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SubjectRe: [PATCH] perf: fix perf.data endianness detection
From
On Tue, Sep 20, 2011 at 8:59 PM, David Ahern <dsahern@gmail.com> wrote:
>
>
> On 09/19/2011 03:56 AM, Stephane Eranian wrote:
>>
>> 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) as known to perf
>> record. If the size does 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 the file and the
>> tool. However, the kernel API is designed to make it possible
>> to extend perf_event_attr for new features. 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 mismatch. The magic number is an eight-byte
>> signature. 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 if
>> the endianness matches. The new signature is "PERFILE2".
>>
>> Backward compatiblity with existing perf.data file is
>> ensured.
>>
>> Signed-off-by: Stephane Eranian <eranian@google.com>
>> ---
>>
>> diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
>> index b6c1ad1..d824502 100644
>> --- a/tools/perf/util/header.c
>> +++ b/tools/perf/util/header.c
>> @@ -57,9 +57,20 @@ char *perf_header__find_event(u64 id)
>>       return NULL;
>>  }
>>
>> -static const char *__perf_magic = "PERFFILE";
>> -
>> -#define PERF_MAGIC   (*(u64 *)__perf_magic)
>> +/*
>> + * 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   (__perf_magic2)
>>
>>  struct perf_file_attr {
>>       struct perf_event_attr  attr;
>> @@ -595,25 +606,57 @@ out_free:
>>       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;
>> +
>> +                     mem_bswap_64(header, offsetof(struct perf_file_header,
>> +                                             adds_features));
>> +                     ph->needs_swap = true;
>> +             }
>> +             return 0;
>> +     }
>> +
>> +     /* check for our magic (same endianness) */
>> +     if (*magic == __perf_magic2)
>> +             return 0;
>> +
>> +     /* check for our magic (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;
>> -
>> -             mem_bswap_64(header, offsetof(struct perf_file_header,
>> -                                         adds_features));
>> -             ph->needs_swap = true;
>> -     }
>> +     if (check_magic_endian(&header->magic, header, ph) < 0)
>> +             return -1;
>>
>>       if (header->size != sizeof(*header)) {
>>               /* Support the previous format */
>> @@ -824,8 +867,13 @@ 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)
>
>
> tip + this patch
>
> new PPC data file created and then the data file is analyzed on x86
> $ perf script -i /tmp/perf-new-ppc.data
>  incompatible file format
>
> on x86:
> $ od -c /tmp/perf-new-ppc.data | head -1
> 0000000   2   E   L   I   F   R   E   P  \0  \0  \0  \0  \0  \0  \0   h
>
> So it is using the new magic.
>
> Same file on ppc:
> # od -c /tmp/perf-new-ppc.data | head -1
> 0000000   2   E   L   I   F   R   E   P  \0  \0  \0  \0  \0  \0  \0   h
>
Ok, not quite right. The signature in the file needs to be different
between PPC and x86. I think I know where this is coming from.
Let me try again....
Thanks for testing, I'll send an updated patch soon.
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