190 | __u16 type;
> | ^~~~
> In file included from util/events_stats.h:6,
> from util/evlist.h:12,
> from builtin-evlist.c:11:
> /git/perf-6.0.0-rc1/tools/lib/perf/include/perf/event.h:190:34: error: packed attribute causes inefficient alignment for 'type' [-Werror=attributes]
> 190 | __u16 type;
> | ^~~~
>
> So probably we need to disable this -Werror=attributes in some
> architectures?

Looks like it. An inefficient load will be better than a SIGBUS.

Thanks,
Ian

> - Arnaldo
>
> > __u16 type;
> > - char data[];
> > + union {
> > + /* Used when type == PERF_CPU_MAP__CPUS. */
> > + struct cpu_map_entries cpus_data;
> > + /* Used when type == PERF_CPU_MAP__MASK and long_size == 4. */
> > + struct perf_record_mask_cpu_map32 mask32_data;
> > + /* Used when type == PERF_CPU_MAP__MASK and long_size == 8. */
> > + struct perf_record_mask_cpu_map64 mask64_data;
> > + };
> > };
> >
> > struct perf_record_cpu_map {
> > diff --git a/tools/perf/tests/cpumap.c b/tools/perf/tests/cpumap.c
> > index f94929ebb54b..7ea150cdc137 100644
> > --- a/tools/perf/tests/cpumap.c
> > +++ b/tools/perf/tests/cpumap.c
> > @@ -17,21 +17,23 @@ static int process_event_mask(struct perf_tool *tool __maybe_unused,
> > struct machine *machine __maybe_unused)
> > {
> > struct perf_record_cpu_map *map_event = &event->cpu_map;
> > - struct perf_record_record_cpu_map *mask;
> > struct perf_record_cpu_map_data *data;
> > struct perf_cpu_map *map;
> > int i;
> > + unsigned int long_size;
> >
> > data = &map_event->data;
> >
> > TEST_ASSERT_VAL("wrong type", data->type == PERF_CPU_MAP__MASK);
> >
> > - mask = (struct perf_record_record_cpu_map *)data->data;
> > + long_size = data->mask32_data.long_size;
> >
> > - TEST_ASSERT_VAL("wrong nr", mask->nr == 1);
> > + TEST_ASSERT_VAL("wrong long_size", long_size == 4 || long_size == 8);
> > +
> > + TEST_ASSERT_VAL("wrong nr", data->mask32_data.nr == 1);
> >
> > for (i = 0; i < 20; i++) {
> > - TEST_ASSERT_VAL("wrong cpu", test_bit(i, mask->mask));
> > + TEST_ASSERT_VAL("wrong cpu", perf_record_cpu_map_data__test_bit(i, data));
> > }
> >
> > map = cpu_map__new_data(data);
> > @@ -51,7 +53,6 @@ static int process_event_cpus(struct perf_tool *tool __maybe_unused,
> > struct machine *machine __maybe_unused)
> > {
> > struct perf_record_cpu_map *map_event = &event->cpu_map;
> > - struct cpu_map_entries *cpus;
> > struct perf_record_cpu_map_data *data;
> > struct perf_cpu_map *map;
> >
> > @@ -59,11 +60,9 @@ static int process_event_cpus(struct perf_tool *tool __maybe_unused,
> >
> > TEST_ASSERT_VAL("wrong type", data->type == PERF_CPU_MAP__CPUS);
> >
> > - cpus = (struct cpu_map_entries *)data->data;
> > -
> > - TEST_ASSERT_VAL("wrong nr", cpus->nr == 2);
> > - TEST_ASSERT_VAL("wrong cpu", cpus->cpu[0] == 1);
> > - TEST_ASSERT_VAL("wrong cpu", cpus->cpu[1] == 256);
> > + TEST_ASSERT_VAL("wrong nr", data->cpus_data.nr == 2);
> > + TEST_ASSERT_VAL("wrong cpu", data->cpus_data.cpu[0] == 1);
> > + TEST_ASSERT_VAL("wrong cpu", data->cpus_data.cpu[1] == 256);
> >
> > map = cpu_map__new_data(data);
> > TEST_ASSERT_VAL("wrong nr", perf_cpu_map__nr(map) == 2);
> > diff --git a/tools/perf/util/cpumap.c b/tools/perf/util/cpumap.c
> > index 12b2243222b0..ae43fb88f444 100644
> > --- a/tools/perf/util/cpumap.c
> > +++ b/tools/perf/util/cpumap.c
> > @@ -22,54 +22,102 @@ static int max_node_num;
> > */
> > static int *cpunode_map;
> >
> > -static struct perf_cpu_map *cpu_map__from_entries(struct cpu_map_entries *cpus)
> > +bool perf_record_cpu_map_data__test_bit(int i,
> > + const struct perf_record_cpu_map_data *data)
> > +{
> > + int bit_word32 = i / 32;
> > + __u32 bit_mask32 = 1U << (i & 31);
> > + int bit_word64 = i / 64;
> > + __u64 bit_mask64 = ((__u64)1) << (i & 63);
> > +
> > + return (data->mask32_data.long_size == 4)
> > + ? (bit_word32 < data->mask32_data.nr) &&
> > + (data->mask32_data.mask[bit_word32] & bit_mask32) != 0
> > + : (bit_word64 < data->mask64_data.nr) &&
> > + (data->mask64_data.mask[bit_word64] & bit_mask64) != 0;
> > +}
> > +
> > +/* Read ith mask value from data into the given 64-bit sized bitmap */
> > +static void perf_record_cpu_map_data__read_one_mask(const struct perf_record_cpu_map_data *data,
> > + int i, unsigned long *bitmap)
> > +{
> > +#if __SIZEOF_LONG__ == 8
> > + if (data->mask32_data.long_size == 4)
> > + bitmap[0] = data->mask32_data.mask[i];
> > + else
> > + bitmap[0] = data->mask64_data.mask[i];
> > +#else
> > + if (data->mask32_data.long_size == 4) {
> > + bitmap[0] = data->mask32_data.mask[i];
> > + bitmap[1] = 0;
> > + } else {
> > +#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
> > + bitmap[0] = (unsigned long)(data->mask64_data.mask[i] >> 32);
> > + bitmap[1] = (unsigned long)data->mask64_data.mask[i];
> > +#else
> > + bitmap[0] = (unsigned long)data->mask64_data.mask[i];
> > + bitmap[1] = (unsigned long)(data->mask64_data.mask[i] >> 32);
> > +#endif
> > + }
> > +#endif
> > +}
> > +static struct perf_cpu_map *cpu_map__from_entries(const struct perf_record_cpu_map_data *data)
> > {
> > struct perf_cpu_map *map;
> >
> > - map = perf_cpu_map__empty_new(cpus->nr);
> > + map = perf_cpu_map__empty_new(data->cpus_data.nr);
> > if (map) {
> > unsigned i;
> >
> > - for (i = 0; i < cpus->nr; i++) {
> > + for (i = 0; i < data->cpus_data.nr; i++) {
> > /*
> > * Special treatment for -1, which is not real cpu number,
> > * and we need to use (int) -1 to initialize map[i],
> > * otherwise it would become 65535.
> > */
> > - if (cpus->cpu[i] == (u16) -1)
> > + if (data->cpus_data.cpu[i] == (u16) -1)
> > map->map[i].cpu = -1;
> > else
> > - map->map[i].cpu = (int) cpus->cpu[i];
> > + map->map[i].cpu = (int) data->cpus_data.cpu[i];
> > }
> > }
> >
> > return map;
> > }
> >
> > -static struct perf_cpu_map *cpu_map__from_mask(struct perf_record_record_cpu_map *mask)
> > +static struct perf_cpu_map *cpu_map__from_mask(const struct perf_record_cpu_map_data *data)
> > {
> > + DECLARE_BITMAP(local_copy, 64);
> > + int weight = 0, mask_nr = data->mask32_data.nr;
> > struct perf_cpu_map *map;
> > - int nr, nbits = mask->nr * mask->long_size * BITS_PER_BYTE;
> >
> > - nr = bitmap_weight(mask->mask, nbits);
> > + for (int i = 0; i < mask_nr; i++) {
> > + perf_record_cpu_map_data__read_one_mask(data, i, local_copy);
> > + weight += bitmap_weight(local_copy, 64);
> > + }
> > +
> > + map = perf_cpu_map__empty_new(weight);
> > + if (!map)
> > + return NULL;
> >
> > - map = perf_cpu_map__empty_new(nr);
> > - if (map) {
> > - int cpu, i = 0;
> > + for (int i = 0, j = 0; i < mask_nr; i++) {
> > + int cpus_per_i = (i * data->mask32_data.long_size * BITS_PER_BYTE);
> > + int cpu;
> >
> > - for_each_set_bit(cpu, mask->mask, nbits)
> > - map->map[i++].cpu = cpu;
> > + perf_record_cpu_map_data__read_one_mask(data, i, local_copy);
> > + for_each_set_bit(cpu, local_copy, 64)
> > + map->map[j++].cpu = cpu + cpus_per_i;
> > }
> > return map;
> >
> > }
> >
> > -struct perf_cpu_map *cpu_map__new_data(struct perf_record_cpu_map_data *data)
> > +struct perf_cpu_map *cpu_map__new_data(const struct perf_record_cpu_map_data *data)
> > {
> > if (data->type == PERF_CPU_MAP__CPUS)
> > - return cpu_map__from_entries((struct cpu_map_entries *)data->data);
> > + return cpu_map__from_entries(data);
> > else
> > - return cpu_map__from_mask((struct perf_record_record_cpu_map *)data->data);
> > + return cpu_map__from_mask(data);
> > }
> >
> > size_t cpu_map__fprintf(struct perf_cpu_map *map, FILE *fp)
> > diff --git a/tools/perf/util/cpumap.h b/tools/perf/util/cpumap.h
> > index 703ae6d3386e..fa8a5acdcae1 100644
> > --- a/tools/perf/util/cpumap.h
> > +++ b/tools/perf/util/cpumap.h
> > @@ -37,9 +37,11 @@ struct cpu_aggr_map {
> >
> > struct perf_record_cpu_map_data;
> >
> > +bool perf_record_cpu_map_data__test_bit(int i, const struct perf_record_cpu_map_data *data);
> > +
> > struct perf_cpu_map *perf_cpu_map__empty_new(int nr);
> >
> > -struct perf_cpu_map *cpu_map__new_data(struct perf_record_cpu_map_data *data);
> > +struct perf_cpu_map *cpu_map__new_data(const struct perf_record_cpu_map_data *data);
> > size_t cpu_map__snprint(struct perf_cpu_map *map, char *buf, size_t size);
> > size_t cpu_map__snprint_mask(struct perf_cpu_map *map, char *buf, size_t size);
> > size_t cpu_map__fprintf(struct perf_cpu_map *map, FILE *fp);
> > diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> > index 0aa818977d2b..d52a39ba48e3 100644
> > --- a/tools/perf/util/session.c
> > +++ b/tools/perf/util/session.c
> > @@ -914,30 +914,30 @@ static void perf_event__cpu_map_swap(union perf_event *event,
> > bool sample_id_all __maybe_unused)
> > {
> > struct perf_record_cpu_map_data *data = &event->cpu_map.data;
> > - struct cpu_map_entries *cpus;
> > - struct perf_record_record_cpu_map *mask;
> > - unsigned i;
> >
> > data->type = bswap_16(data->type);
> >
> > switch (data->type) {
> > case PERF_CPU_MAP__CPUS:
> > - cpus = (struct cpu_map_entries *)data->data;
> > -
> > - cpus->nr = bswap_16(cpus->nr);
> > + data->cpus_data.nr = bswap_16(data->cpus_data.nr);
> >
> > - for (i = 0; i < cpus->nr; i++)
> > - cpus->cpu[i] = bswap_16(cpus->cpu[i]);
> > + for (unsigned i = 0; i < data->cpus_data.nr; i++)
> > + data->cpus_data.cpu[i] = bswap_16(data->cpus_data.cpu[i]);
> > break;
> > case PERF_CPU_MAP__MASK:
> > - mask = (struct perf_record_record_cpu_map *)data->data;
> > -
> > - mask->nr = bswap_16(mask->nr);
> > - mask->long_size = bswap_16(mask->long_size);
> > + data->mask32_data.long_size = bswap_16(data->mask32_data.long_size);
> >
> > - switch (mask->long_size) {
> > - case 4: mem_bswap_32(&mask->mask, mask->nr); break;
> > - case 8: mem_bswap_64(&mask->mask, mask->nr); break;
> > + switch (data->mask32_data.long_size) {
> > + case 4:
> > + data->mask32_data.nr = bswap_16(data->mask32_data.nr);
> > + for (unsigned i = 0; i < data->mask32_data.nr; i++)
> > + data->mask32_data.mask[i] = bswap_32(data->mask32_data.mask[i]);
> > + break;
> > + case 8:
> > + data->mask64_data.nr = bswap_16(data->mask64_data.nr);
> > + for (unsigned i = 0; i < data->mask64_data.nr; i++)
> > + data->mask64_data.mask[i] = bswap_64(data->mask64_data.mask[i]);
> > + break;
> > default:
> > pr_err("cpu_map swap: unsupported long size\n");
> > }
> > diff --git a/tools/perf/util/synthetic-events.c b/tools/perf/util/synthetic-events.c
> > index 0d87df20ec44..4fa7d0d7dbcf 100644
> > --- a/tools/perf/util/synthetic-events.c
> > +++ b/tools/perf/util/synthetic-events.c
> > @@ -1183,27 +1183,33 @@ int perf_event__synthesize_thread_map2(struct perf_tool *tool,
> > return err;
> > }
> >
> > -static void synthesize_cpus(struct cpu_map_entries *cpus,
> > +static void synthesize_cpus(struct perf_record_cpu_map_data *data,
> > const struct perf_cpu_map *map)
> > {
> > int i, map_nr = perf_cpu_map__nr(map);
> >
> > - cpus->nr = map_nr;
> > + data->cpus_data.nr = map_nr;
> >
> > for (i = 0; i < map_nr; i++)
> > - cpus->cpu[i] = perf_cpu_map__cpu(map, i).cpu;
> > + data->cpus_data.cpu[i] = perf_cpu_map__cpu(map, i).cpu;
> > }
> >
> > -static void synthesize_mask(struct perf_record_record_cpu_map *mask,
> > +static void synthesize_mask(struct perf_record_cpu_map_data *data,
> > const struct perf_cpu_map *map, int max)
> > {
> > - int i;
> > + int idx;
> > + struct perf_cpu cpu;
> > +
> > + /* Due to padding, the 4bytes per entry mask variant is always smaller. */
> > + data->mask32_data.nr = BITS_TO_U32(max);
> > + data->mask32_data.long_size = 4;
> >
> > - mask->nr = BITS_TO_LONGS(max);
> > - mask->long_size = sizeof(long);
> > + perf_cpu_map__for_each_cpu(cpu, idx, map) {
> > + int bit_word = cpu.cpu / 32;
> > + __u32 bit_mask = 1U << (cpu.cpu & 31);
> >
> > - for (i = 0; i < perf_cpu_map__nr(map); i++)
> > - set_bit(perf_cpu_map__cpu(map, i).cpu, mask->mask);
> > + data->mask32_data.mask[bit_word] |= bit_mask;
> > + }
> > }
> >
> > static size_t cpus_size(const struct perf_cpu_map *map)
> > @@ -1214,7 +1220,7 @@ static size_t cpus_size(const struct perf_cpu_map *map)
> > static size_t mask_size(const struct perf_cpu_map *map, int *max)
> > {
> > *max = perf_cpu_map__max(map).cpu;
> > - return sizeof(struct perf_record_record_cpu_map) + BITS_TO_LONGS(*max) * sizeof(long);
> > + return sizeof(struct perf_record_mask_cpu_map32) + BITS_TO_U32(*max) * sizeof(__u32);
> > }
> >
> > static void *cpu_map_data__alloc(const struct perf_cpu_map *map, size_t *size,
> > @@ -1247,7 +1253,7 @@ static void *cpu_map_data__alloc(const struct perf_cpu_map *map, size_t *size,
> > *type = PERF_CPU_MAP__MASK;
> > }
> >
> > - *size += sizeof(struct perf_record_cpu_map_data);
> > + *size += sizeof(__u16); /* For perf_record_cpu_map_data.type. */
> > *size = PERF_ALIGN(*size, sizeof(u64));
> > return zalloc(*size);
> > }
> > @@ -1260,10 +1266,10 @@ static void cpu_map_data__synthesize(struct perf_record_cpu_map_data *data,
> >
> > switch (type) {
> > case PERF_CPU_MAP__CPUS:
> > - synthesize_cpus((struct cpu_map_entries *) data->data, map);
> > + synthesize_cpus(data, map);
> > break;
> > case PERF_CPU_MAP__MASK:
> > - synthesize_mask((struct perf_record_record_cpu_map *)data->data, map, max);
> > + synthesize_mask(data, map, max);
> > default:
> > break;
> > }
> > @@ -1271,7 +1277,7 @@ static void cpu_map_data__synthesize(struct perf_record_cpu_map_data *data,
> >
> > static struct perf_record_cpu_map *cpu_map_event__new(const struct perf_cpu_map *map)
> > {
> > - size_t size = sizeof(struct perf_record_cpu_map);
> > + size_t size = sizeof(struct perf_event_header);
> > struct perf_record_cpu_map *event;
> > int max;
> > u16 type;
> > --
> > 2.36.1.476.g0c4daa206d-goog
>
> --
>
> - Arnaldo

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