Re: [PATCH v5 14/18] perf: fix endianness detection in perf.data

[Stephane Eranian <eranian@google.com>]

On Mon, Feb 6, 2012 at 7:17 PM, Arnaldo Carvalho de Melo
<acme@redhat.com> wrote:
> Em Thu, Feb 02, 2012 at 01:54:44PM +0100, Stephane Eranian escreveu:
>> The current version of perf detects whether or not
>> the perf.data file is written in a different endianness
>> using the attr_size field in the header of the file. This
>> field represents sizeof(struct perf_event_attr) as known
>> to perf record. If the sizes do not match, then perf tries
>> the byte-swapped version. If they match, then the tool assumes
>> a different endianness.
>>
>> The issue with the approach is that it assumes the size of
>> perf_event_attr always has to match between perf record and
>> perf report. However, the kernel perf_event ABI is extensible.
>> New fields can be added to struct perf_event_attr. Consequently,
>> it is not possible to use attr_size to detect endianness.
>>
>> This patch takes another approach by using the magic number
>> written at the beginning of the perf.data file to detect
>> endianness. The magic number is an eight-byte signature.
>> It's primary purpose is to identify (signature) a perf.data
>> file. But it could also be used to encode the endianness.
>>
>> The patch introduces a new value for this signature. The key
>> difference is that the signature is written differently in
>> the file depending on the endianness. Thus, by comparing the
>> signature from the file with the tool's own signature it is
>> possible to detect endianness. The new signature is "PERFILE2".
>>
>> Backward compatiblity with existing perf.data file is
>> ensured.
>
> Looks ok, but IIRC David Ahern interacted with you on this specific
> patch in the past, having his Acked-by and/or Tested-by would be great,
> David?
>
I agree, I am still waiting for the results of his test on big-endian systems.
I don't have any unfortunately.

> - Arnaldo
>
>> Signed-off-by: Stephane Eranian <eranian@google.com>
>> ---
>>  tools/perf/util/header.c |   77 ++++++++++++++++++++++++++++++++++++++--------
>>  1 files changed, 64 insertions(+), 13 deletions(-)
>>
>> diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
>> index ecd7f4d..6f4187d 100644
>> --- a/tools/perf/util/header.c
>> +++ b/tools/perf/util/header.c
>> @@ -63,9 +63,20 @@ char *perf_header__find_event(u64 id)
>>       return NULL;
>>  }
>>
>> -static const char *__perf_magic = "PERFFILE";
>> +/*
>> + * magic2 = "PERFILE2"
>> + * must be a numerical value to let the endianness
>> + * determine the memory layout. That way we are able
>> + * to detect endianness when reading the perf.data file
>> + * back.
>> + *
>> + * we check for legacy (PERFFILE) format.
>> + */
>> +static const char *__perf_magic1 = "PERFFILE";
>> +static const u64 __perf_magic2    = 0x32454c4946524550ULL;
>> +static const u64 __perf_magic2_sw = 0x50455246494c4532ULL;
>>
>> -#define PERF_MAGIC   (*(u64 *)__perf_magic)
>> +#define PERF_MAGIC   __perf_magic2
>>
>>  struct perf_file_attr {
>>       struct perf_event_attr  attr;
>> @@ -1620,24 +1631,59 @@ int perf_header__process_sections(struct perf_header *header, int fd,
>>       return err;
>>  }
>>
>> +static int check_magic_endian(u64 *magic, struct perf_file_header *header,
>> +                           struct perf_header *ph)
>> +{
>> +     int ret;
>> +
>> +     /* check for legacy format */
>> +     ret = memcmp(magic, __perf_magic1, sizeof(*magic));
>> +     if (ret == 0) {
>> +             pr_debug("legacy perf.data format\n");
>> +             if (!header)
>> +                     return -1;
>> +
>> +             if (header->attr_size != sizeof(struct perf_file_attr)) {
>> +                     u64 attr_size = bswap_64(header->attr_size);
>> +
>> +                     if (attr_size != sizeof(struct perf_file_attr))
>> +                             return -1;
>> +
>> +                     ph->needs_swap = true;
>> +             }
>> +             return 0;
>> +     }
>> +
>> +     /* check magic number with same endianness */
>> +     if (*magic == __perf_magic2)
>> +             return 0;
>> +
>> +     /* check magic number but opposite endianness */
>> +     if (*magic != __perf_magic2_sw)
>> +             return -1;
>> +
>> +     ph->needs_swap = true;
>> +
>> +     return 0;
>> +}
>> +
>>  int perf_file_header__read(struct perf_file_header *header,
>>                          struct perf_header *ph, int fd)
>>  {
>> +     int ret;
>> +
>>       lseek(fd, 0, SEEK_SET);
>>
>> -     if (readn(fd, header, sizeof(*header)) <= 0 ||
>> -         memcmp(&header->magic, __perf_magic, sizeof(header->magic)))
>> +     ret = readn(fd, header, sizeof(*header));
>> +     if (ret <= 0)
>>               return -1;
>>
>> -     if (header->attr_size != sizeof(struct perf_file_attr)) {
>> -             u64 attr_size = bswap_64(header->attr_size);
>> -
>> -             if (attr_size != sizeof(struct perf_file_attr))
>> -                     return -1;
>> +     if (check_magic_endian(&header->magic, header, ph) < 0)
>> +             return -1;
>>
>> +     if (ph->needs_swap) {
>>               mem_bswap_64(header, offsetof(struct perf_file_header,
>> -                                         adds_features));
>> -             ph->needs_swap = true;
>> +                          adds_features));
>>       }
>>
>>       if (header->size != sizeof(*header)) {
>> @@ -1873,8 +1919,13 @@ static int perf_file_header__read_pipe(struct perf_pipe_file_header *header,
>>                                      struct perf_header *ph, int fd,
>>                                      bool repipe)
>>  {
>> -     if (readn(fd, header, sizeof(*header)) <= 0 ||
>> -         memcmp(&header->magic, __perf_magic, sizeof(header->magic)))
>> +     int ret;
>> +
>> +     ret = readn(fd, header, sizeof(*header));
>> +     if (ret <= 0)
>> +             return -1;
>> +
>> +      if (check_magic_endian(&header->magic, NULL, ph) < 0)
>>               return -1;
>>
>>       if (repipe && do_write(STDOUT_FILENO, header, sizeof(*header)) < 0)
>> --
>> 1.7.4.1