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Subject[PATCH v4 18/18] perf: make perf able to read file from older ABIs
Date
This patches provides a way to handle legacy perf.data
files. Legacy files are those using the older PERFFILE
signature.

For those, it is still necessary to detect endianness but
without comparing their header->attr_size with the
tool's own version as it may be different. Instead, we use
a reference table for all known sizes from the legacy era.

We try all the combinations for sizes and endianness. If we find
a match, we proceed, otherwise we return: "incompatible file format".
This is also done for the pipe-mode file format.

Signed-off-by: Stephane Eranian <eranian@google.com>
---
tools/perf/util/header.c | 126 +++++++++++++++++++++++++++++++++++----------
1 files changed, 98 insertions(+), 28 deletions(-)
diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index 1fb365d..a15f451 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -1630,35 +1630,102 @@ 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)
+static const int attr_file_abi_sizes[] = {
+ [0] = PERF_ATTR_SIZE_VER0,
+ [1] = PERF_ATTR_SIZE_VER1,
+ 0,
+};
+
+/*
+ * In the legacy file format, the magic number is not used to encode endianness.
+ * hdr_sz was used to encode endianness. But given that hdr_sz can vary based
+ * on ABI revisions, we need to try all combinations for all endianness to
+ * detect the endianness.
+ */
+static int try_all_file_abis(uint64_t hdr_sz, struct perf_header *ph)
{
- int ret;
+ uint64_t ref_size, attr_size;
+ int i;

- /* 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;
+ for (i = 0 ; attr_file_abi_sizes[i]; i++) {
+ ref_size = attr_file_abi_sizes[i]
+ + sizeof(struct perf_file_section);
+ if (hdr_sz != ref_size) {
+ attr_size = bswap_64(hdr_sz);
+ if (attr_size != ref_size)
+ continue;

- if (header->attr_size != sizeof(struct perf_file_attr)) {
- u64 attr_size = bswap_64(header->attr_size);
+ ph->needs_swap = true;
+ }
+ pr_debug("ABI%d perf.data file detected, need_swap=%d\n",
+ i,
+ ph->needs_swap);
+ return 0;
+ }
+ /* could not determine endianness */
+ return -1;
+}

- if (attr_size != sizeof(struct perf_file_attr))
- return -1;
+#define PERF_PIPE_HDR_VER0 16
+
+static const size_t attr_pipe_abi_sizes[] = {
+ [0] = PERF_PIPE_HDR_VER0,
+ 0,
+};
+
+/*
+ * In the legacy pipe format, there is an implicit assumption that endiannesss
+ * between host recording the samples, and host parsing the samples is the
+ * same. This is not always the case given that the pipe output may always be
+ * redirected into a file and analyzed on a different machine with possibly a
+ * different endianness and perf_event ABI revsions in the perf tool itself.
+ */
+static int try_all_pipe_abis(uint64_t hdr_sz, struct perf_header *ph)
+{
+ uint64_t ref_size;
+ int i;
+
+ for (i = 0 ; attr_pipe_abi_sizes[i]; i++) {
+ if (hdr_sz != attr_pipe_abi_sizes[i]) {
+ u64 attr_size = bswap_64(hdr_sz);
+
+ if (attr_size != ref_size)
+ continue;

ph->needs_swap = true;
}
+ pr_debug("Pipe ABI%d perf.data file detected\n", i);
return 0;
}
+ return -1;
+}

- /* check magic number with same endianness */
- if (*magic == __perf_magic2)
+static int check_magic_endian(u64 magic, uint64_t hdr_sz,
+ bool is_pipe, 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 (is_pipe)
+ return try_all_pipe_abis(hdr_sz, ph);
+
+ return try_all_file_abis(hdr_sz, ph);
+ }
+ /*
+ * the new magic number serves two purposes:
+ * - unique number to identify actual perf.data files
+ * - encode endianness of file
+ */
+
+ /* check magic number with one endianness */
+ if (magic == __perf_magic2)
return 0;

- /* check magic number but opposite endianness */
- if (*magic != __perf_magic2_sw)
+ /* check magic number with opposite endianness */
+ if (magic != __perf_magic2_sw)
return -1;

ph->needs_swap = true;
@@ -1677,8 +1744,11 @@ int perf_file_header__read(struct perf_file_header *header,
if (ret <= 0)
return -1;

- if (check_magic_endian(&header->magic, header, ph) < 0)
+ if (check_magic_endian(header->magic,
+ header->attr_size, false, ph) < 0) {
+ pr_debug("magic/endian check failed\n");
return -1;
+ }

if (ph->needs_swap) {
mem_bswap_64(header, offsetof(struct perf_file_header,
@@ -1924,21 +1994,17 @@ static int perf_file_header__read_pipe(struct perf_pipe_file_header *header,
if (ret <= 0)
return -1;

- if (check_magic_endian(&header->magic, NULL, ph) < 0)
+ if (check_magic_endian(header->magic, header->size, true, ph) < 0) {
+ pr_debug("endian/magic failed\n");
return -1;
+ }
+
+ if (ph->needs_swap)
+ header->size = bswap_64(header->size);

if (repipe && do_write(STDOUT_FILENO, header, sizeof(*header)) < 0)
return -1;

- if (header->size != sizeof(*header)) {
- u64 size = bswap_64(header->size);
-
- if (size != sizeof(*header))
- return -1;
-
- ph->needs_swap = true;
- }
-
return 0;
}

@@ -1975,6 +2041,10 @@ static int read_attr(int fd, struct perf_header *ph,

/* on file perf_event_attr size */
sz = attr->size;
+ if (sz != our_sz)
+ pr_debug("on file attr=%zu vs. %zu bytes,"
+ " ignoring extra fields\n", sz, our_sz);
+
if (ph->needs_swap)
sz = bswap_32(sz);

--
1.7.1


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