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Message-ID: <CABPqkBRDhKUZf0FvShGbQWoPPAWAHA+XbUUqxDY+M1J9_gDiuQ@mail.gmail.com>
Date: Tue, 20 Sep 2011 21:02:22 +0200
From: Stephane Eranian <eranian@...gle.com>
To: David Ahern <dsahern@...il.com>
Cc: linux-kernel@...r.kernel.org, peterz@...radead.org,
acme@...hat.com, mingo@...e.hu
Subject: Re: [PATCH] perf: fix perf.data endianness detection
On Tue, Sep 20, 2011 at 8:59 PM, David Ahern <dsahern@...il.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@...gle.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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