Messages in this thread | | | From | Ian Rogers <> | Date | Fri, 19 Aug 2022 10:09:09 -0700 | Subject | Re: [PATCH v2 4/6] perf cpumap: Fix alignment for masks in event encoding |
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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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