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SubjectRe: [PATCH v4 14/18] perf: fix endianness detection in perf.data
On Saturday 28 January 2012 02:26 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_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.
>
> 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;
In perf context, the variable '__perf_magic2_sw' (I guess 'sw' stands for switch)
sounds something related to SW events. Could we change this to something like
'__perf_magic2_revend' or simply '__perf_magic2_rev' which would mean reverse endianness ?
>
> -#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)


--
Anshuman Khandual
Linux Technology Centre
IBM Systems and Technology Group



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