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


On August 19, 2022 2:09:09 PM GMT-03:00, Ian Rogers <irogers@google.com> wrote:
>On Fri, Aug 19, 2022 at 8:58 AM Arnaldo Carvalho de Melo
><acme@kernel.org> wrote:
>>
>> Em Thu, Aug 18, 2022 at 06:50:00PM -0300, Arnaldo Carvalho de Melo escreveu:
>> > 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@google.com>
>> > > ---
>> > > 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?
>>
>> Slapped this there:
>>
>> #pragma GCC diagnostic push
>> #pragma GCC diagnostic ignored "-Wpacked"
>> #pragma GCC diagnostic ignored "-Wattributes"
>>
>> struct __packed perf_record_cpu_map_data {
>> __u16 type;
>> 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;
>> };
>> };
>>
>> #pragma GCC diagnostic pop
>
>Perhaps we should also carry a comment like:
>perf_record_cpu_map_data is packed as unfortunately an earlier version
>had unaligned data and we wish to retain file format compatibility.


Thanks, I'll add it.

- Arnaldo

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