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

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